1995_revievghfjmvhgfw_peo-ppo-peo block copolymer surfactants in aqueous solutions and at interfaces
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8/16/2019 1995_revievghfjmvhgfw_PEO-PPO-PEO Block Copolymer Surfactants in Aqueous Solutions and at Interfaces
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C L LS A
AND
Colloids and Surfaces SURFACES
LS VI R A: Physicochemical and Engineering Aspects 96(1995) I 46
e v i ew
Poly ethy lene ox ide)-poly pro py lene oxide ) -poly ethylene oxide)
block copolymer surfactants in aqueous solutions and at interfaces:
thermodynam ics, structure, dynam ics, and m ode ling
Paschalis Alexandridis, T. Alan Hat ton
Department ~?/Chemical Engineering, Massachusetts Institute of Teclmolo~y, 77 Massachusetts At:emte, Camhrid:,,e,
MA 02139,
USA
Received 18 July 1994: accepted 17 September 1994
bstract
The association properties of poly(ethylene oxide)-b/ock-poly(propylene oxidel block-poly(ethylene oxidel IPEO
PPO PEOI copolymers (commercially available as Poloxamers and Pluronics) in aqueous solutions, and the adsorpt ion
of these copolymers at interfaces are reviewed. At low temperatures and/'or concentrations the PEO PPO PEO
copolymers exist in solution as individual coils (unimers). Thermodynami cally stable micetles are formed with increasing
copolymer conce ntrat ion and/or solution temperature, as revealed by surface tension, light scattering, and dye
solnbili zation experiments. The unimer-to-micelle trans ition is not sharp, but spans a concentr ation decade o1 10 K.
The critical micellization concen trat ion (CMC) and temperature ( CMTt decrease with an increase in the copolymer
PPO content or molecular weight. The dependence of CMC on temperature, together with differential scanning
calorimetry experiments, indicates that the micellization process of PE O- PP O PEO copolymers in water is endothermic
and driven by a decrease in the polarity of ethylene oxide (EO) and propylene oxide (PO) segments as the temperature
increases, and by the ent ropy gain in water when unimers aggregate to form micelles (hyd rophobic effect I. The free
energy and enthalpy of micellization can be correlated to the total numb er of EO and PO segments in the copolymer
and its molecular weight. The micelles have hyd rodyna mic radii of approximately 10 nm and aggregation numbers in
the order of 50. The aggregation number is thought to be independent of the copolymer concentration and to increase
with temperature. Phenomenological and mean-field lattice models for the formation of micetles can describe
qualitatively the trends observed experimentally. In addition, the lattice models can provide information on the
distr ibution of the EO and PO segments in the micelle. The PEO PPO-- PE() copolymers adsorb on both air water
and solid water interfaces: the PPO block is located at the interface while the PEO block extends into the solution,
when copolymers are adsor bed at hydrophobic interfaces. Gels are formed by certain PEO P PO PEO block
copolymers at high concent rations, with the micelles rema ining apparent ly intact in the form of a 'crystal . -Vhe
gelation onset temperature and the thermal stability range of the gel increase with increasing PEO block length. A
comparison of PEO -P PO copolymers with PEO -P BO and P EO PS block copolymers and C~E/surfactants is made,
and selected applications of PEO PPO -P EO block copolymer solutions (such as solubilization of organics, protection
of microorganisms, and biomedical uses of micelles and gels) are presented.
Kevword,s: Block copolymer surfactants; Dynamics: Interfaces: Modeling: Structure: Thermodynamics
* Corresponding author.
092%7757:95 $29.00 ,c: 1995 Elsevier Science B.V. All rights reserved
S,S I)I 0927-7757( 94 )03028-6
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2 P. Alexandridis, T.A . Hatton/Colloids Surfaces A: Physicochem. Eng . Aspects 96 1995) 1 46
1 . I n t r od u c t i on
Copolymers are synthesized by the simultaneous
polymerization of more than one type of monomer.
The result of such a synthesis is called a block
copolymer if the individual monomers occur as
blocks of various lengths in the copolymer mole-
cule. The different types of blocks within the copol-
ymer are usually incompatible with one another
and, as a consequence, block copolymers self
assemble in melts and in solutions. In the case of
amphiphilic copolymers in aqueous solutions, the
copolymers can assemble in microstructures that
resemble micelles formed by low-molecular-weight
surfactants.
Water-soluble triblock copolymers of poly(eth-
ylene oxide) (PEO) and poly(propylene oxide)
(PPO), often denoted PEO PP O- PE O or (EO),I
(PO)m(EO),2, are commercially available non-
ionic macromolecular surface active agents.
Variation of the copolymer composition (PPO/
PEO ratio) and molecular weight (PEO and PPO
block length) during synthesis leads to the
production of molecules with optimum properties
that meet the specific requirements in various areas
of technological significance. As a result, PEO
PPO PEO block copolymers are an important
class of surfactants and find widespread industrial
applicat ions in [-1,2] detergency, dispersion stabili-
zation, foaming, emulsification, lubrication, and
formulation of cosmetics [3,4] and inks [5,6], etc.,
along with more specialized applications in, for
example, pharmaceuticals (drug solubilization and
controlled release [7 10] and burn wound cover-
ing [11]), bioprocessing (protecting microorga-
nisms against mechanical damage [12-14]), and
separations (solubilization of organics in aqueous
solutions [15 17]). Commercial names for these
surfactants are Poloxamers (manufactured by ICI)
and Pluronics (manufactured by BASF).
2 . S y n t h e s i s n o m e n c l a t u r e a n d p h y s i c a l p r o p e r t ie s
The PEO-PPO-PEO triblock copolymers are
synthesized by the sequential addition of first pro-
pylene oxide (PO) and then ethylene oxide (EO)
to a low molecular weight water-soluble propylene
glycol, a poly(propylene oxide) oligomer (note tha t
propylene glycol changes from water soluble to
water insoluble as the molecular weight increases
beyond about 740). The oxyalkylation steps are
carried out in the presence of an alkaline catalyst,
generally sodium or potassium hydroxide. The
catalyst is then neutralized and removed from the
final product [1]. The equations representing the
two steps in the synthesis of the PE O- PP O- PE O
copolymers are shown in Fig. 1.
The Pluronic PEO-PPO PEO block copoly-
mers are available in a range of molecular weights
and PPO/PEO composition ratios [1,2,18]. The
Pluronic copolymers are presented in Fig. 2,
arranged in the so-called Pluronic grid [2,18].
The copolymers along the vertical lines have the
same PPO/PEO composition, while the copoly-
mers along the horizontal lines have PPO blocks
of the same length. The notation for the Pluronic
triblock copolymers starts with the letters L (for
liquid), P (for paste), or F (for flakes). The first
one or two numbers are indicative of the molecular
weight of the PPO block, and the last number
signifies the weight fraction of the PEO block. For
example, Pluronics P104 and F108 have the same
molecular weight of PPO (in the order of 3000),
but P104 has 40 wt.% PEO and F108 80 wt.%
PEO. The notation of Fig. 2 will be used in the
ensuing discussion.
Addit ion of PO to form the PPO middle block
H O 'C H -C H 2" O HI + ( m I ) C H 3 C H ? H 2
C H 3 O
HO[-ClH-CH2-O-]rnH
CH 3
Addit ion of EO to form the PEO s ide block
HO[-CH-CH2-O-]mH + (2n)CH2-CH 2
\ /
OH 3 O
HO[-CH2-CH2-O-]n[-CH-CH2-O-]m[-CH2-CH2-O-]nH
CH 3
Fig. 1 . Equ at ions represent ing the synthes is of the P E O
P P O P E O c o p o l y m e r s .
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P . A l e x a n d r i d i s , T . A . H a t t o n / C o l l o i d s
> ; u r /u c e s
A : P h v s i c o c h e m . E n g .
A,v~ects
9 6 ( 1 9 9 5 ) 1 4 6 3
4 ~
36 5
3 2 5 0 ~ ' - - - - - -
~ , 2 7 5 , 0
o 2 2 5 0
2 0 5 1 3 1 - -
~ 1 7 5 0
E
E
t .~_
c ~
>
~ 2 0 0
9 5 0 ' - - - - - - - -
° /o ~ 0
- - - 4 p 85 't
J
i , ~ / /
I
_ _ _ _
1 0 2 0 3 0 q O 5 0 6 0 7 0 8 0
Percen tage o f e thy len e ox ide in a mo lecu le - the P lm on ic grM
Fig. 2 . P luronic PE O -P PO PE O copo lymers arranged in the Pluronic grid . The copolym ers ahm g the vert ical lines have t h e
s a m e P P O , / P E O c o m p o s i t io n r a ti o, w h i l e t h e c o p o ly m e r s a h m g t h e h o r i z o n t a l li ne s
h a v e P P O b l o ck s o f
the sa me length. I Reprinted
with permission from
R e f, [ 2 ] ; c o p y r i g h t C a r l
Hanser Verlag,
1991.
S o m e o f t h e p h y s i c a l p r o p e r ti e s ( av e r a g e m o l e c u -
l a r w e i g h t , m e l t i n g / p o u r p o i n t , v i s c o s i t y , s u r f a c e
t e n s io n , fo a m h e i gh t , c l o u d p o i n t a n d h y d r o p h i l i ~
l ip o p h i li c b a la n c e ) o f t h e P l u r o n i c P E O - P P O -
P E O c o p o l y m e r s a r e li st e d i n T a b l e 1 ( d a t a s u p -
p l ie d b y t h e m a n u f a c t u r e r [ 1 8 ] ) . T h e c l o u d p o i n t ,
t h e te m p e r a t u r e a t w h i c h t h e c o p o l y m e r s p h a s e
s e p a r a t e f r o m w a t e r , r a n g e s f r o m a p p r o x -
i m a t e l y 1 0 C f or c o p o l y m e r s w i t h l o w P E O
c o n t e n t to a b o v e 1 0 0 C f o r c o p o l y m e r s w i t h h i g h
P E O c o n t e n t. T h e P E O c o n t e n t a l so i n fl u en c e s
t h e r a t e o f d i s s o l u t i o n : t h i s r a t e d e c r e a s e s a s t h e
r e l a t iv e p r o p o r t i o n o f t h e P E O b l o c k i n c r e a s e s .
T h e d i s s o l u t i o n r a t e a l s o d e c r e a s e s a s t h e c o -
p o l y m e r m o l e c u l a r w e i g h t i n c r e a s e s f o r P l u r o n i c
c o p o l y m e r g r o u p s w i t h t h e s a m e P P O / P E O c o rn -
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4 P. Alexandridis , T .A . Ha t ton/Co l lo ids Sur fa ces A: Phys icochem. Eng. Asp ects 96 1995) 1 46
T a b l e 1
P r o p e rt i es o f t h e P l u r o n ic P E O - P P O - P E O c o p o l y m e r s
A B C D E F G H I
L 3 5 1 9 0 0 5 0 7 3 7 5 4 9 2 5 7 3 1 8 - 2 3
F 3 8 4 7 0 0 8 0 4 8 2 6 0 5 2 35 > 1 0 0 > 2 4
L 4 2 1 6 3 0 2 0 - 2 6 2 8 0 4 6 0 3 7 7 - 1 2
L 4 3 1 8 5 0 3 0 - 1 3 1 0 4 7 0 4 2 7 - 1 2
L 4 4 2 2 0 0 4 0 1 6 4 4 0 4 5 2 5 6 5 1 2 - 1 8
L 6 2 2 5 0 0 2 0 - 4 4 5 0 4 3 2 5 3 2 1 - 7
L 6 3 2 6 5 0 3 0 1 0 4 9 0 4 3 3 0 3 4 7 - 1 2
L 6 4 2 9 0 0 4 0 1 6 8 5 0 4 3 4 0 5 8 1 2 - 1 8
P 6 5 3 4 0 0 5 0 2 7 1 8 0 4 6 7 0 8 2 1 2 - 1 8
F 6 8 8 4 0 0 8 0 5 2 1 0 00 5 0 3 5 > 1 0 0 > 2 4
L 7 2 2 7 5 0 2 0 - 7 5 1 0 3 9 1 5 2 5 1 - 7
P 7 5 4 1 5 0 5 0 2 7 2 5 0 4 3 1 0 0 8 2 1 2 - 1 8
F 7 7 6 6 0 0 7 0 4 8 4 8 0 4 7 1 0 0 > 1 0 0 > 2 4
P 8 4 4 2 0 0 4 0 3 4 2 8 0 4 2 9 0 7 4 1 2 - 1 8
P 8 5 4 6 0 0 5 0 3 4 3 1 0 4 2 7 0 8 5 1 2 - 1 8
F 8 7 7 7 0 0 7 0 4 9 7 0 0 4 4 8 0 > 1 0 0 > 2 4
F 8 8 1 1 4 0 0 8 0 5 4 2 3 0 0 4 8 8 0 > 1 0 0 > 2 4
F 9 8 1 3 0 0 0 8 0 5 8 2 7 0 0 4 3 4 0 > 1 0 0 > 2 4
P 1 0 3 4 9 5 0 3 0 3 0 2 8 5 3 4 4 0 8 6 7 - 1 2
P 1 0 4 5 9 0 0 4 0 3 2 3 9 0 3 3 5 0 8 1 1 2 - 1 8
P I 0 5 6 5 0 0 5 0 3 5 7 5 0 3 9 4 0 9 1 1 2 - 1 8
F 1 0 8 1 4 6 0 0 8 0 5 7 2 8 0 0 4 1 4 0 > 1 0 0 > 2 4
L 1 2 2 5 0 0 0 2 0 2 0 1 7 5 0 3 3 2 0 1 9 1 - 7
P 1 2 3 5 7 5 0 3 0 3 1 3 5 0 3 4 4 5 9 0 7 - 1 2
F 1 2 7 1 2 6 0 0 7 0 5 6 3 1 0 0 4 1 4 0 > 1 0 0 1 8 - 2 3
A : c o p o l y m e r . B : a v e r a g e m o l e c u l a r w e i g h t . C : P E O w t . .
D : m e l t i n g p o u r p o i n t (' ~C ) . E : v i s c o s i t y ( B r o o k f i e l d ) ( c p s ;
l i q u i d s a t 2 5 ~ C , p a s t e s a t 6 0 '~ C , s o l i d s a t 7 7 : C ) . F : s u r f a c e
t e n s i o n a t 0 .1 , 2 5 '~ C ( d y n c m ~ ). G : f o a m h e i g h t ( m m ) ( R o s s
M i l e s , 0 . 1 a t 5 0 : C ) . H : c l o u d p o i n t i n a q u e o u s 1 s o l u t i o n
( : C ) . l : H L B ( h y d r o p h i l i c - l i p o p h i l i c b a l a n c e ) .
posit ion ratio [- 1]. This is probably a result of the
degree of hydrogen bonding between the copoly-
mer molecules, and is also reflected in the physical
form of the copolymers liquid for low molecular
weight, low PEO content; solid for high molecular
weight, high PEO content copolymers). The PEO
PPO-PEO copolymers exhibit maximum foam
height at a PP O/ PEO ratio of 40 : 60; foam proper-
ties of each copolymer series increase and then
decrease slightly as the molecular weight of the
PPO segment increases; copolymers with high
PPO content are effective defoamers [, 1]. Attempts
to correlate the emulsification properties with
PPO/PEO ratio and PPO molecular weight have
not been very successful; copolymers with PPO
blocks of high molecular weight are generally
better emulsifiers than the lower-molecular-weight
homologs [ 1 ]. The thickening power of each series
of copolymers increases as the PP O block molecu-
lar weight increases and as the PPO/PEO ratio
decreases.
3 . M i c e ll e f o rm a t io n in P E O P P O P E O b lo ck
copolym er aqueous solutions
3 1 T e c h ni q ue s f o r d e te c ti n g C M C a n d C M T
The critical micellization concentration CMC),
the amphiphile concentration at which micelles
thermodynamically stable polymolecular aggre-
gates) start forming, is a parameter of great fun-
damental value [19,20]. The micellization of
amphiphilic block copolymers is inherently more
complex than that of conventional, low-molecular-
weight surfactants. The composition polydispersity
could be appreciable even for a copolymer with a
narrow molecular weight distribution and, accord-
ingly, no sharp CMC or CMT critical micell-
ization temperature, the copolymer solution tem-
perature at which micelles form) have been
observed for block copolymers. In practice, a cer-
tain CMC range with some notable uncertainty is
usually detected. A large difference is often noted
between the CMC values determined by different
methods because the sensitivity of the techniques
to the quantity of molecularly dispersed copoly-
mers unimers) present in solution may vary [21],
For common surfactants, a considerable amount
of CMC data has been collected and summarized
by Mukerjee and Mysels [,22], whereas for block
copolymers only scarce CMC data have been
available in the literature so far see, for example,
Refs. [-23] and [-24]), Further, the values reported
in the literature differ substantially: the lack of
sufficient temperature control, in conjunction with
batch-to-batch variations, may be responsible for
the observed variations [25].
Light scatter ing and fluorescence spectroscopy
experiments have shown that PEO-PPO-PEO
block copolymers of suitable PPO/PEO composi-
tion and molecular weight do indeed form polymo-
lecular aggregates in solution. As discussed below,
the CMC of aqueous PEO PPO PEO copolymer
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P . . 4 1 e . w m d r i d is . T . A . H a t t o n / C o l h f i d ~ S u d ~ t c e s A : Phvs i c ochem. Eng . ,4.~pect.v 96 17 95 ) 1 4:~
s o l u t i o n s d e c r e a s e s w i t h i n c r e a s i n g t e m p e r a t u r e
[ 2 1 , 2 6 - 2 8 ] . M i c e l l a r g r o w t h w a s o b s e r v e d f o r
P l u r o n i c L 6 4 w i t h a n i n c r e a s e i n t h e c o p o l y m e r
c o n c e n t r a t i o n a t 2 5 < C [ 2 9 ] ; m i c e ll e f o r m a t i o n b y
P l u r o n i c P 8 5 w a s i n v e s t i g a t e d b y B r o w n e t a l .
[ 2 7 ] u s i n g s t a t i c a n d d y n a m i c l i g h t s c a t t e r i n g . I t
w a s f o u n d t h a t u n i m e r s , m i c e l l e s , a n d " ' m i c e l l a r
a g g r e g a t e s " c o e x i st e d ; t h e r e la t iv e p r o p o r t i o n s o f
e a c h s p e c i e s d e p e n d e d s t r o n g l y o n t e m p e r a t u r e
a n d c o p o l y m e r c o n c e n t ra t io n . M i c e ll a r g ro w t h
o c c u r r e d w i t h i n c r e a s i n g t e m p e r a t u r e a s e v i d e n c e d
w h e n t h e s c a t t e r i n g i n t e n s i t y o f a 4 .8 % P 8 5 s o l u t i o n
w a s r e c o r d e d a s a f u n c t i o n o f t e m p e r a t u r e ; a s h a r p
i n c r e a s e i n t h e s c a t t e r i n g i n t e n s i t y a t a p p r o x i m a t e l y
3 0 ' C d e n o t e d t h e f o r m a t i o n o f m i c e ll e s. A s ig n i fi -
c a n t t e m p e r a t u r e d e p e n d e n c e w a s al s o o b s e r v e d i n
t h e m i c e l li z a ti o n b e h a v i o r o f P l u r o n i c I :' 6 8 [ 2 1 ] .
U s i n g s t a t i c a n d d y n a m i c l i g h t s c a t t e r i n g , Z h o u
a n d C h u [ 2 1 ] d e t e c t e d t h r e e t e m p e r a t u r e r e g io n s ,
w h i c h t h e y c a l l e d t h e u n i m e r , t r a n s i t i o n , a n d
m i c e l le r e g i o n s , re s p e c t i v e ly . A t r o o m t e m p e r a t u r e ,
" p a r t ic l e s " w i t h a h y d r o d y n a m i c r ad i u s o f a r o u n d
2 .3 n m a n d a b r o a d p o l y d i s p e r s i t y w e r e d e t e c t e d .
A b o v e 5 0 C , m i c e l le m o l e c u l a r w e i g h t s w e r e
f o u n d t o i n c r e a s e l i n e a r l y w i t h t e m p e r a t u r e , w h i l e
t h e h y d r o d y n a m i c r a d i u s re m a i n e d c o n s t a n t , a t
a p p r o x i m a t e l y 8 n m [ 2 1 ] . W a n k a e t a l. [- 30 ] co n -
f i rm e d t h e f in d i n g o f Z h o u a n d C h u , t h a t a g g r e g a -
t i o n n u m b e r s i n c r e a s e w i t h t e m p e r a t u r e , w h i l e t h e
m i c e l l e r a d i u s r e m a i n s a p p r o x i m a t e l y c o n s t a n t .
M o r e i n f o r m a t i o n o n P l u r o n i c c o p o l y m e r m i c el le
s iz e a n d a g g r e g a t i o n n u m b e r s , d e t e r m i n e d b y s c a t-
t e r i n g t e c h n i q u e s , i s p r e s e n t e d i n S e c t i o n 5 . 1 .
S p e c t r o s c o p i c te c h n i q u e s , b a s e d e i t h e r o n o p t i c a l
a b s o r p t i o n o r o n e m i s s io n o f l ig h t fr o m s o m e
" ' p r o b e " m o l e c u l e , a r e n o w w e l l e s t a b l i s h e d f o r
i n v e s t i g a t i n g a w i d e r a n g e o f p h y s i c a l p r o p e r t i e s
o f m i c e l l a r s o l u t i o n s [ 1 9,3 1 ] . F l u o r e s c e n c e p r o b e s
w e r e e m p l o y e d b y T u r r o a n d C h u n g [ 3 2 ] i n
s t u d y i n g t h e b e h a v i o r o f a p o l y ( e t h y l e n e o x i d e) -
p o l y ( p r o p y l e n e o x i d e ) b l o c k c o p o l y m e r i n w a t e r .
T h e p o l y m e r th e y u s e d h a d a t o ta l m o l e c u l a r
w e i g h t o f 3 0 00 , w i t h a P E O / P P O m o l a r r a ti o o f
0 .8 ; i t wa s no t spe c i f i e d i n t he pa pe r whe the r i t
w a s a d i - o r t ri b l o c k c o p o l y m e r . R e g i o n s o f n o
a g g r e g a t i o n , " m o n o m o l e c u l a r ' " m i c el le s ( a t t r ib u t e d
t o a c h a n g e i n t h e c o p o l y m e r se g m e n t c o n f o r m a -
t i o n t o a m o r e c o m p a c t s t r u c t u r e ) , a n d p o l y -
m o l e c u l a r m i c e l l e s w e r e i d e n t i f i e d w i t h i n c r e a s i n g
p o l y m e r c o n c e n t r a ti o n . T h e a g g r e g a t io n n u m b e r
o f t h e m i c el le s f o r m e d a t h i g h p o l y m e r c o n c e n -
t r a t i o n w a s d e t e r m i n e d t o b e 5 2 . W i t h a n i n c r e a s e
i n te m p e r a t u r e , a h y d r o p h o b i c f l u o r e s c e n ce p r o b e
( e x p e c t e d t o r e s id e i n t h e m i c e l le c o r e ) e x p e r i e n c e d
a m o r e h y d r o p h i l i c e n v i r o m n e n t , w h e r e a s a h y d r o -
p h i l i c p r o b e ( e x p e c t e d t o r e s i d e i n t h e i n t e r f a c i a l
r e g i o n ) e x p e r i e n c e d a m o r e h y d r o p h o b i c e n v i r o n -
m e n t . T h i s o b s e r v a t i o n w a s a t t r i b u t e d t o a g r e a te r
m i x i n g o f Pl - O c h a i n s i n th e h y d r o p h o b i c m i c e l le
c o r e a t h i g h e r t e m p e r a t u r e s o w i n g t o i n c r e a s e d
t h e r m a l a g i t a t i o n .
A h y d r o p h o b i c f l u o r e s c en c e p r o b e ( d i p h e n y l h e x -
a t r i e n e , D P H ) w a s u s e d b y A l e x a n d r i d i s e t a l. [ 2 8 ]
t o d e t e r m i n e t h e o n s e t o f m i c e l l i z a ti o n f o r a
n u m b e r o f P l u r o n i c b l o c k c o p o l y m e r s
i n
a q u e o u s
s o l u t i o n s . T h e f l u o r e s c e n c e ef f ic i e n c y o f D P H is
z e r o
i r i
a h y d r o p h i l i c e n v i r o n m e n t a n d u n i t y in
a
h y d r o p h o b i c e n v i r o n m e n t ( s u ch a s t h e c o r e o f a
m i c e l l e ) , t h u s p r o v i d i n g a s e n s i t i v e i n d i c a t o r o f
m i c e l l e f o r m a t i o n w i t h i n c r e a s i n g s o l u t i o n t e m p e r -
a t u re a n d c o p o l y m e r c o n c e n t r at i o n . C M C a n d
C M T d a t a w e r e o b t a i n e d a n d c o r r e l a t e d t o th e
P l u r o n i c m o l e c u l a r w e ig h t a n d P P O ' P E O r a ti o
[ 2 8 ] . F i g. 3 s h o w s t h e i n t e n s i t y o f a b s o r b e d l ig h t
( a r b i t r a r y u n i t s ) d u e t o D P H a s a f u n c t i o n o f
t e m p e r a t u r e [ o r a q u e o u s P h i r o n i c
P 1 0 4
s o l u t i o n s
g
03
g
E
o
2~
F
O..
E3
0.3
0.25
02
0.15
01
0.05
0
10
/ / ~ / / , / o , , , ' " 0 2 5 ~< ~ • 0 ' :' ,~
/ . 5 mt
,,,,, , ,,
.
0 05o,, j
/ , j ' . / +
/ ' 1 T ' /
/ , ' / , " / ' . o o ~ 5 ~ : ' :
i / /% I / ' : ~ i { ) 0 1 r,~
5 . S e t . / / ,' J , ....
2 5 = ~ x / . . I / . A / . / ~ / - - i ~ , -
• ~ " ~ ' < ~ , ~ : : - 0 ,0 0 5% 'i
i i i i L L . . . . Z . . . . .
20 25 30 35 40 45 50
5
T e m p e r a t u r e ( C )
F i g . 3 . L i g h t a b s o r p t i o n h l r b i t r a r y u n i t s l d u c t o D P l t a s a
f u n c t i o n o f l e m p e r a t u r e f o r a q u e o u s P l u r o n i c P I 0 4 s o l u t i o n s
a t v a r i o u s c o p o l y m e r c o n c e n t r a t i o n s : t h e c ri t ic a l m i c e l l i z a ti o n
t e m p e r a t u r e v a l u e s w e r e o b t a i n e d l 'r o m th e f ir s t b r e a k i n th e
s i g m o i d a l c u r v e s . ( A d a p t e d w i t h p e r m i s s i o n t 'r o m R e [. [ 2 8 ] :
c o p ) r i g h t A m e r i c a n C h e m i c a l S o c i e t y , 1~ )9 4.1
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6 P. Alexandridis , T .A . Ha t ton/Co l lo ids Sur fac es A: Phys icochem. Eng. Asp ects 96 1995) 1 46
at various copolymer concentrations [28]; the
critical micelle temperature values were obtained
from the first break in the intensity vs. temperature
sigmoidal curves. Critical micelle concentrations
can be obtained in a similar manner from intensity
vs. log(concentration) plots. Chattopadhyay and
London [33] used the fluorescence emission of
DPH to determine the CMC of various surfactants,
and obtained values tha t were within 10 of those
determined by other techniques. Surface tension
results showed that probe molecules, at the levels
used to determine the CMC by fluorescence (probe
concentration < 10 6 kmol m-3), did not notice-
ably affect the surfactant properties, notably sur-
face tension, nor the surface tension-derived CMC
value [34]. Edwards et al. [35] reported that, in
general, the CMC values they obtained from the
surface tension data for solutions of deionized
water and non-ionic surfactants showed fairly close
agreement with the CMC values inferred from
polyaromatic hydrocarbon solubilization data.
Similar values for the CMC of the PEO-
PPO -P EO block copolymers measured by surface
tension and dye solubilization methods have been
reported by Schmolka and Raymond [36]. The
use of surface tension measurements for the deter-
mination of CMC in PEO PPO PEO copolymer
solutions is discussed in detail in Section 7.1.
Alexandridis et al. [37] reported the effects of
temperature on the micellization properties and
the structure of the micelles for Pluronics P104
and F108, PEO-PPO-PEO block copolymers
having similar size hydrophobic (PPO) block and
different size hydrophilic (PEO) blocks. CMC and
CMT data for aqueous copolymer solutions were
obtained from a dye solubilization method [28]
and corroborated with differential scanning calo-
rimetry (DSC), surface tension, density, light scat-
tering intensity, and fluorescence spectroscopy
experiments. Best agreement between the different
techniques was achieved when the critical micelli-
zation data were estimated from (i) the onset of
solubilization of a hydrophobic dye, (ii) the onset
of the endothermic transition observed in DSC,
(iii) the high-concentration break in the surface
tension vs. polymer concentration curve, (iv) the
first break in the partial specific volume vs. temper-
ature curve, (v) the onset of increased light scatter-
ing intensity, and (vi) the inflexion in the pyrene
fluorescence emission intensity 1 1 / 1 3 ratio (see
Section 5.2.) vs. temperature sigmoidal curve [37].
These guidelines should facilitate the reporting of
consistent CMC and CMT data for amphiphilic
copolymer solutions.
3 2 Ef fe ct o f copolymer com pos i t ion and molecular
we igh t on m ice ll e fo r m a t ion
It is apparent from the experimental studies that
PEO-PPO-PEO copolymers which are relatively
less hydrophobic, either due to a high PEO content
or a low molecular weight, do not form micelles
at room temperature, but do start to aggregate at
higher temperatures. This can be explained by the
fact that water becomes a poorer solvent for both
ethylene oxide and propylene oxide segments at
higher temperatures. As shown by Alexandridis
et al. [28], the micellization process is strongly
driven by entropy, and the free energy of micelliza-
tion is mainly a function of the PPO block, the
hydrophobic part of the copolymer.
For groups of PEO-PPO PEO copolymers
with PEO blocks of constant molecular weight
and PPO blocks of varying molecular weight, the
CMC values for the Pluronic solutions (at a given
solution temperature) decreased with increasing
number of PO segments, indicating that polymers
with a larger hydrophobic (PPO) domain form
micelles at lower concentrations [28]. Higher tem-
peratures resulted in lower CMC values, with the
slope of the log(CMC) vs. PO-segment-number
curve increasing with temperature. The CMT
values for the Pluronic copolymer solutions (at a
given copolymer concentration) also decreased as
a function of the number of PO segments, indicat-
ing that polymers with a larger hydrophobic
domain form micelles at lower temperatures. The
CMC and CMT values for groups of Pluronic
copolymers with hydrophobic blocks of constant
length and hydrophilic blocks of varying length,
e.g. with 30 (L64, P65, F68), 40 (P84, P85, F88),
and 60 (P104, P104, F108) PO segments, showed
a small increase in the CMC and CMT with
increasing number of EO segments. This would
indicate that micelle formation becomes more
difficult the more hydrophilic the molecules,
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P. Alexandridis , T .A . Hat ton/ Col lo id~ Sur / iw es A: Phy,~icochem. En~ . A,~pects 96 (19 95 ) 1 46 7
I.--
: s
o
60 i
5 5 5 0 %
P E O
5 0 ) 0 1 %
4 5 , ~
4 0 01 '~k
3 5 \
1 0 % - -~ Q
3 0
o 1
25 40 \
20 P E O ~ k 1 0 ~,;
%
50 1 O 0
0 . 1 %
1 0 % %
100:o ~.. ..
I
1 5 0
8 0 % P E O
\
\
•
" ~
x \ \ \ •
\.j,
,
- - •
2 0 0 2 5 0
N u m ber o f E O s eg m en t s , N EO
3 0 0
Fig. 4. Critical micellization temperatures at various PEO
PPO PgO copolymer concentrations, as a function of the
number of EO segments, for copolymers with 40, 50, and 80
PEO content. IReprinted with permission from Ref.[28]:
copyright American Chemical Society, 19943
a l t h o u g h t h e ef fe ct o f P E O o n t h e C M C a n d C M T
is le ss p r o n o u n c e d t h a n t h a t o f P P O , i n t i m a t i n g
t h a t P P O is a p r i m a r y f a c t o r in t h e m i c e l li z a ti o n
p r oc e ss . T h e C M C a n d C M T v a lu e s f o r c o p o l y -
m e rs o f c o n s ta n t P P O / P E O c o m p o s i t i o n r at io
d e c r e a s e d w i t h i n c r e a s i n g m o l e c u l a r w e i g h t . T h e
C M T v a l u e s w e re in f l u e n c e d m o r e b y m o l e c u l a r
w e i g h t t h e l o w e r t h e r e l a t i v e P E O c o n t e n t a n d t h e
l o w e r t h e c o p o l y m e r c o n c e n t r a t i o n , a s i s e v i d e n c e d
b y t h e s t e e p e r s l o p e s f o r t h e c u r v e s f o r t h e s e
c o n d i t i o n s . T h e s e t r e n d s a r e s h o w n i n F ig . 4 [ 2 8 ] .
4 M i ce l li za t io n therm o dy na m i cs
4.1. Asso c iat io n model , d f ferent ial scan ning
c a lor ime t ry
I f i s w e l l e s t a b l i s h e d t h a t b l o c k c o p o l y m e r s o f
the A B or A B A typ e f o r m m ic e l l e s i n se l e c ti ve
s o lv e n ts w h i c h a r e t h e r m o d y n a m i c a l l y g o o d
s o l v e n l s f o r o n e b l o c k a n d p r e c i p i t a n t s f o r t h e
o t h e r , i n g e n e r a l , m i c e l l i z a t i o n o f b l o c k c o p o l y -
m e r s , a s in t h e c a s e o f c o n v e n t i o n a l s u r f a c t a n t s ,
o b e y s t h e c l o s e d a s s o c i a ti o n m o d e l , w h i c h a s s u m e s
a n e q u i l i b r i u m b e t w e e n m o l e c u l a r l y d i s p e r s e d
c o p o l y m e r ( u n i m e r ) a n d m u l t i m o l e c u l a r a g g r e g a t es
( m ic el le s) [2 4 , 3 8 ] . T h e r e a r e t w o m a i n a p p r o a c h e s
t o t h e t h e r m o d y n a m i c a n a l y s is o f t h e m i c e l li z a ti o n
p r o c e s s : t h e p h a s e s e p a r a t i o n m o d e l , i n w h i c h t h e
m i c e l l e s a r e c o n s i d e r e d t o f o r m a s e p a r a t e p h a s e
a t th e C M C , a n d t h e m a s s - a c ti o n m o d e l t h a t
c o n s i d e r s m i c e l l e s a n d u n a s s o c i a t e d u n i m e r s t o b e
i n a n a s s o c i a t i o n - d i s s o c i a t i o n e q u i l i b r i u m [ 3 9 ] .
I n b o t h a p p r o a c h e s , t h e s t a n d a r d f re e e n e r g y
c h a n g e f o r th e t r a n s f e r o f 1 t o o l o f a m p h i p h i l e f r o m
s o l u t i o n t o t h e m i c e l la r p h a s e , A G ( t h e f r ee e n e r g y
o f m i c e l l iz a t i o n ) , in t h e a b s e n c e o f e l e c t r o s t a t i c
i n t er a c ti o n s ( th e P E O P P O P E O c o p o l y m e r s a re
non - ion i c ) i s g ive n b y [ 19 ,39]
A G = R T lnlXcM c) ( 1 )
w h e r e R i s t h e g a s la w c o n s t a n t , 7 is t h e a b s o l u t e
t e m p e r a t u r e , a n d X c M c is t h e c r i ti c a l m i c e l l i z a t io n
c o n c e n t r a t i o n i n m o l e f r a c t i o n u n i t s . I n o r d e r t o
o b t a i n t h i s s i m p l e e x p r e s s i o n , t h e a s s u m p t i o n h a s
b e e n m a d e t h a t t h e c o n c e n t r a t i o n o f f re e s u r f a c t a n t
( u n i m e r ) i n t h e p r e s e n c e o f m i c e ll e s is c o n s t a n t a n d
e q u a l t o t h e C M C v a l u e , i n t h e c a s e o f t h e p h a s e -
s e p a r a t i o n m o d e l , o r t h a t t h e m i c e ll e a g g r e g a t i o n
n u m b e r i s l a r g e f o r t h e m a s s - a c t i o n m o d e l .
A p p l y i n g t h e G i b b s H e l m h o l t z e q u a t i o n , w e c a n
e x p r e s s t h e s t a n d a r d e n t h a l p y o f m i c e l l i z a ti o n ,
A H ' , a s [ 1 9 , 3 9 ]
A H = - R T 2 [ ? l n (X c M c ) /T T ] p
= R [ ? l n ( X c M c ) / ? ( l / T ) ] e (2)
F i n a ll y , t h e s t a n d a r d e n t r o p y o f m i c e ll i za t i o n p e r
m o l e o f s u r f a c t a n t , A S ', c a n b e o b t a i n e d f r o m
A S = ( A H - A G ' ) /T ( 3)
I t h a s b e e n s h o w n f o r b lo c k c o p o l y m e r m i c e l li z a-
t i o n [ 4 0 ] t h a t , w i t h i n e x p e r i m e n t a l e r r o r ,
? l n ( X c M c ) / ( ( I / T ) = (~ ln(X ) : ? ( l /T c~ lr ) (41
w h e r e X i s t h e c o n c e n t r a t i o n e x p r e s s e d a s a m o l e
f r a c t i o n , a n d Tc Mr i s t he c r i t i c a l m ic e l l i z a t i on t e m -
p e r a t u r e ; t h u s E q . ( 2 ) b e c o m e s
AH = R [ ~ I n ( X l /? ( l /TcMr t ie {5 }
I n a c c or da nc e w i th E q . ( 5 ) , t he i nve r se T oM ,
v a l u e s a r e s h o w n a s a f u n c t i o n o f t h e l o g a r i t h m o f
c o p o l y m e r c o n c e n t r a t i o n ( m o l e f r a c t i o n u n i t s ) f o r
a n u m b e r o f P l u r o n i c c o p o l y m e r s in F i g . 5 [ 2 8 ] .
The To,M y d a t a , p l o t t e d i n t h i s m a n n e r , g e n e r a l l y
s h o w e d l i n e a r i t y . T h e u s e o f t h e c l o s e d a s s o c i a t i o n
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8 P. Alexandridis, T. A. Hatton/Colloids Surfaces A: Physicochem. Eng. Aspe cts 96 1995) 1-46
0 . 0 0 3 5 ~ , ~ f
P , ~ 4
~ 0 . 0 0 3 3
o
0 . 0 0 3 2
0 . 0 0 3 1
F 8 8 F 6 8
0 . 0 03 I I L J
- 1 7 - 1 5 - 1 3 - 1 1 - 9 - 7
In C (mole fraction)
Fig. 5. R eciproc al TCMT VS. copolym er concentration plots for
various Pluronic PE O-P PO PEO copolym ers; he plots were
used for determination of the micellization enthalpy in terms
of the closed association model. (Reprinted with permission
from Ref. [-28]; copyrigh t American C hemical So ciety, 1994.)
m o d e l a p p e a r e d w e ll j u s t if i ed b a s e d o n t h e g o o d
fit; s u c h a m o d e l w a s c o n s e q u e n t l y e m p l o y e d f o r
e s t i m a t in g A H ° a n d A S ° o f t h e m i c e l l iz a t io n p r o -
c e s s [ 2 8 ] . N o t e t h a t t h e m a j o r a s s u m p t i o n
i n v o l v e d i n d e r i v in g E q s . ( 1 ) - ( 5 ) i s b a s e d o n t h e
m i c e l l e a g g r e g a t i o n n u m b e r ( N ) b e i n g s u f f i c i e n t l y
l a r g e t h a t t h e t e r m s R T N ~ I n N a n d
R T N - ~ In
Xmic w her e Xmic
i s t h e m o l e f r a c t i o n o f
m i c e l l e s ) c a n b e o m i t t e d f r o m t h e r i g h t - h a n d - s i d e
o f E q . (1 ) [ 1 9 ] . F o r a t y p i c a l a g g r e g a t i o n n u m b e r
o f 5 0, N 1 I n N ~ 0 .0 8 , in d e e d n e g l i g i b l e c o m p a r e d
to t he v a lue o f [ ln Xc M c [ ( i n t he r a ng e 10 15) .
U s i n g t h e s a m e a g g r e g a t i o n n u m b e r a n d c o n c e n -
t r a t i o n s X c M c = 6
x 10 -6
a n d
Xmi c=6 x 10 -1°
t h e
m a g n i t u d e o f t h e I N - 1 I n X m i¢ [ t e r m is 0 .4 , a p p r o x i -
m a t e l y 3 % o f th e [ln X cM c [ ( ~ 1 2 ) t e r m ; t h e t e m -
p e r a t u r e d e r i v a ti v e s o f t h e R T N - 1 1 n N a n d
R T N - 1 1 n X m i c t e r m s t h a t s h o u l d a p p e a r i n E q .
( 2 ) a r e a l so ne g l ig ib l e .
T h e s t a n d a r d e n t h a l p y o f m i c e l li z a ti o n ,
A H ~ ,
w a s c a l c u l a t e d f r o m t h e i n v e r s e s lo p e o f t h e l i n e a r
f i t t o t he 1/TcMT v s . l n ( m o l e f r a c t i o n ) d a t a , i n
a c c o r d a n c e w i t h E q . (5 ) . A H ° v a l u e s , t o g e t h e r w i t h
A G ° a n d A S ° ( c a l c u l a t e d f r o m E q s . ( 1 ) a n d ( 3 ) ,
r e s p e c t i v e l y , a t t h e c r i t i c a l m i c e l l i z a t i o n t e m p e r -
a t u r e f o r 1 % c o p o l y m e r s o l u t i o n s ) , a r e l i s t e d i n
R ef. [ 2 8 ] f o r v a r i o us P l u r o n i c P E O - P P O - P E O
c o p o l y m e r s . T h e t a b u l a t e d v a l u e s o f A H ' - , A G ° ,
a n d A S ~ r a n g e d f r o m 1 69 t o 3 3 9 k J m o 1 - 1 , - 2 4 . 5
t o - 2 8 . 8 k J m o 1 - 1 , a n d 0 . 6 3 8 t o 1 .2 44 k J t o o l - 1
K - 1 , r e sp e c ti v e ly . T h e h i g h e r v a l u e s o f A H °, A G ' 2
a n d A S ° w e r e o b s e r v e d f o r P l u r o n i c s P 1 0 3 a n d
L 6 4 , w h i l e t h e l o w e r v a l u e s w e r e o b s e r v e d f o r t h e
r e l a t i v e l y h y d r o p h i l i c P l u r o n i c c o p o l y m e r s F 6 8
a n d F 8 8 . T h e s t a n d a r d e n t h a l p y o f m i c e l li z a ti o n ,
A H ° , is p o s i t i v e , i n d i c a t i n g t h a t t h e t r a n s f e r o f
u n i m e r s f r o m s o l u t i o n t o t h e m i c e l le i s a n e n t h a l p i -
c a l l y u n f a v o r a b l e e n d o t h e r m i c p r o c e s s . T h e f r e e
e n e r g y , A G ° , i s n e g a t i v e , s i n ce t h e r m o d y n a m i c a l l y
s t a b l e m i c e l l e s a r e f o r m e d s p o n t a n e o u s l y . T h u s , i t
b e c o m e s c le a r t h a t a n e g a t i v e e n t r o p y c o n t r i b u t i o n
m u s t b e t h e d r i v i n g f o r c e f o r m i c e l l iz a t i o n o f t h e
b l o c k c o p o l y m e r s ; n o t e t h a t , i n c o n t r a s t t o t h e
e n t r o p y - d r i v e n m i c e l l i z a t i o n i n w a t e r , t h e m i c e l l i -
z a t i o n o f c o p o l y m e r s i n n o n - p o l a r s o l v e n t s o r ig i -
n a t e s f r o m e n t h a l p y i n t e r a c t i o n s b e t w e e n t h e
c o p o l y m e r se g m e n ts a n d t h e s o lv e n t [ 2 4 ] . T h e
t r a d i t i o n a l v i e w o f m i c e ll e f o r m a t i o n 1 -1 9,4 1] is
b a s e d o n t h e h y d r o p h o b i c e f fe c t [ 4 2 , 4 3 ] . T h e
p r e s e n c e o f h y d r o c a r b o n m o l e c u l e s in w a t e r c a u s e s
a s i g n i f i c a n t d e c r e a s e in t h e e n t r o p y o f t h e l a t te r ,
s u g g e s t i n g a n i n c r e a s e in t h e d e g r e e o f s t r u c t u r i n g
o f t h e w a t e r m o l e c u le s . W h e n h y d r o c a r b o n r e s i-
d u e s a g g r e g a t e i n a q u e o u s s o l u t i o n t o f o r m a
m i c e l l e , t h e h y d r o g e n b o n d i n g s t r u c t u r e i n t h e
w a t e r i s r e s t o r e d a n d t h e w a t e r e n t r o p y i n c r e a s e s ,
o v e r c o m i n g t h e e n t r o p y l o ss d u e t o t h e l o c a l i z a ti o n
o f th e h y d r o p h o b i c c h a i n s i n t h e m i c el le s . T h e
e n t r o p y c o n t r i b u t i o n u s u a l ly d o m i n a t e s t h e m i c el li -
z a t i o n p r o c e s s i n a q u e o u s s u r f a c t a n t s o l u t i o n s ,
w i t h th e e n t h a l p y p l a y i n g a m i n o r r o l e [ 1 9 ] . T h e
m a g n i t u d e o f th e h y d r o p h o b i c e f fe c t i n c re a s e s w i th
t e m p e r a t u r e E 4 3 ] , i n a g r e e m e n t w i t h t h e o b s e r v e d
i n c r e a s e d t e n d e n c y f o r m i c e l l e f o r m a t i o n w i t h
i n c r e a si n g t e m p e r a t u r e ; h o w e v e r , s u ch a n e x p l a n a -
t i o n d o e s n o t t a k e i n t o a c c o u n t t h e s p e ci fi c n a t u r e
o f t he P E O - P P O - P E O c o po ly m e rs .
S o l u t e - s o l v e n t [ 4 4 , 4 5 ] o r s o l u t e - s o l u t e i n t e r -
a c t i o n s [ 4 6 ] h a v e a l s o b e e n p r o p o s e d i n o r d e r t o
i n t e r p r e t t h e ~ m o l e c u l a r- l ev e l m e c h a n i s m b e h i n d
t h e t e m p e r a t u r e d e p e n d e n c e o b s e r v e d i n t h e m i c el -
l iz a ti o n o f P E O - P P O P E O c o p o l ym e r s in w a te r
( n o t e t h a t t h e h y d r o p h o b i c e ff e ct o u t l i n e d a b o v e i s
b a s e d o n s o l v e n t s o l v e n t i n t e r a c t i o n s ) . K j e l a n d e r
a n d F l o r in [ 4 4 ] a t t e m p t e d t o r e p r o d u c e t h e n e g a-
t iv e e n t r o p y a n d e n t h a l p y o f w a t e r - P E O m i x in g
a n d t h e p h a s e d i a g r am o f t h e P E O - w a t e r s y st em ,
8/16/2019 1995_revievghfjmvhgfw_PEO-PPO-PEO Block Copolymer Surfactants in Aqueous Solutions and at Interfaces
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P. A lexan dr id is . T .A . Ha t ton /C ol lo i ds Sur lac es A . Phys icochem . Eng . A ,v~ec ts 96 ( 1995 , 1 46 9
a s s u m i n g a z o n e w i t h i n c re a s e d s t r u c t u r i n g o f
w a t e r t o e x is t a r o u n d t h e P E O c h a i n . T h e p h a s e
s e p a r a t i o n t h a t t a k e s p l a c e a t h i g h t e m p e r a t u r e s
w a s a t t r i b u t e d t o a b r e a k d o w n o f t h e z o n e s o f
e n h a n c e d w a t e r s t ru c t u r e . K j e l a n d e r a n d F l o r i n
[ -4 4 ] c l a im t h a t w h e n P P O is i n t r o d u c e d i n t o
w a t e r i t a l s o d e v e l o p s a h y d r a t i o n s h e l l w i t h a n
e n h a n c e d w a t e r s t r u c t u r e , b u t s i n c e t h e m e t h y l
g r o u p s o f P P O p r o v i d e s t e ri c h i n d r a n c e , t h e w a t e r
s t r u c t u r e i s w e a k a n d l e a d s t o p h a s e s e p a r a t i o n .
K a r l s t r o m [ - 4 6 ] p r e d i c t e d t h e P E O w a t e r p h a s e
d i a g r a m u s i n g F l o r y H u g g i n s t h e o r y a n d a s s u m -
i n g t h a t e a c h s e g m e n t o f t h e P E O c h a i n c a n e x i st
i n t w o f o r m s , o n e p o l a r w i t h a l o w e n e r g y a n d a
l o w s t a t i s t i c a l w e i g h t , a n d o n e l e s s p o l a r , o r n o n -
p o l a r , w i t h a h i g h e r e n e r g y a n d s t a t i s t i c a l w e i g h t .
T h e p o l a r c o n f o r m a t i o n s d o m i n a t e a t lo w t e m p e r -
a t u r e s m a k i n g t h e s o l u t e - s o l v e n t i n t e ra c t i o n f a v o r -
a b le , w h e r e a s a t h i g h e r t e m p e r a t u r e s t h e n o n - p o l a r
c l a ss o f s t a t e s b e c o m e s i n c r e a s i n g l y p o p u l a t e d .
r e n d e r i n g t h e s o l u t e s o l v e n t i n t e r a c t i o n l e s s f a v o r -
a b l e [ 4 7 ] . T h e m o d e l f o r p r e d i c t in g t h e s o l u t i o n
b e h a v i o r o f b l o c k c o p o l y m e r m i c e ll e s d e v e l o p e d
by Hur t e r e t a l . [ - 48 ,49] a nd L inse [ - 50 ,51]
( se e S e c t i o n 6 . 2. f o r a d e s c r i p t i o n o f th i s m o d e l )
i n c o r p o r a t e d K a r l s t r o m ' s i d ea s t o a c c o u n t f o r t h e
c o n f o r m a t i o n a l d i s tr i b u ti o n in P E O a n d P P O ; t h e
m o d e l p r e d i c t i o n s a g r e e w i t h t h e t r e n d s o b s e r -
v e d e x p e r i m e n t a l l y , t h u s s u p p o r t i n g t h e p o l a r -
n o n - p o l a r s t a t e m o d e l a s a n e x p l a n a t i o n o f t h e
e ff ec t o f t e m p e r a t u r e o n t h e s o l u t i o n b e h a v i o r o f
P E O a n d P P O .
A n o t h e r m e t h o d f o r o b t a i n i n g e s t i m a t e s o f t h e
e n t h a l p y o f m i c e l li z a ti o n , A H ~ is D S C . D S C m e a s -
u r e m e n t s f o r a q u e o u s P l u r o n i c c o p o l y m e r s o l u -
t i o n s s h o w e n d o t h e r m i c p e a k s [ 3 0 , 5 2 ] , t y p i c a l f o r
a f i r s t - o r d e r p h a s e t r a n s i t i o n , a t c o n c e n t r a t i o n -
d e p e n d e n t c h a r a c t e r i s t i c t e m p e r a t u r e s a s s h o w n i n
F ig . 6 [ 3 7 ] . T h e p e a k s y i e l d ra t h e r h i g h e n t h a l p y
v a l u e s a n d a r e b r o a d , e x t e n d i n g m o r e t h a n 1 0 K .
T h e l a t t e r o b s e r v a t i o n h a s b e e n a t t r i b u t e d t o t h e
f a ct t h a t t h e c o p o l y m e r s a r e n o t p u r e c o m p o u n d s
b u t s h o w a b r o a d m o l a r w e i g h t d i s t r i b u t i o n ; i t i s
k n o w n t h a t m e l t i n g p e a k s b e c o m e b r o a d i n th e
p r e s e n c e o f i m p u r i t ie s [ 3 0 ] . I t s h o u l d b e p o i n t e d
o u t t h a t t h e e n t h a l p y c h a n g e m e a s u r e d b y th e
p e a k a r e a i n D S C i s n o t t h e s t a n d a r d e n t h a l p y
c h a n g e , b u t d e p e n d s u p o n t h e r e al s t at e s o f t h e
0 3 - - ~ i i i T ~ i - -
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Temperature (C)
Fig. 6. Differential scann ing ca lorimet ry curves fl~r Pluronic
P104 aqueous solutions at various copolymer concentrations:
the endothermic peak is indicative of miccllc formation.
(Adapted from Rcf. [31].}
c o p o l y m e r m o l e c u le s b e f o r e a n d a f t er m i c e l li z a ti o n .
T h e s t a n d a r d s t a t e e n t h a l p y c h a n g e i s d e f i n e d f o r
t r a n s f e r o f 1 t o o l o f c o p o l y m e r s f r o m t h e i d e a l ly
d i l u t e s o l u t i o n t o t h e s o l v a t e d m i c e l l a r s t a t e . I n
t h e i d e a l l y d i l u t e s o l u t i o n , c o p o l y m e r s e g m e n t s
i n t e r a c t o n l y w i t h t h e s o l v e n t , w h e r e a s i n r e a l
s o l u t i o n s s e g m e n t s a l s o i n t e r a c t w i t h e a c h o t h e r ,
a n d t h i s m a y c a u s e d i s c r e p a n c i e s b e t w e e n m i c e l l i -
z a t i o n e n t h a l p i e s o b t a i n e d f r o m D S C a n d t h o s e
d e r i v e d f r o m a n a n a l y s i s s i m i l a r t o t h a t o f
A l e x a n d r i d i s e t a l . [ 2 8 ] . I t h a s a l s o b e e n r e p o r t e d
b y H i e m e n z [ 4 1 ] t h a t A H ' v a l u e s c a l c u l a te d b y a
m i c e l li z a ti o n t h e r m o d y n a m i c s m o d e l g e n e r a l l y gi v e
p o o r a g r e e m e n t w i th t h o s e d e t e r m i n e d c a l o r i m e t r i -
c a l l y , a t l e a s t f o r i on i c su r f a c t a n t s .
F r o m a r a t h e r l i m i te d s e t o f d a t a ( P l u r o n i c s
P 1 2 3 , F 1 2 7 , a n d P 1 0 4 ) , W a n k a e t a l . [ 3 0 ] o b s e r v e d
n o p r o p o r t i o n a l i t y b et w e en t h e D S C e n t h a l p y
v a l u e s a n d t h e s iz e o f t h e P E O b l o c k o f t h e
m o l e c u l e s , b u t n o t e d t h a t , t o a f i r s t a p p r o x i m a t i o n ,
t h e r e w a s p r o p o r t i o n a l i t y b e t w e e n t h e s e w d u e s a n d
t h e n u m b e r o f P O s eg m e n ts . T h e y c o n c l u d e d f r o m
t h i s t h a t t h e t r a n s i t i o n w a s p r o b a b l y d u e t o t h e
d e h y d r a t i o n o r m e l t i n g o f t h e P O s eg m e n t s .
S i m i l a r t r e n d s w e r e o b s e r v e d i n t h e
l l
d a t a o f
A l e x a n d r i d i s e t a l. [ 2 8 ] , a l t h o u g h a w e a k e f fe c t o f
P E O b l o c k si ze o n A H w a s a p p a r e n l f r o m t h e
l a t te r d a t a s et , F o r P l u r o n i c c o p o l y m e r s w i th
t h e s a m e s iz e h y d r o p h o b i c ( P P O ) b l o c k a n d v a r y -
i n g s iz e h y d r o p h i l i c ( P E O ) b l o c k ( i.e . P I 0 3 , P I 0 4 ,
8/16/2019 1995_revievghfjmvhgfw_PEO-PPO-PEO Block Copolymer Surfactants in Aqueous Solutions and at Interfaces
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10 P. A lexandridis, T .A . H atton/Colloids Surfaces A. Physicochem. E ng. Aspe cts 96 (1995) 1 46
P 1 0 5 , a n d F 1 0 8 ) , A H '~ d e c r e a s e d b y a p p r o x i m a t e l y
1 5 % as th e n u m b e r o f E O s e g m e n t s in c r e a s e d f r o m
2 x 7 t o 2 x 1 32 . F o r P l u r o n i c c o p o l y m e r s w i th t h e
s a m e s iz e h y d r o p h i l i c ( P E O ) b l o c k a n d v a r y i n g
s iz e h y d r o p h o b i c ( P P O ) b l o c k ( i.e. P 6 5 , P 8 4 , a n d
P 1 2 3 ) , A H ° i n c r e a se d b y a p p r o x i m a t e l y 1 0 0% a s
t h e n u m b e r o f P O s e g m e n t s i n c re a s e d f r o m 3 0 t o
7 0 . T h e P P O e f fe c t w a s m o r e s i g n i fi c a n t, le a d i n g
t o t h e c o n c l u s i o n t h a t P P O i s m a i n l y r e s p o n s i b l e
f o r t he m ic e ll iz a ti o n o f P E O - P P O - P E O c o p o ly -
m e r s i n w a t e r . D S C w a s a l s o u s e d b y B e e z e r e t al .
[ 5 3 ] a n d M i t c h a r d e t al. [ 5 4 ] t o s tu d y P E O -
P P O - P E O b l o ck c o p o l y m e r a q u e o u s so lu ti on s .
T h e o b s e r v e d p h a s e t r a n s i t io n s w e r e i n t h e
1 0 0 3 0 0 k J m o l - ~ r a n g e a n d w e r e i n it ia l ly t h o u g h t
t o r e s u l t f r o m c h a n g e s i n t h e p o l y m e r s o l v a t i o n a s
t h e t e m p e r a t u r e c h a n g e d . B e e z e r e t al . [ 5 3 ] a n d
M i t c h a r d e t a l . [ 5 4 ] c l a i m e d t h a t , a t t h e c o n c e n -
t r a t i o n u s e d ( 5 g 1 ~ ), o n l y u n i m e r s w e r e p r e s e n t
i n s o l u t i o n . I n a m o r e r e c e n t p a p e r f r o m t h e s a m e
g r o u p , h o w e v e r , A r m s t r o n g e t al . [ 5 2 ] c o n c l u d e d
t h a t t h e o b s e r v e d e n t h a l p y c h a n g e i s i n d i c a t i v e
o f a n a g g r e g a t i o n p r o c e s s , a c c o m p a n i e d b y d e s o l -
v a t i o n a n d c h a n g e i n c o n f o r m a t i o n o f t h e
h y d r o p h o b e . A l t h o u g h d i r e c t c o m p a r i s o n b e t w e e n
t h e c a l o r i m e t r y A H v a l u e s o f R efs . [ 5 2 ] - [ - 5 4 ] a n d
t h e A H :' o f R ef. [ 2 8 ] c a n n o t b e m a d e a s t h e
c o p o l y m e r s u s e d w e r e n o t t h e s am e , th e d a t a o f
t h e l a t t e r p a p e r s t r o n g l y s u g g e s t t h a t t h e m i c e l l i z a -
t i o n p r o c e s s is t h e m a j o r c o n t r i b u t o r t o t h e c a l o r i -
m e t r i c a l l y - o b s e r v e d e n t h a l p y c h a n g e .
4 2 E f f ec t o f copo lym er com pos i t ion and m o lecu lar
we igh t on m ice l l iz a t ion ther m odyna m ics
F r e e e n e rg i e s o f m i c e l li z a t io n p e r m o l o f P l u r o n i c
c o p o l y m e r i n s o l u t i o n , A G : ( c a l c u l a te d a t t h e C M T
f o r t h e v a r i o u s c o p o l y m e r s a t d i f f e r e n t s o l u t i o n
c o n c e n t r a t i o n s ) , e x h i b i t s o m e s c a t t e r w h e n p l o t t e d
a s a fu n c t i o n o f c o p o l y m e r m o l e c u l a r w e i g h t , b u t
t h e y d o i n d i c a t e a t e n d e n c y f o r h ig h e r m o l e c u l a r
w e i g h t c o p o l y m e r s t o h a v e m o r e n e g a t i v e f r e e
e n e r g ie s o f m i c e l l iz a t i o n [ 2 8 ] . T h e A G ~ d a t a c o l -
l a p s e i n t o a s i n g l e s m o o t h c u r v e , i n d e p e n d e n t o f
t h e c o p o l y m e r c o m p o s i t io n r a ti o P P O / P E O , w h e n
n o r m a l i z e d w i t h r e s p e c t t o t h e t o t a l n u m b e r o f
m o n o m e r s e g m e n t s i n t h e p o l y m e r , N ~ o + N p o ,
a s s h o w n i n F i g . 7 [ 2 8 ] . I t c a n b e s e e n c l e a r l y
t h a t l o w e r m o l e c u l a r w e i g h t , m o r e h y d r o p h o b i c
P l u r o n i c s y i e ld m o r e n e g a t iv e A G ° v a l u e s p e r
m o n o m e r s e g m e n t. T h e d e p e n d e n c e o f th e t h e r-
m o d y n a m i c p a r a m e t e r s A H ° a n d A G ° o n t h e
P P O / P E O c o m p o s i t io n r a ti o w as al so e x a m i ne d .
T h e n o r m a l i z e d A H ( e x p re s s e d i n k J m o l - ~ o f
a v e r a g e m o n o m e r s e g m e n t ) a p p r o a c h e d z e r o a s
t h e P P O / P E O r a ti o w e n t t o z e r o [ 2 8 ] . I t c a n th u s
b e i n f e r r e d t h a t t h e m i c e l l i z a t i o n p r o c e s s i s d o m i -
n a t e d b y t h e P P O ( h y d r o p h o b i c ) p a r t o f t h e c o p o l-
y m e r . A s i m i l a r t r e n d w a s o b s e r v e d b y A r m s t r o n g
e t a l. [ 5 2 ] b a s e d o n D S C e n t h a l p y c h a n g e s o f I C I
P o l o x a m e r s o l u t i o n s , a n d b y W i l l i a m s e t a l . [ 5 5 ]
u s i n g t h e i n c r e m e n t i n a p p a r e n t m o l a r v o l u m e o n
t h e r m a l t r an s i t io n o f P o l y S c ie n c e s P E O - P P O
c o p o l y m e r s a s a f u n c ti o n o f P P O / P E O . T h e f ac t
t h a t t h e n o r m a l i z e d A H ° v a lu e s a re a p p r o x i m a t e l y
t h e s a m e f o r P l u r o n i c c o p o l y m e r s o f t h e s a m e
P P O / P E O c o m p o s i t io n r a ti o a n d d i ff er en t m o l e c u -
l a r w e i g h t s w o u l d i n d i c a t e t h a t t h e m i c e l l i z a t i o n
e n t h a l p y p e r m o n o m e r s e g m e n t i s i n d e p e n d e n t o f
m o l e c u l a r w e i g h t.
I n c o n t r a s t t o t h e t r e n d o f d e c r e a s in g A G ° w i t h
i n c r e a s i n g P l u r o n i c m o l e c u l a r w e i g h t , t h e r e i s n o
d e f i n i t e d e p e n d e n c e o f A H ~' o n m o l e c u l a r w e i g h t
o v e r t h e r a n g e c o v e r e d i n t h e s t u d y o f A l e x a n d r i d is
e t a l. [ 2 8 ] . T h e A H '~ d a t a p r e s e n t e d i n R e f. [ 2 8 ]
r e v e a le d t w o m a i n g r o u p s o f c o p o l y m e r s : t h e r e la -
t iv e l y h y d r o p h o b i c P l u r o n i c s P 1 0 3 , P 1 0 4 , P 1 0 5 ,
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F i g . 7 . E f fe c t o f P E O - P P O P E O c o p o l y m e r m o l e c u la r w e ig h t
o n t h e f r e e en e r g i e s o f m i c e l l i z a ti o n f o r v a r i o u s c o p o l y m e r
c o n c e n t r a t i o n s e x p r e s s e d a s AG : / ( N Eo + N p o ) k J m o l 1 o f
m o n o m e r s e g m e n ts ) . R e p r i n t e d w i t h p er m i s s i o n f r o m
R e f. [ 2 8 ] ; c o p y r i g h t A m e r i c a n C h e m i c a l S o c i e ty , 1 9 94 .)
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P. Alexandridis, T .A. Hatton/Colloids SutJiwes A. P hysicochem. Etlg. Aspects 96 /g? 5; l 4 5 11
a n d P 1 2 3 w i t h A H '~' i n t h e 3 0 0 - 3 5 0 k J m o l
r a n g e , a n d t h e r e la t iv e l y h y d r o p h i l i c c o p o l y m e r s
L 64 , P65 , P84 , a nd P85 wi th A H '~ i n t h e
1 8 0 - 2 3 0 k J m o l ~ r a n g e : A H '~ f o r t h e o t h e r f o u r
c o p o l y m e r s r a n g e d b e t w e e n 1 7 0 a n d 2 7 0 k J t o o l ~.
A p l o t o f A G o f m i c e l l i z a t i o n ( e x p r e s s e d i n
k J t o o l ~ o f a v e r a g e m o n o m e r s e g m e n t ) a s a
f u n c ti o n o f P P O / P E O s h o w e d t he A G - '/ s eg m e n t
v a lu e s a p p r o a c h i n g z e r o as th e P P O / P E O r a ti o
w e n t t o z e r o [ 2 8 ] . T h e s e g m e n t f re e e n e r g y , h o w -
e v e r , d e c r e a s e d ( b e c a m e m o r e n e g a t i v e ) w i t h a
d e c re a s e in m o l e c u l a r w e i gh t f o r P E O P P O P E O
c o p o l y m e r s h a v in g t he s am e P P O / P E O r at io . In
a d i f f e r e n t i nve s t i ga t ion , Re ddy e t a l . [ - 26] e s t i -
m a t e d t h e A G o f m i c e l li z a ti o n a t - 2 0 k J t o o l -
a n d t h e A H ' o f m i c e l l i z a t i o n a t 2 0 0 k J t o o l 1 f o r
a s o l u t i o n o f p u r i f i e d P l u r o n i c L 6 4 , u ti l iz i n g
I n ( C M C ) v s . l I T d a t a ( o n l y th r e e d a t a p o i n t s,
t h o u g h ) . T h e s a m e g r o u p [ - 4 0 ] r e p o r t e d t h e A H '
o f m i c e l l iz a t i o n a t 3 1 6 + 2 0 k J t o o l - ~ f o r a s o l u t i o n
o f p u r i fi e d F 1 2 7 ( f r o m In ( c o n c e n t r a t i o n ) v s. 1 /T cM r
d a t a ) . T h e s e v a l u e s v a r y b y a p p r o x i m a t e l y 2 0
f ro m A G = - 2 4 . 5 k J m o l - ~ , a n d
A H ' =
2 3 0 k J t o o l 1 f o r L 6 4 a n d A H - = 2 5 3 f o r F 1 2 7
r e por t e d i i a Re f . [ 28] .
5 Structure of block copolym er micel les
5 .1 . A¢ greg a t ion munbe ; mice l l e s i ze and shape
T h e a s so c ia ti o n o f P E O P P O - P E O b l oc k
c o p o l y m e r s i n t o m i c e ll es a n d t h e s t r u c t u r e o f t h e s e
m i c e l l e s h a v e b e e n i n v e s t i g a t e d b y m a n y r e s e a r c h -
e r s [ -2 1,2 6 3 0 , 3 7 , 4 0 , 5 6 - 7 0 ] . W e p r e s e n t b e l o w
i n f o r m a t i o n o n m i c e l l e s f o r m e d b y P l u r o n i c s L 6 4 ,
P 8 5, F 8 8 , F 6 8 , a n d F 1 2 7 P E O - P P O P E O b l o c k
c o p o l y m e r s , f o r w h i c h a c o n s i d e r a b l e n u m b e r o f
s t u d i e s h a v e b e e n p u b l i s h e d . S i g n i f i ca n t i n f o r m a -
t i o n h a s b e e n o b t a i n e d f r o m l i g h t a n d n e u t r o n
sc a t t e r ing s tud i e s . I n ge ne r a l , t he un im e r s i z e i s
f o u n d t o b e a p p r o x i m a t e l y 1 n m a n d t h e m i c el le
s iz e 1 0 n m , i n d e p e n d e n t o f c o p o l y m e r c o n c e n -
t r a t i o n ( a l t h o u g h , t h e c o n c e n t r a t i o n s u s e d w e r e
u s u a ll y h ig h ( a b o v e a p p r o x i m a t e l y 2 ) t o a l lo w
g o o d s i g n a l - t o - n o i s e r a t i o i n t h e s c a t t e r i n g e x p e r i -
m e n t s ) . T h e e f f e ct o f t e m p e r a t u r e i s i n te r e s t in g : a n
i n c r e a s e i n t h e a g g r e g a t i o n n u m b e r w i t h t e m p e r -
a t u r e h a s b e e n o b s e r v e d , w h i l e t h e m i c e l l a r r a d i u s
r e m a i n s c o n s t a n t . T h e c o n c l u s i o n s a r e c o m p l i c a t e d
b y t h e r a t h e r b r o a d C M T t r a n s it i o n , a n d t h e fa c t
t h a t d y n a m i c l i g h t s c a t t e r i n g d e t e c t s t h e m i c e l l e
h y d r o d y n a m i c r a d i u s w h i c h in c l u d e s t h e w a t e r
h y d r a t i n g t h e E O s e g m e n t s .
P l u r o n i c L 6 4 s h o w e d d e t e c t a b l e a g g r e g a t e s a t
2 5 ' C o n l y a t co n c e n t r a t io n s a b o v e a p p r o x i m a t e l y
6 : th e m i c e l le s i ze i n c r e a s e d w i t h c o n c e n t r a t i o n
( 1 0 n m a t 8 1 2 . 5 n m a t 2 0 ) a n d ex h i b i te d
s i g n i f ic a n t p o l y d i s p e r s i t y , p r o b a b l y i n d i c a t i n g a
m u l t i p l e a s s o c i a t i o n p r o c e s s [ 2 9 ] . A t 3 5 C , h o w -
e v e r , e s s e n t i a l l y i n v a r i a n t v a l u e s f o r t h e h y d r o d y -
n a m i c r a d iu s w e r e f o u n d o v e r a w i d e c o n c e n t r a t i o n
r a n g e a n d t h e m i c e l l e s w e r e m o n o d i s p e r s e : t h e s e
s y s t e m s a r e m o r e l i k e l y r e p r e s e n t e d b y a c l o s e d
a s s o c i a ti o n m o d e l [ -2 9 ]. Z h o u a n d C h u [ 5 6 ] f o u n d
t h a t m i c e ll a r m o l e c u l a r w e i g h t i n c r ea s e d e x p o n e n -
t i a l l y w i t h i n c r e a s i n g t e m p e r a t u r e ; c o m p o s i t i o n
h e t e r o g e n e i t y ( p r e s e n c e o f h y d r o p h o b i c i m p u ri ti es )
c a u s e d s t r o n g a n g u l a r a s y m m e t r y o f s c a t t e re d l i g h t
b y L 6 4 c o p o l y m e r s o l u t io n s . T h e m o d e o f a ss o c ia -
t i o n o f a p u r i fi e d L 6 4 s a m p l e i n a q u e o u s s o l u t i o n
h a s b e e n e x a m i n e d b y R e d d y e t a l . [ 2 6 ] ; a s s o c i a -
t i o n a t 34 .5 a n d 4 0 C w a s d e s c r i b e d b y a c o o p e r a -
t i v e a s s o c i a t i o n m o d e l w h i c h a s s u m e s a g g r e g a t e
g r o w t h b y s t e p w is e a d d i t i o n o f u n i m e r s ; n o t e t h a t
t h i s i s i n c o n t r a s t t o t h e r e s u l t s r e p o r t e d b y
A1- Sa de n e t a l . [ 29] . A lm gr e n e t a l . [ - 58] c a l c u l a t e d
( f r o m s t a t i c l i g h t s c a t t e r i n g d a t a ) t h e a g g r e g a t i o n
n u m b e r s o f L 6 4 a t t h e t e m p e r a t u r e s 2 1 . 0 , 2 5 . 9 ,
40.0. an d 6 0.0 C to be 2, 4 .
19,
a nd 85 r e spe c t ive ly ,
i n d i c a t i n g a s m o o t h i n c r e a s e w i t h t e m p e r a t u r e .
M o r e r e c en t ly , th e e f f ec t o f t e m p e r a t u r e in a q u e o u s
s o l u t i o n s o f P l u r o n i c L 6 4 w a s e x a m i n e d b y m e a n s
o f v i sc o s i ty , s e d i m e n t a t i o n , s c a t t e r i n g . a nd s o u n d
v e l o c i t y m e a s u r e m e n t s [ -6 7 ] . T h e m i c e l le s g r e w
l a r g e , p a r t i c u l a r l y a t t e m p e r a t u r e s n e a r t h e c l o u d
p o i n t . T h e v i s c o s i t y d a t a fl )r L 6 4 w e r e a n a l y z e d
t o e s t i m a t e v a r i o u s p a r a m e t e r s , i n c l u d i n g t h e
h y d r a t e d m i c e l l a r v o l u m e , h y d r a t i o r l n u m b e r ,
h y d r o d y n a m i c r a d i u s , e t c .
D y n a m i c l i g h t s c a t t e r i n g e x p e r i m e n t s i n
P l u r o n i c P 8 5 s o l u t i o n s i n d i c a t e d t h e c o e x i s t e n c e
o f u n i m e r s ( h y d r o d y n a m i c r a d i u s 1 .8 n m ) , m i c el le s
( h y d r o d y n a m i c r a d i u s 8 n m ) , a n d m i c e l la r a g g re -
g a te s , at c o p o l y m e r c o n c e n t r a t i o n s l e ss t h a n 1 0
a n d a t l o w t e m p e r a t u r e s ( 2 5 C ) [ - 2 7 ] . M i c e l l e s
8/16/2019 1995_revievghfjmvhgfw_PEO-PPO-PEO Block Copolymer Surfactants in Aqueous Solutions and at Interfaces
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2 P. Alexandridis, T. A. Hatton/Colloids Surfaces A: Physicochem. En g. Aspects 96 1995) 1 .4 6
w e r e f o rm e d a t a c o p o l y m e r c o n c e n t r a t i o n o f
a p p r o x i m a t e l y 5 % a t 2 5 ° C ; a t t e m p e r a t u r e s o f
4 0 ° C a n d h i g h e r , m i c e l l e s w e r e p r e s e n t a t a l l t h e
c o n c e n t r a t i o n s s t u d i ed (C > 0 . 3 % ) . T h e h y d r o d y -
n a m i c r a d i u s o f t h e m i c e ll e s ( c a l c u l a t e d a t i n f i n i te
d i l u t io n ) w a s a p p r o x i m a t e l y c o n s t a n t ( 8 n m ) o v e r
t h e t e m p e r a t u r e r a n g e 1 5 - 5 0 ° C ( a g g re g a ti o n
n u m b e r 2 0 - 4 0 ) . A t fi n it e c o n c e n t r a t i o n s t h e a p p a r -
e n t m i c e l l a r r a d i u s i n c r e a s e d w i t h i n c r e a s i n g t e m -
p e r a t u r e . T h e h a r d - s p h e r e r a d i u s o f a c o p o l y m e r
m o l e c u l e o f M w = 4 5 0 0 w o u l d b e 1 .1 5 n m ; t h u s ,
f r o m t h e 1 .8 n m r a d i u s o b s e r v e d e x p e r i m e n t a l l y
o n e m a y c o n c l u d e t h a t t h e u n i m e r i s h i g h l y c o m -
p a c t , p o s s ib l y w i th t h e P E O c h a i n s f o r m i n g a t ig h t
s h el l a r o u n d t h e n o n - h y d r a t e d P P O c o r e [ 2 7 ] .
I n t r i n s i c v i s c o s i t y m e a s u r e m e n t s ( c a p i l l a r y v i s c o m -
e t ry ) w e r e m a d e o n d i l u t e P 8 5 s o l u t i o n s i n o r d e r
t o o b t a i n m o r e i n f o r m a t i o n o n t h e m o l e c u l a r
d i m e n s i o n s . T h e i n t ri n s ic v i s c o s i t y d e c r e a s e d f r o m
1 6 m l g - 1 a t 1 5 ° C t o 6 . 5 m l g - 1 a t 5 0 ° C . T h e
i n t r i n s i c v i s c o s i t y o f 6.5 m l g 1 a t 5 0 ° C s u g g e s t e d
a v e r y c o m p a c t p a r t i cl e ( a lt h o u g h w i t h s o m e d i s-
s y m m e t r y a n d / o r s o l v a t i o n ); th e r a d i u s o f g y r a t i o n ,
R g , w a s e s t i m a t e d a t 8 . 3 n m .
V a l u es f o r th e h y d r o d y n a m i c r a d i i o f t h e m i c e l la r
c o m p o n e n t f r o m s e l f - d i f f u s i o n c o e f f i c i e n t s ( m e a -
s u r e d b y p u l s e d - f ie l d - g r a d ie n t N M R a n d e x t r a p o -
l a t e d t o i n f i n i t e d i l u t i o n ) v a r i e d b e t w e e n 6 . 2 a n d
9 .0 n m i n t h e te m p e r a t u r e r a n g e 2 1 .3 3 5 . 7 ° C , w i th
a n a v e r a g e v a l u e o f 7.7 r im . T h i s v a r i a t i o n w i t h
t e m p e r a t u r e c o n t r a d i c t e d t h e c o n s t a n t m i c e l l e
r a d i u s d e t e c t e d u s i n g d y n a m i c l i g h t s c a t t e r i n g . T h i s
w a s p o s s i b l y t h e r e s u l t o f p o l y d i s p e r s i t y , s in c e t h e
n u m b e r - a v e r a g e q u a n t i t y o b t a i n e d w i t h N M R
w o u l d b e v e r y s e n s i t i v e t o e v e n s m a l l a m o u n t s o f
m o n o m e r [ 2 7 ] ; t h e d if f er e n c e b e t w e e n t h e h y d r o -
d y n a m i c r a d i i d e t e r m i n e d f r o m p u l s e d f i e l d g r a d i -
e n t ( P F G ) N M R a n d d y n a m i c li g ht s ca t te r in g w a s
s m a l l e r a t t h e h i g h e r t e m p e r a t u r e s a t w h i c h t h e
s u s p e n s i o n i s n e a r l y m o n o d i s p e r s e . T h e s e l f -
d i f fu s i o n o f P l u r o n i c P 8 5 c o p o l y m e r i n a q u e o u s
s o l u ti o n w as in v e s ti g a te d w it h P F G - N M R
[ -6 5 ,6 6 ]. T h e h y d r o d y n a m i c r a d ii o f th e u n i m e r
a n d t h e m i c e l l e s w e r e d e t e r m i n e d t o b e 1 . 5 a n d
4 .5 n m , r e sp e c t iv e l y , a n d f o u n d t o b e i n d e p e n d e n t
o f t e m p e r a t u r e a n d c o n c e n t r a t i o n i n t h e c o n c e n -
t r a t i o n r a n g e b e t w e e n 1 a n d 1 0% . M o r t e n s e n a n d
c o - w o r k e r s [ , 61 , 6 2 ] s tu d i e d t h e s t r u c t u r e o f P 8 5
a q u e o u s s o l u t i o n s u s i n g n e u t r o n s c a t t e r i n g . T h e
r a d i u s o f g y r a t i o n o f t h e f r ee c o p o l y m e r w a s 1 .7 n m .
T h e m i c e l l a r s i z e s ( m i c e l l e c o r e r a d i u s a n d h a r d -
s p h e r e i n t e r a c t io n r a d iu s ) a p p e a r e d t o b e i n d e p e n -
d e n t o f p o l y m e r c o n c e n t r a t i o n , b u t s h o w e d a s m a ll
t e m p e r a t u r e d e p e n d a n c e r e fl e ct in g c h a n g e s i n
a g g r e g a t i o n n u m b e r . T h e m i c e ll e c o r e r a d iu s a n d
h a r d - s p h e r e i n t e r a c t i o n r a d i u s w e r e 3 .8 a n d 6 .0 n m
a t 2 0 ° C , r e s p e c t i v e l y , a n d i n c r e a s e d t o 5 .1 a n d
7 . 5 n m a t 5 0 C ; t h e a g g r e g a t i o n n u m b e r w a s 3 7 a t
2 0 ° C , i n c r e a s i n g t o 7 8 a t 4 0 ° C [ 6 4 ] .
B r o w n e t a l. [ 5 7 ] a n d M o r t e n s e n a n d B r o w n
[ 6 3 ] p u b l i s h e d s tu d i e s c o m p a r i n g t h e P l u r o n i c
P E O - P P O - P E O c o p o l ym e r s P 8 5, F 8 7, a n d F 8 8.
R e l a x a t i o n t i m e d i s t r i b u t i o n s o b t a i n e d b y L a p l a c e
i n v e r s io n o f t h e d y n a m i c l ig h t s c a t ti n g ( D L S )
c o r r e l a t i o n f u n c t i o n s d e m o n s t r a t e d c o m p l e x s t a t e s
o f a g g r e g a t i o n i n s o l u t i o n [ -5 7 ] . U n i m e r , m i c e ll e s,
a n d l a r g e r ag g r e g a t e s c o e x is t e d i n p r o p o r t i o n s t h a t
d e p e n d e d s t r o n g l y o n t e m p e r a t u r e a n d c o n c e n -
t r a t i on , a s se e n in F ig . 8 f o r P85 so lu t ions [ - 27].
T h e u n i m e r s h a d h y d r o d y n a m i c ra d i i i n t h e si ze
r a n g e 1 . 5 - 3 . 0 n m ; t h e m i c e ll e r a d i i w e r e 8 1 3 n m
( in s e q u e n c e o f i n c r e a s i n g P E O b l o c k l e n g t h ) ,
w h e r e a s t h e r a d i i o f t h e c l u s t e r s w e r e g r e a t e r t h a n
8 0 n m . T h e m i c el le a g g r e g a t i o n n u m b e r f o r F 8 8
w a s 1 7 a t 40 ~ 'C , a n d t h e h y d r o d y n a m i c r a d i i 1 3
a n d 1 0 . 5 n m a t 4 0 a n d 5 0~ 'C , r e s p e c t iv e l y .
M o l e c u l a r w e i g h t s , g y r a t i o n r a d i i , e t c . , c o u l d n o t
b e d e t e r m i n e d f r o m s t a t i c l i g h t s c a t t e r i n g m e a s u r e -
m e n t s o w i n g t o t h e h i g h l y p o l y d i s p e r s e c h a r a c t e r
o f t h e s o l u t i o n s . T h e i n v e r s e o s m o t i c c o m p r e s s i b i l -
i t y w a s e v a l u a t e d f r o m t h e a b s o l u t e s c a t t e r i n g
i n t e n s i t y a t z e r o a n g l e, p r o v i d i n g a m e a s u r e o f t h e
r e l a t iv e s t r e n g t h s o f s o l u t e / s o l v e n t a n d s o l u t e /
s o l u t e i n t e r a c t i o n s ; f o r m a t i o n o f m i c e l le s a n d
a g g r e g a t e s w a s c o n s i s t e n t w i t h l o w v a l u e s o f t h e
i n v e r s e o s m o t i c c o m p r e s s i b i l i t y . P l u r o n i c s P 8 5 ,
F 8 7 , a n d F 8 8 h a d a p p r o x i m a t e l y th e s a m e te n -
d e n c y f o r m i c e l l e f o r m a t i o n w h e n c o m p a r e d a t t h e
s a m e m o l a r c o n c e n t r a t i o n ; f o r t h e s a m e w e i g h t
c o n c e n t r a t i o n , P 8 5 f o r m e d m i c e l l e s m o r e r e a d i l y
[ - 5 7 ] . M o r t e n s e n a n d B r o w n [ - 6 3 ] f o u n d t h a t
t h e c r i t i c a l m i c e l l i z a t i o n t e m p e r a t u r e v a l u e s o f
P l u r o n i c s L 8 1 , P 8 5 , P 8 7 a n d F 8 8 f ell o n a c o m m o n
l i n e w h e n p l o t t e d a g a i n s t t h e c o n c e n t r a t i o n o f
P P O i n s o l u t i o n ; i t t h u s a p p e a r e d t h a t t h e P P O
c o n c e n t r a t i o n w a s t h e r e l e v a n t p a r a m e t e r i n t h e
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431
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F i g . 8 . R e l a x a t i o n t i m e d i s t r i b u t i o n s o b t a i n e d b y L a p l a c e
i n v e r s i o n o f d y n a m i c l i g ht s c a t t e r i n g d a t a , r A ( r ) v s. l o g l r , y s ) ,
f o r P l u r o n i c P 8 5 a t l o w c o n c e n t r a t i o n s ( < 1 0 % ) a n d a t
t e m p e r a t u r e s b e t w e e n 5 a n d 5 0 C [ A( r) i s t h e a u t o c o r r e l a t i o n
f u n c t i o n L P e a k s a t t r i b u t e d t o u n i m e r a n d m i c e l le s a r e i n d i c a t e d ,
i n a d d i t i o n t o t h e p e a k s a t l o w c o n c e n t r a t i o n s w h i c h p r o b a b l y
r e f le c t c l u s t e rs o f d i b lo c k c o n t a m i n a n t . ( R e p r i n t e d w i t h p e r m i s -
s i o n f r o m R e f. [ 2 7 1 ; c o p y r i g h t A m e r i c a n C h e m i c a l S o c i e ty ,
1991.)
100
80.
~-
0 .
40.
20.
O.
O.
100.
8 0 ,
~ 60
40.
20.
O.
O.
100.
80.
~ 60
40.
20.
O.
O.
o , 1
Y ~ P85
• C = ~ 5 %
C =25~
®C=16 ~,
C= 97,
D C = 8 ~
2 C = 4 ~
I
20. 40 . 6 0 I 8 0 .
T E M P E R A T U R E C)
|
Eli
20.
c 8 v
I ~ C = 2 6 X
® C = 2 O ~
7 C = 1 3 ~ ,
D C = 9 ~
I
4 0 . 6 0 . 8 0 ,
T E M P E R A T U R E C )
F 8 8
C = 2 BY;,
® ¢ = 2 0 ~
C = 9 Y ,
, 2 C = 8 %
100
100 .
I
20. 40 . 6 . 8 ~0 0 .
T E M P E R A T U R E C)
d e t e r m i n a t i o n o f t h e C M T . P l u r o n i c L 8 1 f o r m e d
a g g re g a te s a t t e m p e r at u r es b e l o w t h e c o m m o n
c u r v e , p r e s u m a b l y d u e t o t h e s i g n i f i c a n t l y g r e a t e r
h y d r o p h o b i c n a t u r e o f L 8 1 ( L 8 1 c o n t a i n s 9 0
P P O ) : t h e re w a s a l s o s o m e d e v i a t i o n f r o m t h e
c u r v e f o r P 8 5 a t lo w P P O c o n c e n t r a t i o n s .
T h e h a r d - s p h e r e i n t e r a c t i o n r a d i u s , Rh s f o r
P l u r o n i c s P 8 5 , P 8 7 a n d F 8 8 , o b t a i n e d b y f it t in g
t o t h e e x p e r i m e n t a l s c a t t e r i n g d a t a , is s h o w n i n
F i g . 9 [ 6 3 ] . T h e r a d i i a r e p l o t t e d a s a f u n c t i o n
F i g . 9 . H a r d - s p h e r e i n t e r a c t i o n r a d i u s , R h ~ , f o r P l u r o n i c P 8 5 ,
P 8 7, a n d F 8 8 P E O P P O P E O c o p o l y m e r s , o b t a i n e d by f i tt in g
t h e h a r d - s p h e r e P e r c u s Y e v ic k m o d e l t o e x p e r i m e n t a l n e u t r o n
s c a t t e r i n g d a t a : t h e r a d ii a r e p l o t t e d a s a f u n c t i o n o f t e m p e r a t u r e
f o r d i f f e re n t c o p o l y m e r c o n c e n t r a t i o n s . I R e p r i n t e d w i t h p e r m i s -
s i o n f r o m R e f. [ 6 3 ] ; c o p y r i g h t A m e r i c a n C h e m i c a l S o c i e ty ,
1993.1
o f t e m p e r a t u r e f o r d if fe r en t c o p o l y m e r c o n c e n -
t r a t i o n s . T h e m i c e l l a r c o r e r a d i u s , Re a n d t o s o m e
e x t e n t a l s o t h e h a r d - s p h e r e i n t e r a c t i o n r a d iu s , R h s,
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14 P. Alexandridis , T .A. Ha t ton/Co l lo ids Sur fac es A. Phys icochem. Eng. Asp ects 96 (1995) 1 46
appeared to be essentially independent of copoly-
mer concentration but showed a significant
increase with temperature. The difference between
R c and Rh, remained practical ly unaffected by the
temperature. When Rc was plotted against reduced
temperature T-TcMv, the data for solutions of
P85, P87, and F88 followed a common master
curve. A double logarithmic plot of R¢ vs. T - Tcuc
gave the empirical scaling relation R~
T- TCMT) 2 as seen in Fig. 10 [63]. The aggrega-
tion number was very small, close to TcMx, and
increased continuously following a N
T- TcMT) 6 relationship to N ~ 200 at the highest
temperature, where spherical micellar aggregates
were present. From the hard-sphere da ta analysis,
it appears that the main difference between P85,
P87, and F88 micelles at a given temperature is
the size of the micelles; the larger the PEO block,
the smaller the core and the aggregation number
[63]. The dependence of the core radius and the
aggregation number on the degree of polymeriza-
tion of both EO and PO [63] contradicted the
Halperin [71] star model of polymeric micelles,
which was recently shown to be valid for the
system PEO-PS-PEO [72]. The relationship of
increasing micellar core with decreasing number
of EO segments would lead to a limitation in the
formation of spherical micelles; this would happen
if the number of EO segments were so small that
the core radius extended the length of a stretched
PPO chain. Extrapolation of the core radii data
40 .
~ z
g % 2 O X
v •
F 8 8
20 . 1 0 .
1o . 1oo.
T T c r n I
F i g . 1 0. D o u b l e l o g a r i t h m i c p l o t o f R c v s . r e d u c e d t e m p e r a t u r e ,
T - T c M c ( d a t a s h o w n fo r 9 a n d 20 a q u e o u s s o l u ti o n s o f
P l u r o n i c P 8 5 , F 8 7 , a n d F 8 8 ) . ( R e p r i n t e d w i t h p e r m i s s i o n f r o m
R e f . [ 6 3 ] ; c o p y r i g h t A m e r i c a n C h e m i c a l S o c i e t y , 19 93 .)
for Pluronics P85, F87, and F88 to the number of
EO segments corresponding to L81 would lead to
a value for the core approaching or exceeding the
maximum value given by the length of a fully
extended PPO chain; this is probably the reason
why Pluronic L81 does not form spherical micellar
aggregates [63].
Static and dynamic light scattering experiments
on Pluronic F68 aqueous solutions revealed three
temperature regions, those of unimer, transition,
and micelle [21]. The apparent hydrodynamic
radii of F68 solutions at three different copolymer
concentrations are presented in Fig. ll a) [21].
A transition corresponding to the CMT was
observed; below the CMT, low scattering intensity
and small particle size 2.3nm) were detected,
1
(a)
/ /
, - ' . /
2
i i I i i
3 0 4 0 5 0 6 0 7 0
Temperature ('C)
6
E
o
I
@ -
0 i t ~ i i
5 0 5 5 6 0 6 5 7 0 7 5
(b) Tem perature ( 'C)
16
12
8
B
0
8 0
F i g . 11 . ( a t T e m p e r a t u r e d e p e n d e n c e o f a p p a r e n t h y d r o d y n a m i c
r a d i u s o f P l u r o n i c F 6 8 s o l u t i o n s a t t h r e e d i f f e r e n t c o n c e n -
t r a t i o n s : ~ , 5 1 .7 m g m l - 1; @ , 2 5 . 0 m g m l 1 ; I I , 1 2 . 5 m g m l ~ .
( b ) L i n e a r i n c r e a s e o f F 6 8 m i c e l l a r w e i g h t s w i t h i n c r e a s i n g
t e m p e r a t u r e , w i t h t h e h y d r o d y n a m i c r a d i u s o f t h e m i c e ll e s
r e m a i n i n g a p p r o x i m a t e l y c o n s t a n t . ( R e p r i n t e d w i t h p e r m i s s io n
f r o m R e f . [ 2 1 ] ; c o p y r i g h t A c a d e m i c P r e s s , 1 9 8 8. )
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P. A h ' xa n 'i d i s , T . A . H a t t o n / Co l l o i d s S u rJa cc s ,4: Ph y s i co ch em . EH k '. A S lWCt S 9 6 ( 1 9 ~ 5 ~ 1 4 t5 1 5
s h o w i n g l i tt le t e m p e r a t u r e d e p e n d e n c e . O n l y u n i m -
e rs w e r e p r e s e n t b e l o w t h e C M T ; m i c e ll e f o r m a t i o n
b e c a m e a p p r e c i a b l e a b o v e t h e C M T . I n t h e m ic e ll e
r e g i o n , t h e m i c e l l a r w e i g h t s m e a s u r e d w e r e o f t h e
o r d e r o f 1 05 g m o l ~ ( a v e ra g e a g g r e g a t i o n n u m b e r
w a s 6 5 ) a n d i n c r e a s e d l i n e a r l y w i t h t e m p e r a t u r e ,
w h i le t h e h y d r o d y n a m i c r a d ii o f t h e m i c el le s
r e m a i n e d n e a r l y c o n s t a n t ( 8 . 0 n m ) a s s e e n i n
F i g . l l ( b ) [ 2 1 ] ( n o t e , h o w e v e r , t h a t A 1 - S a d e n e t a l.
[ 2 9 ] r e p o r t e d th e a v e ra g e h y d r o d y n a m i c ra d iu s t o
i n c re a s e w i t h t e m p e r a t u r e f o r a g i v e n c o n c e n -
t r a t i o n ) . S u c h a d u a l e f f e c t o f t e m p e r a t u r e w a s
i n t e r p r e t e d in t e rm s o f e n h a n c e d d e h y d r a t i o n o f
t h e m i c e ll e c o r o n a ( c o n s is t in g m a i n l y o f P E O ) w i t h
t e m p e r a t u r e [ 2 1 ] . I n s t a t i c l i g h t s c a t t e r i n g m e a s -
u r e m e n t s , t h e s c a t t e r e d l i g h t f ie l d i s a m e a s u r e o f
t h e p o l a r i z a b i l i t y d i f f er e n c e b e t w e e n t h e d i s p e r s e d
p a r t i c l e a n d t h e s u r r o u n d i n g m e d i u m , t h u s p r o v i d -
i n g u s e fu l i n f o r m a t i o n c o n c e r n i n g t h e ' d r y d i s-
p e r s e d p a r t i c l e s . I n t h e d y n a m i c l i g h t s c a t t e r i n g
e x p e r i m e n t s , t h e B r o w n i a n m o t i o n o f t h e w e t
d i s p e r s e d p a r t i c l e s i s m o n i t o r e d t o p r o v i d e s i z e
i n f o r m a t i o n o n t h e s o l v a t e d p a r t i c l e s . I t i s w e l l
e s t a b li s h e d t h a t d e h y d r a t i o n o f n o n - i o n i c s u r f a c-
t a n t s g r a d u a l l y o c c u r s w i t h i n c re a s i n g t e m p e r a t u r e ,
w h i c h r e s u l t s i n a n e n h a n c e d t e n d e n c y t o s e p a r a t e
f r o m t h e s o l v e n t e n v i r o n m e n t a n d , c o n s e q u e n t l y ,
i n a n in c r e a s e i n th e a g g r e g a t i o n n u m b e r . S u c h a
d u a l e f fe c t o f t e m p e r a t u r e i n c r e a s e o n a n i n c r e a s e d
a g g r e g a t i o n n u m b e r a n d a d e c r e a s e d m i c e l l a r
h y d r a t i o n m a k e s i t p o s s i b l e f o r t h e h y d r o d y n a m i c
r a d iu s o f P E O P P O P E O c o p o l y m e r m i ce ll es t o
r e m a i n a h n o s t i n d e p e n d e n t o f t e m p e r a t u r e [ 2 1 ] .
T h e h y d r o d y n a m i c r a d ii o f m i c el le s f o r m e d b y
P l u r o n i c F 1 2 7 i n w a t e r , a s c a l c u l a t e d f r o m d i f f u -
s i o n ( Q E L S ) d a t a a t t h e C M C , r e m a i n e d c o n s t a n t
a t 1 0. 2 n m , o v e r t h e 3 5 4 5 C t e m p e r a t u r e r a n g e
[ 7 3 ] . V i s c o m e t r ic s tu d i es , h o w e v e r , s h o w e d a p r o -
g r e s s iv e d e h y d r a t i o n o f t h e m i ce l le s w i t h t e m p e r -
a t u r e i n c r e a s e : t h e a g g r e g a t i o n n u m b e r w a s 3 a t
3 5 C , 9 a t 4 0 C , a n d 1 2 a t 4 5 C [ 7 3 ] . M a l m s t e n
a n d L i n d m a n [ 5 9 ] u s ed N M R t o s t u d y d i ff u si o n
o f m o l e c u l e s in F 1 2 7 s o l u t i o n s . T h e d i f f u s i o n o f t h e
p o l y m e r m o l e c u l es w a s s l ow
D p = 1 0 - 1 1 1 0 - 1 2 m 2
s ~) a n d d e c r e a s e d w i t h i n c r e a s i n g c o p o l y m e r c o n -
c e n t r a t i o n ( r o u g h l y a s D p ~ c ~) u p t o 2 0 w t. % :
the d i f f us ion c oe f f i c i e n t o f wa te r wa s Dw = 10 - 9 m - '
s 1 [5 91 ]. W a t e r s e l f- d i ff u s io n d e c r e a s e d m o n o t o n i -
c a ll y w it h i n c r e a s in g c o p o l y m e r c o n c e n t r a t i o n ,
g i v i n g D / D o = 0 .5 a t 4 0 w t . % . T h e d e c r e a s e i n D / D .
w i t h in c r e a s in g c o p o l y m e r c o n c e n t r a t i o n c o u l d b e
r e p r o d u c e d b y' t a k i n g i n t o a c c o u n t t h e o b s t r u c t i o n
d u e t o e x c l u d e d v o l u m e a n d t h e h y d r a t i o n o f t h e
p o l y m e r m o l e c u le s . T h e a n a l y s is y i e l d e d t h a t s o m e -
w h e r e b e t w e e n t w o a n d f i v e w a t e r m o l e c u l e s p e r
E O s e g m e n t w e r e p e r t u r b e d . T h e w a t e r d i f f u s i o n
i n c r ea s e d w i th t e m p e r a t u r e . T h e d a t a o f M a l m s t e n
a n d L i n d m a n [ 5 9 , 6 0 ] a r e c o n s i st e n t w it h a g r a d u a l
d e h y d r a t i o n o f t h e p o l y m e r m o l e c u l es w i th i n c re a s -
i n g t e m p e r a t u r e , a n d a r e i n a g r e e m e n t w i t h t h o s e
o f A t t w o o d e l a l. [ 7 3 ] . S u c h a d e h y d r a t i o n , h o w -
e v e r , d i d n o t c o r r e l a t e w i t h t h e o c c u r r e n c e o f t h e
g e l r e g i o n [ 6 0 ] .
5 .2 . M i c e l l e m i c r o e n v i r o m m n t
F l u o r e s c e n c e s p e c t r o s c o p y t e c h n i q u e s h a v e b e e n
d e v e l o p e d a n d o p t i m i z e d o v e r t h e p a s t 1 5 y e a r s
f o r t h e s t u d y o f c o l l o i d a l s o l u t i o n s , a n d m a n y
f l u o r e s c e n t m o l e c u l e s a re n o w a v a i l a b l e f o r p r o b i n g
s t r u c t u r a l i n f o r m a t i o n i n s u c h s y s t e m s [ 3 4 , 7 7 ] .
P y r e n e , a m o n g s t o t h e r s , i s a w e l l - c h a r a c t e r i z e d
p o l a r i t y - s e n s i t i v e p r o b e . T h e p y r e n e f l u o r e s c e n c e
e m i s s i o n s p e c t r u m c o n s i s t s m a i n l y o f fi ve b a n d s
r e f e r r e d to a s , 1 1 , I,_ . . . . . I s , f r o m s h o r t e r t o l o n g e r
w a v e l e n g th s . T h e
I 1 / I 3
i n t e n s i t y r a t i o o f t h i s v i b r a -
t i o n a l f in e s t r u c t u r e d e p e n d s s t r o n g l y o n t h e p o l a r -
i t y o f t h e m e d i u m : t h e l a r g e r t h e r a t i o , t h e m o r e
p o l a r t h e m e d i u m . E l e c t r o n i c a l l y e x c i t e d p y r e n e is
a r e p o r t e r o f th e a v e r a g e m i c r o p o l a r i t y o f th e
e n v i r on m e n t i t v i si t s dur in g i ts l i f e t im e ( v 300 ns) .
r e n d e r i n g p y r e n e a t t r a c t i v e f o r s t u d y i n g r e s t r i c t e d
m i c r o e n v i r o n m e n t s s u c h a s m i c e l l a r s y s t e m s
[ 3 4 , 7 7 ] .
A l e x a n d r i d i s a n d c o - w o r k e r s [ 3 7 , 7 0 , 7 6 ] u s e d
p y r e n e t o p r o b e t h e m i c r o p o l a r i t y in P l u r o n i c
c o p o l y m e r s o lu t i o n s, b y c o n d u c t i n g a s y s t e m a t i c
i n v e s t i g a t i o n o f t h e v a r i a t i o n o f t h e p y r c n e f l u o r e s -
c e n c e e m i s s i o n i n t e n s i t y r a t i o I~ 13 i n d i f f e r e n t
m e d i a a s a f u n c t i o n o f t e m p e r a t u r e . S p e c i fi c a ll y ,
s t u d i e s w e r e c o n d u c t e d i n w a t e r , o r g a n i c s o l v e n t s ,
b u lk P E O a n d P P O h o m o p o l y m e r s , a n d a q u eo u s
s o l u ti o n s o f P E O P P O P E O c o p o l y m e r s . A
s h a r p d e c r e a s e i n
I1 , , I ,~
w a s o b s e r v e d f o r P E O
P P O P E O t r i b l o c k c o p o l y m e r s o l u ti o n s , a t t e m -
p e r a t u r e s c h a r a c t e r i s t i c o f e a c h p o l y m e r , f o l l o w e d
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16 P. Alexandridis, T.A . Hatton/Colloids Surfaces A: Physicochem. En g. Aspe cts 96 1995) 1 46
by a less dramatic linear decrease as the temper-
ature was further increased (see Figure 12(a)). This
sharp decrease is attributed to the formation of
micelles with a well-defined hydrophobic core into
which pyrene partitions preferentially. The temper-
ature dependence of the
11/13
intensity ratio can
be used for the determination of the CMT [37],
as with the I 1 /13 vs. concentration plots in the
determination of the CMC in surfactant systems
[34]. The smoother linear decrease observed at
temperatures higher than the CMT reflects a less
polar microenvironment with increasing temper-
(a)
2 . 1
2
1 . 9
1 . 8
1 . 7
1 . 6
1 . 5
1 . 4
1 . 3
i i i r
t
1 5 2 5 3 5 4 5
Temperature ( 'C)
5 5
2 6 i i i i i
• PPO 425
.4
\
~ % . "m - m - P PO 2 0 0 0
2 . 2
\
" ~ . \ q . - -* - F 1 0 8 1 %
1 8
1 4
1.2 t I I I a
3 4 3 7 4 0 4 3 4 6 4 9 5 2
(b) Tempe rature ( 'C)
F i g. 12 . (a ) M i c r o p o l a r i t y i n P E O P P O P E O c o p o l y m e r s o l u-
t i o n s : p y r e n e f l u o r e s c e n c e e m i s s i o n i n t e n s i t y r a t i o 11/13) a s a
f u n c t i o n o f t e m p e r a t u r e f o r 1 a q u e o u s s o l u t i o n s o f P l u r o n i c s
P 1 0 5 , P 8 5 , a n d P 6 5 . ( A d a p t e d f r o m R e f . [ -7 6 ]; c o p y r i g h t
M a s s a c h u s e t t s I n s t i t u t e o f T e c h n o l o g y , 1 99 4.) ( b t M i c r o v i s c o s i t y
i n P E O - P P O P E O c o p o l y m e r m ic e ll es : d i p y m e f lu o r es c e n ce
e m i s s i o n m o n o m e r / e x c i m e r i n te n s i ty r a ti o IM/IE) a s a f u n c t i o n
o f te m p e r a t u r e f o r P l u r o n i c F 1 0 8 m i c e ll e s; a l s o s h o w n i n t h e
s a m e g r a p h f o r c o m p a r i s o n p u r p o s e s a r e d a t a f or b u l k P P O
h o m o p o l y m e r . ( A d a p t e d f r o m R e f . [ 7 9 ] . )
ature. This was attributed to a temperature-
induced change in the solution micropolarity [70].
The influence of molecular weight on the micro-
polarity afforded by copolymer micellar solutions
is depicted in Fig. 12(a) for a series of Pluronic
copolymers (P105, P85, P651 with the same
PP O/PE O composition ratio (50%). The highest
molecular weight P105 exhibited the lowest CMT,
in accordance with the findings of Ref. [28]. At
temperatures higher than the CMT, the
11/13
ratios
decreased linearly with increasing temperature for
all copolymers, but their values remained in the
order: 1 1 /1 3 P 6 5 ) > I 1 / I 3 P 8 5 ) > I 1 / I 3 (P105).
This is an indication that micelles assembled from
higher-molecular-weight Pluronics (maintaining
constant PPO/PEO copolymer composition)
exhibit more hydrophobic microdomains [70]. In
addition to probing the microenvironment, time-
resolved fluorescence quenching from pyrene has
been used to determine the aggregation number of
PEO-PPO-PEO micelles [70,75].
The microviscosity in the interior of block copol-
ymer micelles can be probed using the hydrophobic
molecule bis(1-pyrenylmethyl)ether (dipyme) that
exhibits intramolecular excimer fluorescence
(intensity Is) in competition with fluorescence from
the locally excited pyrene chromophore ( mon-
omer emission, intensity IM; 15 is used here to
represent IM) [78]. The extent of excimer emission
in dipyme depends upon the rate of conformational
change, the motion of the pyrene segments of
dipyme being restricted by the local friction
imposed by the environment. As a consequence,
the intensity ratio I M / I E provides a measure of the
microviscosity of its environment (the larger the
IM/I~ ratio, the more viscous the environment in
which the probe is located), and is a particularly
powerful means of monitoring changes in microvis-
cosity as the system is subjected to external stim-
uli [78].
Quali tative changes in the microviscosity experi-
enced by dipyme are reported by Nivaggioli et al.
[79] for PPO homopolymers and PEO-
PP O- PE O copolymer micellar solutions, as a
function of a copolymer composition and sample
temperature. Values of the monomer/excimer
intensity ratio I M / I E ) of dipyme in bulk PPO
homopolymers and Pluronic F108 micelles are
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P. Alexandridis. T. ,4. Hatton/Colloi& Surliwes A: Phvsicochem . EiJg.,4.g~ects96 (1995) 1 4~, 17
s h o w n i n F i g . 1 2 ( b l [ 7 9 ] . I M / I E d a t a a r e r e p o r t e d
f o r th e 3 5 5 0 ' C t e m p e r a t u r e r a n g e , s o t h a t
m i c e l l e s a r e p r e s e n t i n w h i c h d i p y m e c a n b e s o l u b i -
l iz e d [ t h e s o l u b i l i ty o f d i p y m e i n w a t e r i s v e r y
l im i t e d) . M i c r o v i s c o s i ty d e c r e a s e d w i th t e m p e r -
a t u r e f o r a l l t h e s y s t e m s s t u d i e d : h i g h e r m i c r o v i s -
c o s i t y v a l u e s w e r e o b s e r v e d f o r t h e h i g h e r M w
P P O . T h e l u / I F v a l u e s f o r t h e E l 0 8 m i c e l l e s a r e
c o m p a r a b l e w i t h th o s e fo r b u l k P P O , s u g g e s ti n g
t h a t t h e e n v i r o n m e n t i n w h i c h d i p y m e i s l o c a t e d
is s i m i l a r t o b u l k P P O . N o t e t h a t t h e m i c r o v i s c o s -
i ty e x p e r i e n c e d b y d i p y m e i n t h e i n t e r i o r o f F I 0 8
m i c e l l e s a p p e a r s t o b e s i m i l a r t o t h a t s e n s e d i n l o w
Mw ~7 25 ) P P O a t 3 5 C , w h i l e i t e x c e e d s t h e
m i c r o v i s c o s i t y in h i g h M 2 ( 2 0 0 0 ) P P O a t 5 0 C .
T h i s c o u l d b e a n i n d i c a t i o n t h a t t h e m i c e l le c o r e
b e c o m e s m o r e c o m p a c t a s t h e t e m p e r a t u r e
i n c r e a s e s . S u c h a n o b s e r v a t i o n c a n b e r e l a t e d t o
t h e fa c t t h a t t h e a g g r e g a t i o n n u m b e r o f P l u ro n i c
c o p o l y m e r m i c e l l e s i n c r e a s e s w i t h t e m p e r a t u r e ,
w h i l e t h e h y d r o d y n a m i c r a d i u s r e m a i n s a p p r o x i -
m a t e l y c o n s t a n t ( s e e S e c t i o n 5 . 1 ) . A s e m i l o g p l o t
o1 IM~I~ VS. t e m p e r a t u r e c a n b e f i t te d t o a l i n e a r
r e l a t i o n s h i p [ 7 9 ] : a c t i v a t i o n e n e r g y ( E a ) v a l u e s f o r
t h e m i c r o v i s c o s i ty e x p e r ie n c e d b y d i p y m e c a n b e
e s t i m a t e d f r o m s u c h a p l o t. I n c r e a s e o f t h e c o p o l y -
m e t m o l e c u l a r w e i gh t a t c o n s t a n t P P O / P E O c o m -
p o s i t i o n , a n d i n c r e a s e o f t h e P P O b l o c k s i z e f o r
c o n s t a n t P E O b l o c k s iz e , r e s u l t e d in in c r e a s i n g
m i c r o v i s c o s i t y a n d a c t i v a t i o n e n e r g i e s . T h e e f f e c t
o f P E O b l o c k o n m i c e ll e m i c r o v i s c o s i ty w a s l e ss
s i g n i f i c a n t .
T h e h y d r a t i o n a n d m i c r o v i s c o s i t y i n th e m i c e l la r
s o l u l io n s a n d g e ls f o r m e d b y P l u r o n i c F 1 2 7 h a v e
b e e n s t u d i e d u s i n g t h e f l u o r e s c e n t p r o b e m o l e c u l e s
p y r e n e a n d 8 - a n i l i n o - l - n a p h t h a l e n e s u lf o n ic a ci d
[ 7 4 ] : t h e r e s u l t s i n d i c a t e d t h a t m i c r o v i s c o s i t y
d e c r e a s e s w i t h i n c r e a s i n g t e m p e r a t u r e . R e c e n t l y ,
N a k a s h i m a e t a l . [ 8 0 ] r e p o r t e d f l u o r e s c e n c e
a n i s o t r o p y d a t a f r o m o c t a d e c y l r h o d a m i n e B in
P l u r o n ic F 6 8 P E O P P O P E O m i ce ll es ; t he m i c r o -
v i s c o s i t y o f t h e l a t t e r w a s e s t i m a t e d f r o m t h e s e
m e a s u r e m e n t s . T h e m i c r o v i s c o s i t y i n c r e a se d f r o m
1 .9 c P a t 2 0 'C t o 1 6 c P a t 4 0 ': C : t h is i n c r e a s e w a s
a t t r i b u t e d t o a c o n f o r m a t i o n a l c h a n g e o f t h e
m i c e l l e s f r o m a l o o s e l y c o i l e d a g g r e g a t e t o a m o r e
c o m p a c t s tr u c t u re . N o t e h o w e v e r , th a t o c t a d e c y l
r h o d a m i n e B is a c a t io n i c a m p h i p h i l i c m o l e c u l e
t h a t w i l l m o s t l i k e l y a s s o c i a t e w i t h t h e m i c e l l e
c o r o n a , t h u s c o m p l i c a t i n g t h e m i c r o v i s c o s i t y
s tud ies .
5 .3 . P h a s e d ia g r a m
T h e p h a s e b e h a v i o r o f P E O P P O P E O tr i bl o c k
c o p o l y m e r s d i ss o l ve d i n w a t e r h a s b e e n s t u d i e d b y
M o r t e n s e n a n d c o - w o r k e r s u s i n g s m a l l - a n g l e n e u t -
r o n s c a t t e r i n g a n d d y n a m i c l i g h t s c a t t e r i n g
[ 6 1 , 6 3 , 6 4 ] . T h e s t r u c t u r a l p r o p e r t i e s h a v e b e e n
s t u d ie d a s a f u n c t io n o f p o l y m e r c o n c e n t r a t i o n a n d
t e m p e r a t u r e f o r a s er ie s o f P l u r o n ic c o p o l y m e r s
w i t h P P O b l o c k s o f c o n s t a n t s i ze a n d P E O b l o c k s
o f v a r y i n g s i te . A t l o w t e m p e r a t u r e [T -< ~ 15 C ) a n d
l o w p o l y m e r c o n c e n t r a t i o n s , t h e u m m e r s w e r e f ul ly
d i s s o l v e d g a u s s i a n c h a i n s w i t h r a d i u s R g = 1 .7 n m
C l o s e t o a m b i e n t t e m p e r a t u r e , t i l e h y d r o p h o b i c
n a t u r e o f P P O c a u s e d a g g r e g a t i o n o f t h e p o l y m e r s
i n t o s p h e r i c a l m i c e l l e s w i t h c o r e s i z e s o f t h e o r d e r
o f 4 5 r a n . s o m e w h a t t e m p e r a t u r e d e p e n d e nt .
A c c o r d i n g t o t h e d a t a a n a l y s i s , t h e c o r e s i z e
i n c r e a s e d w i t h d e c r e a s i n g P E O b l o c k s iz e a n d w i t h
i n c r e a s in g t e m p e r a t u r e . T h e c o p o l y m e r w i th t h e
l a r g e s t P E O b l o c k a g g r e g a t e d in m i c e ll e s w i t h a
c o r e d i a m e t e r w h i c h , w i t h i n t h e w h o l e l e m p e r a t u r e
r e g i m e , w a s s m a l l e r t h a n t h e l e n g th o f a s t r e t c h e d
P P O c h a in . M i c el le s f o r m e d b x c o p o l y m e r s o f
i n t e r m e d i a t e P E O s iz e h a d a c o r e d i a m e t e r w h i c h
a t h i g h t e m p e r a t u r e a p p r o a c h e d t h e si ze o f a fu l ly
s t r e t c h e d P P O c h a i n , t h u s c a u s i n g a n a b r u p t
c h a n g e f r o m a s p h e r i c a l t o a r o d - l i k e s t r u c t u r e ( a s
o b s e r v e d b o l h b y n e u t r o n s c a t t e ri n g a n d d e p o l a r -
i z ed l ig h t s c a t te r i n g ) . I t a p p e a r e d f r o m t h e h a r d -
s p h e r e d a t a a n a l y s is t h a t t h e m i c e l l e - f o r m m g p o l y -
m e r s c o u l d a l l b e s c a l e d t o a c o m m o n p h a s e
b e h a v i o r w h e r e t h e c r i t i c a l m i c e l l i z a t t o n t e m p e r -
a t u r e is d e t e r m i n e d b y th e P P O c o n c e n t r a t i o n .
w h e r e a s t h e c r y s t a l l i z a t i o n ( g e l a t i o n l t e m p e r a t u r e
is d e t e r m i n e d b y t h e to t a l c o p o l y m e r c o n c e n t r a t i o n
[ 6 3 ] . T h e c o n c e n t r a t i o n o f m i c e ll e s i n c re a s e d
r o u g h l y l i n e a r l y w i t h t e m p e r a t u r e u n t i l e i t h e r a
s a t u r a t i o n , a a s r e a c h e d ( w h e r e al l p o l y m e r s w e r e
p a r t o f a m i c e l l el o r t h e v o l u m e d e n s i t y o f m i ce l le s
w a s s o h i g h t h a t t h e y ' l o c k e d i n t o a c r y s t a l l in e
s t r u c t u r e o f h a r d s p h e r e s . I n t h e 60 7(1 C t e m p e r -
a t u r e r a n g e , t h e m i c e l l a r s t r u c t u r e c h a n g e d f l ' o m
s p h e r i c a l f o r m t o p r o l a t c e l l i p s o i d , l e a d i n g t o a
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18 P. Alexandridis . T .A. H at ton/C ol lo ids SurJaces A: Phys icochem. Eng. Asp ects 96 1995) 1 46
decreasing intermicelle interaction. At high copoly-
mer concentration this caused melting of the cubic
lattice and led successively to the formation of
a rod-like structure with hexagonal symmetry.
Large aggregates of PE O- PP O- PE O copolymers
ordered in lamellae structure were formed close to
95°C, leading to an opaque suspension [64].
The phase diagram of Pluronic P85 PEO-
PPO-PEO copolymer in water is presented in
Fig. 13, in which the concentration-temperature
regions where the different aggregates (discussed
in the previous paragraph) exist are also depicted
[61]. The micellization boundary (phase diagram)
for a number of Pluronic PEO PP O- PE O copoly-
mers in aqueous solutions is shown in Fig. 14 [76]
(the CMC CMT data are extracted from
Ref. [28]). The free polymer (unimer) is in solution
at copolymer concentrations and temperatures
lower than the CMC-CMT boundary; micelles in
equilibrium with unimers are present above the
boundary. Recently, Wanka et al. [69] reported
the phase diagrams of several PEO-PPO-PEO
copolymers in water. Unimer, micellar, cubic (gel),
hexagonal, and lamellar phases were observed;
8 0 . 0
6 0 . 0
40.0
E
2 0 . 0
0 . 0
0 . 0 0
liquid ~
-c~ -~ ~ hexagonal rods
I I
I 1 1
micellar
~c.c- ryst al
. . . . . . . . ~ . . . . . . . . 1_ ,.~ ¢ __ J ~ . ~
polymer in so lu t ion ~ ~ 0 ~ ~ ~
0 . 1 0 0 . 2 0 0 . 3 0 0 . 4 0
p o l y m e r c o n c e n t r a t i o n
F i g . 1 3. P h a s e d i a g r a m o f a q u e o u s P l u r o n i c P 8 5 s o l u t i o n ,
s h o w i n g t h e f u l ly d i s s o l v e d p o l y m e r s ( u n i m e r s ) a t l o w t e m p e r -
a t u r e s a n d c o n c e n t r a t i o n s, t h e C M C C M T l in e ( ¢ = 0 ) , t h e
¢ ~ = 0 . 5 3 t r a n s i t i o n t o b . c .c , m i c e l l a r c r y s t a l , a n d t h e h i g h -
t e m p e r a t u r e h e x a g o n a l p h a s e ( ¢ i s t h e h a r d - s p h e r e m i c e l l e
v o l u m e f r a c t i o n ). A t T ~ 6 0 ~ C a l i q u i d p h a s e a p p e a r s b e t w e e n
t h e c r y s t a l l i n e b . c . c , a n d h e x a g o n a l p h a s e s . ( R e p r i n t e d w i t h
p e r m i s s i on f ro m R e f . [ 6 1 ] ; c o p y r i g h t E u r o p e a n P h y s i ca l
Soc ie ty , 1992 . )
6O
i i
m i c e l l e s
C ~
f r e e p o l y m e r ~ - - , _ - - ~ -
1 0 . . . . . . . . J . . . . . . . . J . , , L ~ i ,
0 . 0 1 0 . 1 1 1 0
C o p o l y r n e r c o n c e n t ra t i o n ( % w / v )
F i g. 1 4. P h a s e d i a g r a m o f a q u e o u s P l u r o n i c P E O . - P P O P E O
c o p o l y m e r s o l u ti o n s , s h o w i n g t h e C M C C M T m i c e ll i za t io n
b o u n d a r y . T h e f r e e p o l y m e r ( u n i m e r ) i s i n s o l u t i o n a t c o p o l y m e r
c o n c e n t r a t i o n s a n d t e m p e r a t u r e s l o w e r t h a n t h e C M C C M T
b o u n d a r y : m i c e l le s in e q u i l i b r i u m w i t h u n i m e r s a r e p r e s e n t
a b o v e t h e b o u n d a r y . ( R e p r i n t e d w i t h p e r m i s s i o n f r o m R ef . [ 7 6 ] ;
c o p y r i g h t M a s s a c h u s e t t s I n s t i t u t e o f T e c h n o l o g y , 1 9 94 .)
their occurrence depended on the copolymer
PPO/PEO composition ratio. Copolymers with a
small hydrophilic PEO block usually formed a
hexagonal phase as the first liquid crystalline meso-
phase. The lyotropic mesophases exhibited thermo-
tropic behavior: reversible transitions between the
different phases occurred with increasing temper-
ature at constant copolymer concentration. The
phase behavior of a monoolein-rich (89.5 wt.%)
monoglyceride blend based on pine needle oil,
Pluronic F127, and water has been investigated by
small-angle X-ray diffraction and reported recently
by Landh [81]: an extended cubic phase region
was observed.
5 4 E f f e c t o f add i t i v e s on m ic e ll i za t ion
The phase behavior of Pluronic L64 PEO
PPO PEO block copolymer in the presence of
various additives was studied by Pandya et al.
[67] using cloud point and phenol index (i.e. the
amount of phenol added to give a turbid solution)
measurements. Addit ion of electrolytes, having
anions and cations of different sizes and polariz-
abilities, resulted in either an increase or a decrease
in cloud point: the effect of salts was discussed in
terms of salting in and 'salting out and followed
the Hofmeister series [67]. The effect of various
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P . A l e . v a m b M i s , T . A . H a t t o n / C o l l o i d v S t o ; l ia ' e s < 4. P h v . s i c o c h e m . E r a ,, . A s p ec t, ~ ~ )6 : 1 7 ~ 5 : 1 4 ~
19
n o n - e l e c t r o l y te s (e .g . h y d r o x y c o m p o u n d s a n d
a m i d e s ) o n t h e c l o u d p o i n t o f L 6 4 w a s a l s o e x a m -
i n e d a n d d i s c u s s e d i n t e r m s o f t h e i r i n fl u e n c e o n
w a t e r s t r u c t u r e [ 6 7 ] . T h e a g g r e g a t i o n b e h a v i o r o f
P l u r o n i c s P 8 5 a n d L 6 4 in a q u e o u s s o l u t i o n h a s
b e e n i n v e s t i g a t e d i n th e p r e s e n c e o f a d d e d s a l ts
{ K C N S , K I , K B r , K C I a n d K F ) b y v is c os it y, c l o ud
p o i n t , l i g h t s c a t t e r i n g , p u l s e g r a d i e n t s p i n e c h o
N M R , a n d s o l u b i l i z a t i o n m e a s u r e m e n t s [ 8 2 ] . T h e
s a l ts h a v e a s t r o n g e f f e ct o n t h e c l o u d p o i n t o f t h e
P E O P P O P E O c o p o l y m e r s . A s l ig h t l in e a r
i n c r e a s e i n c l o u d p o i n t w a s o b s e r v e d w i t h a n
i n c r e a s e i n c o p o l y m e r c o n c e n t r a t i o n i n t h e c a s e o f
L 6 4 , l \ q l o w i n g a n i n i t i a l d e c r e a s e u p t o a c o p o l y -
m e r c o n c e n t ra t i o n o f a p p r o x i m a t e l y 2 . K C N S ,
w h i c h i s k n o w n t o i n c r e a s e t h e c l o u d p o i n t , s h o w e d
a s i m i l a r t r e n d i n b o t h L 6 4 a n d P 8 5 s o l u t i o n s ;
a d d i t i o n o f K B r , K C 1, a n d K F d e c r e a s e d t h e c l o u d
p o i n t . A l i n e a r c h a n g e ( i n c r e a s e o r d e c r e a s e ) i n t h e
c l o u d p o i n t w i t h in c r e a s i n g s a lt c o n c e n t r a t i o n w a s
o b s e r v e d . B o t h P l u r o n i c P 8 5 a n d L 6 4 f o r m e d
m i c e l le s w h i c h i n c r e a s e d in s iz e a n d b e c a m e e l o n -
g a t e d i n s h a p e w h e n t h e c l o u d p o i n t w a s
a p p r o a c h e d . T h e c h a n g e s o f si ze a n d s h a p e o f th e
mi c e l l e s , r e v e a l e d b y t h e i n t r i n s i c v i s c o s i t y a n d
r h e o l o g i c a l p r o p e r t i e s o f t h e s o lu t i o n , s e e m e d t o
o c c u r a t t h e s a m e t e m p e r a t u r e r e la t iv e t o t he c l o u d
p o i n t , i n d e p e n d e n t o f t h e n a t u r e o f t h e s al t. T h e
o n s e t o f m i c e l l e f o r m a t i o n w a s a l s o s h i f te d i n t h e
s a m e d i r e c t i o n a s t h e c l o u d p o i n t b y t h e s a l t s , b u t
t o a l e ss e r d e g r e e [ 8 2 ] .
T h e c l o u d p o i n t o f P h i r o n i c F 6 8 w a s r e d t i c e d
b y 50 C w h e n 1.0 M K F w a s a d d e d t o t h e a q u e o u s
s o l u t i o n [ 8 3 ] . T h e s i z e o f t h e u n i m e r s a n d m i c e ll e s
i n t h e s o l u t i o n , a s e s t i m a t e d f r o m d i f f u s i o n c o e f f i -
c i e n ts ( d e t e r m i n e d b y d y n a m i c l i g ht s c a t t e ri n g a n d
N M R s e lf - di ff u si o n m e a s u r e m e n t s ) o r m i c e ll e
m o l e c u k u w e i g h t s t f r o m s t a t i c l i g h t s c a t t e r i n g ) ,
w a s s i m i l a r in t h e p r e s e n c e a n d a b s e n c e o f sa l ts ,
b u t t h e g r o w t h o f t h e m i c e l le s s t a r t e d a t a l o w e r
t e m p e r a t u r e 1 25 C i n 1 .0 M K F , 4 4 - C i n w a l e r t
a n d c o n t i n u e d o v e r tt w i d e r t e m p e r a t u r e r a n g e i n
t h e s tilt s o l u t i o n [ 8 3 ] . T h e i n f lu e n c e o f t e m p e r a t u r e
o n P h i r o n i c I -'1 27 m i c e t l a r si z e a n d h y d r a t i o n w a s
l es s p r o n o u n c e d i n t h e p r e s e n c e o f 0 .5 m o l d m
N a C I i n t h e a q u e o u s s o l u t i o n [ 7 3 ] . T h e i n f l u e n c e
o f a g a r o s c o n t h e c lo u d i n g b e h a v i o r a n d t h e d if fu -
s i o n ~ f E O i ,~PO ,~< ,E O I, ~ c o p o l y m e r , a s w e l l a s t h e
e ff ec t o f th e c o p o l y m e r o n t h e g e l a t io n p r o p e r t i e s
o f a g a r o s e w e r e r e c e n t l y r e p o r t e d b y P e n d e r s e t al .
[ 8 4 ] . T h e c l o u d p o i n t o f
EOI PO oEOI on
COOl-
i n g a n d t h e s e lf - d if f u si o n o f t h e c o p o l y m e r
d e c r e a s e d i n t h e p r e s e n c e o f a g a r o s e g e l s. T h e
p r e s e n c e o f 1.0 M N a S C N s u p p r e s s e d t h e f o r m a -
t i o n a n d g r o w t h o f E O 1 3 P O 3 n E O I 3 m i c e l l e s [ 8 4 ] .
T h e e f fe c ts o f u r e a o n t h e m i c e l l i z a t io n p r o p e r t i e s
o f P l u r on i c P 105 P E O - P P O P E O c o p o l y m e r , a n d
o n t h e st r u c t u r e a n d m i c r o e n v i r o n m e n t o f t h e
m i c e l le s w e r e i n v e s t i g a t e d b y A l e x a n d r i d i s e t al .
[ 8 5 ] . C M C a n d C M T d a t a f o r P h i ro n i c P 1 05
d i s so l v e d m u r e a / w a t e r m i x t u r e s ( u r ea c o n c e n -
t r a t i o n : 0 , 1 , 2 , a n d 4 M I w e r e o b t a i n e d u s i n g a
d y e s o l u b i ti z a t io n m e t h o d a n d c o r r o b o r a t e d w i th
s u r f a c e t e n s i o n , d e n s i t y , a n d f l u o r e s c e n c e s p e c t r o -
s c o p y e x p e r i m e n t s . A d d i t i o n o f u r e a i n c r e a s e d t h e
C M C a n d C M T o f t he P F, O P P O P E O c o p ol y -
m e t : t h e e f fe c t o f u r e a o n t h e ( ' M T w a s m o r e
p r o n o u n c e d a t lo w c o p o l y m e r c o n c e n t r a ti o n s a n d
d i m i n i s h e d a t c o n c e n t r a t i o n s o f a p p r o x i m a t e l y
2 . 5 ° ; .
T h e t h e r m o d y n a m i c p a r a m e t e i s o f P 1 05
m i c e l l e f o r m a t i o n i n t h e p r e s e n c e o f u r e a w e r e
e s t i m a t e d u s i n g a c l o s e d a s s o c i a t i o n m o d e l : t h e
e n t h a t p y o f m i c e l l i z a t i o n w a s p o s i t i v e ( e n d o t h e r -
m i c l a n d d e c r e a s e d u p o n i n c r e as i n g t h e u r e a c o n -
c e n t r a t i o n . ~ I h e s u r f a c e a c t i v i t y ' a n d t h e p a r t i a l
s p e c if i c v o l u m e o f P l u r o n i c P I 0 5 d e c r e a s e d w i t h
a n i n c r e a s e i n t h e u r e a c o n c e n t r a t i o n , w h e r e a s t h e
h y d r o d y n a m i c r a d i i o f t h e c o p o l y m c r m i c el le s .
d e t e r m i n e d u s i n g d y n a m i c l i g h t s c a t t e r i n g ,
r e m a i n e d u n a f f e c t e d b y t h e p r e s e n c e o f 4 M u r e a
i n t h e s o l u t i o n . T h e m i c r o p o h u i t y i n c o p o l y m e r
s o l u t i o n s i n u r e a / w a t e r w a s p r o b e d a s a f u n c t i o n
o f t e m p e r a t u r e u s in g t h e
l
' I. ~ i n t e n s i t y r a t i o o f
t h e p y r e n e f l u o r e s c e n c e e m i s s i o n s p e c t r a : a s m a l l
d e c r e a s e i n th e m i c r o p o l a r i t y o f t h e m i c e l te c o r e
w a s o b s e r v e d i n t h e p r e s e n c e o f u r e a [ : S 5 ].
S o d i t u n d o d e c y l s u l f a t e I S D S t i n c r e a s e d t h e
c l o u d p o i n t o f P l u r o n i c L 6 4 , w h i c h o t h e r w i s e
d e c r e a s e d i n t h e p r e se n c e o f s m a l l a m o u n t s o f
e l e c t r o ly t e s . T h e c l o u d i n g b e h a v i o r o f a m i x e d
L 6 4 . S D S s y s t e m in th e a b s e n c e a n d p r e s e n ce o f
s a l t w a s d i s c u s s e d b y P a n d y a e t a l. [6 7 11 , p r i m a r i l y
i n t e r ms o f e l e c t r i c a l c h a r g e o n t h e mi c e l l e su r f a c e .
N M R c h e m i c a l s h i f t m e a s u r e m e n t s a n d f l u o r e s -
c e n c e q u e n c h i n g h a v e b e e n c o m b i n e d i n a s t u d y
o f t h e a g g r e g a t i o n b e h a v i o r a n d p r t+ p e r ti e s o f
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2
P. Alexandridis , T .A . Ha t ton/C ol lo ids Sur fac es A: Phys icochem. Eng. Aspe cts 96 1995) 1 46
P l u r o n i c s L 6 4 a n d F 6 8 i n t h e p r e s e n c e o f S D S a t
2 0 an d 4 0 ° C [ 8 6 ] . W h i le t h e c o p o l y m e r s d i d n o t
f o r m m i c e l l e s a t 2 0 ° C , t h e y f o r m e d m i x e d m i c e l l e s
w i t h S D S , a t S D S c o n c e n t r a t i o n s a s l o w a s 1 r a M .
T h e a g g r e g a t i o n n u m b e r s o b t a i n e d f r o m f l u o r e s -
c e n c e q u e n c h i n g s t u d i e s in d i c a t e th a t a b o u t t h r e e
S D S m o l e c u l e s p e r c o p o l y m e r m o l e c u l e a r e n e e d e d
t o g l u e t o g e t h e r t h e P P O b l o c k s i n t o a h y d r o -
p h o b i c c o r e at 2 0 ~ C [ 8 6 ] . T h e m i c e ll e s f o r m e d a t
l o w S D S c o n c e n t r a t i o n s w e r e s m a ll ( 1 5 S D S m o l e -
c u le s a s s o c i a te d w i t h f o u r t o f iv e c o p o l y m e r m o l e -
c u l es ) ; t h e m i c e l le s c o n s i s t e d m a i n l y o f S D S a t h i g h
S D S c o n c e n t r a t i o n s . L 6 4 f o r m e d m i c e l l e s a t 4 0 u C
t h a t w e r e l a r g e b u t d e c r e a s e d r a p i d l y i n s i z e u p o n
a d d i t i o n o f S D S . T h e 1 3C c h e m i c a l s h if ts f o r t h e
m e t h y l c a r b o n s i n t h e P P O b l o c k w e r e i n d i c a ti v e
o f a c h a n g e f r o m a c o i l e d c o n f o r m a t i o n i n s m a l l
m i c e l l e s , u n i m e r s , a s w e l l a s i n t h e b u l k P P O
l i q u i d , t o a m o r e e x t e n d e d P P O c o n f o r m a t i o n i n
t h e l a r g e m i c e ll e s. T h e c h a n g e o f th e s h i ft s o f th e
i n d i v i d u a l c a r b o n s i n th e a l k y l c h a i n o f S D S o n
a d d i t i o n o f t h e c o p o l y m e r s t o S D S m i c el le s fo l -
l o w e d a s i m il a r p a t t e r n f o r b o t h L 6 4 a n d F 6 8
P E O P P O - P E O c o p o ly m e rs [ 8 6 ] .
R e c e nt ly , H e c h t a n d H o f f m a n n [ 8 7 ] r e p o r t e d
t h e i n fl u e n c e o f S D S o n t h e a g g r e g a t i o n b e h a v i o r
o f P l u r o n i c F 1 2 7 . F o r b l o c k c o p o l y m e r s o l u t io n s
i n t h e c o n c e n t r a t i o n r a n g e o f 1 - 5 % a n d i n c r e a s i n g
S D S c o n c e n t r a t i o n , t h e l i g h t s c a t t e r i n g i n t e n s i t y
p a s s e d t h r o u g h a d e e p m i n i m u m , t h e e l ec t ri c b i re -
f r in g e n c e p a s s e d o v e r a m a x i m u m , a n d t h e e n d o -
t h e r m i c p e a k o f t h e D S C s ig n a ls ( d u e to t h e
m i c e l l iz a t i o n o f t h e b l o c k c o p o l y m e r ) d i s a p p e a r e d .
T h e s e r e s u l t s i n d i c a t e t h a t S D S b i n d s t o u n i m e r s
o f F 1 2 7 a n d t h e r e b y s u p p r e s s e s c o m p l e t e l y t h e
f o r m a t i o n o f F 12 7 m ic e ll es . A p p r o x i m a t e l y s i x S D S
m o l ec u le s b i n d t o o n e F1 2 7 P E O - P P O P E O
c o p o l y m e r m o l e c u l e at s a t u r a t i o n . T h e b i r e f ri n -
g e n c e d a t a i n d i c a t e d t h a t th e F 1 2 7 / S D S c o m p l e x
e x is ts in a m o r e o r l e ss e x t e n d e d c o n f o r m a t i o n a n d
n o t a s a c o il . T h e b i n d i n g o f S D S o n F 1 2 7 w a s
a l s o c o n f i r m e d b y s u r f a c e t e n s i o n m e a s u r e m e n t s ,
w h i c h s h o w e d t h e s u r f a c e a c t i v e b l o c k c o p o l y m e r s
t o b e d i s p l a c e d f r o m t h e a i r - w a t e r i n t e r f a c e s b y
S D S m o l e c u le s . T h e b i n d i n g o f S D S o n F 1 2 7
c o m m e n c e d a t a c o n c e n t r a t io n f a r b e l ow t h e C M C
o f S D S . A d d i t i o n o f C T A B N-cetyl-N,N,N-tri-
m e t h y l a m m o n i u m b r o m i d e ) t o F 1 2 7 s o l u t i o n s
d e c r e a s e d th e m a g n i t u d e o f t h e D S C p e a k , s h o w i n g
t h a t C T A B ( a c a t i o n i c s u r f a c t a n t ) a l s o s u p p r e s s e s
t h e m i c e l l i z a ti o n o f F 1 2 7 . T h e z w i t t e r i o n i c s u r f a c -
t a n t C a 4 D M A O N,N-dimethyl-l-tetradecanam-
i n e - N - o x i d e ) s u p p r e s s e s F 1 2 7 m i c e l l i z a t i o n a t
c o n c e n t r a t i o n s t e n t i m e s a b o v e i t s
C M C ,
sugge s t -
i n g a d if f e re n t m o d e o f a s s o c i a t i o n t h a n S D S ;
m a y b e , F 1 2 7 m o l e c u l e s b i n d t o t h e s u r f a c e o f
C 1 4 D M A O m i c e l l e s [ 8 7 ] .
6 M o de l i ng o f b l o ck co po l y m er m i ce l l iza t i o n a nd
micel l e s tructure
T h e o r i e s o f m i c e ll e f o r m a t i o n i n s o l u t i o n s o f
b l o c k c o p o l y m e r s h a v e b e e n s u g g e s t e d b y a
n u m b e r o f re s e a r c h e rs o v e r t h e p a s t d e c a d e . L e i b l er
e t a l. [ 8 8 ] , N o o l a n d i a n d H o n g [ 8 9 ] , M u n c h a n d
G a s t [ 9 0 ] , a n d N a g a r a j a n a n d G a n e s h [ 9 1 ] c o m -
p u t e d t h e f r ee e n e r g y o f m i c el le f o r m a t i o n a s s u m i n g
u n i f o r m c o p o l y m e r s e g m e n t c o n c e n t r a t i o n s i n t h e
c o r e a n d c o r o n a r e g i o n s , r es p e c t i v e ly . S c a l i n g t h e o -
r i e s f o r p o l y m e r i c m i c e l l e s w i t h a n i n s o l u b l e c o r e
a n d a n e x t e n d e d c o r o n a w e r e d e v e l o p e d b y
H a l p e r in [ 7 1 ] , M a r q u e s [ 9 2 ] , S e m e n o v [ 9 3 ] , a n d
Z h u l i n a a n d B i r s h te i n [ 9 4 ] . I n a t h ir d a p p r o a c h ,
v a n L e n t a n d S c h e u t j e n s [ 9 5 ] , L i n s e [ 5 0 , 5 1 ] , a n d
H u r t e r e t a l . [ 4 8 , 4 9 ] u s e d a s e l f - c o n s i s t e n t m e a n -
f ie l d t h e o r y t o d e t e r m i n e t h e d e t a i l e d s e g m e n t
d e n s i t y p r o f i l e s w i t h i n a m i c e ll e , m a k i n g n o a p r i o r i
a s s u m p t i o n s a s t o t h e l o c a t i o n s o f t h e m i c e l l a r
c o m p o n e n t s . T h e m i c e l l e s t r u c t u r e s d e t e r m i n e d
f r o m t h i s m o d e l h a v e b e e n c o n f i r m e d r e c e n t l y b y
M o n t e C a r l o s i m u l a t i o n s ( e . g . W a n g e t a l . [ 9 6 ] ) ,
a n d s h o w t h a t t h e i n t e r f a c i a l r e g i o n b e t w e e n t h e
c o r e a n d c o r o n a o f t h e m i c e l l e is d i f fu s e a n d n o t
s h a r p a s i s a s s u m e d i n t h e o t h e r m o d e l i n g
a p p r o a c h e s .
6.1. Phenomenological models
M u n c h a n d G a s t [ 9 0 ] a p p l ie d th e th e o r y o f
L e i b l e r e t al . [ 8 8 ] , w h i c h t r e a t e d m i c e ll e f o r m a t i o n
i n m i x t u r e s o f b l o c k c o p o l y m e r s a n d h o m o -
p o l y m e r s , t o d e s c r i b e t h e m i c e l l iz a t i o n o f d i b l o c k
A B c o p o l y m e r s in s o l u ti o n . T h e m i ce l le c o r e w a s
a s s u m e d t o b e a m e l t o f B c h a in s , w h i l e t h e c o r o n a
c o n t a i n e d A c h a i n s w i t h s o l v e n t S . T h e i n t e r a c ti o n
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P . A l e .w m ~ h ' f l li s , T . , 4 . H a t t o # i / C o l l o i d ~ " S u d d c e . ~ A . " P h v s i c o c h c m . E H g , . . ~l .S o c ct s 9 6 ( 1 9 9 5 ~ 4 6 2 1
b e t w e e n t h e s o l v e n t S a n d t h e B s e g m e n t w a s
d e s c r i b e d b y t h e Z us i n t e r a c t i o n p a r a m e t e r , w h e r e a s
a t h e r m a l i n t e r a c t i o n s b e t w e e n t h e A s e g m e n t a n d
s o l v e n t w e r e a s s u m e d ( Z A S = 0 L T h e e n e r g y o f a
s i n g l e m i c e l l e w a s e x p r e s s e d a s a c o n t r i b u t i o n o f
t h e c o r e s o l v e n t i n t e r f a c i a l e n e r g y , t h e e n e r g y d u e
t o d e f o r m a t i o n o f t h e c o p o l y m e r c h a i n s in t h e
m i c e l le , a n d t h e f r e e e n e r g y o f m i x i n g o f s o l v e n t
m o l e c u l e s w i t h t h e A m o n o m e r s i n t h e c o r o n a ( ' / ~ o :
v o l u m e f r a c t i o n o f A in t h e c o r o n a l . T h e t o t a l f r e e
e n e r g y o f t h e s y s t e m i n c l u d e d t h e e n e r g y o f t h e
m i c e l l e s , t h e f r e e e n e r g y o f m i x i n g i n d i v i d u a l
d i b l o c k c o p o l y m e r s ( u ni m e r s ) w it h s o l v e n t m o l e -
c u le s ~ q ~ : v o l u m e f r a c t io n o f c o p o l y m e r s o u t s i d e
t h e m i c e l l e s ) , a n d a n e n e r g y c o n t r i b u t i o n a r i s i n g
f r o m t h e t r a n s l a ti o n a l e n t r o p y o f th e g a s o f
m i c e l l e s . T h e t o t a l f r e e e n e r g y w a s m i n i m i z e d w i t h
r e s p e c t t o t h e m i c e l l e a g g r e g a t i o n n u m b e r p , a n d
t h e w f l u m e f r a c t i o n s qS~ a n d ~0 . O n e u s e f u l f e a t u r e
o f th e t h e o r y o f L e i b l e r e t a l . [ 8 8 ] i s t h a t , f o r a
h i g h d e g r e e o f i n c o m p a t i b i l i ty b e t w e e n B a n d S ,
t h e f r ee e n e r g y ' m i n i m i z a t i o n w i t h r e s p e c t t o ~
r e s u l t s i n a n a n a l y t i c a l e s t i m a t e f o r
C M C a s 4 1
~+0
[ 8 8 , 9 0 ] :
4>: ~
exp{.l: k
T + N -
ZllsNl~ ) ( 13 )
w h e r e
[ , k T
is th e e n e r g y p e r c o p o l y m e r c h a i n i n
a n i s o l a t e d m i c e ll e , N i s t h e t o t a l n u m b e r o f
s e g m e n t s in t h e c o p o l y m e r m o l e c u l e , N , is t h e
n u m b e r o f B t c o r e ) s e g m e n t s , a n d Z R sN ~ is t h e
t o t a l e f f e c t iv e in t e r a c t i o n p e r c h a i n ( s in c e t h e A
c h a i n is in a n a t h e r m a l e n v i r o n m e n t l . T h e l e a d i n g
o r d e r t e r m s i n t h e / / k T e x p r e s s i o n a r e [ 9 0 ]
[ 5 k T ~ { 0 j T 2 . ~ B } 1 6 ( ~ . B S n l g } 1 2 1 .) 1 3
+ N i I + N I , , , 'N ~ , I ~ (1 -q ~oj ~b o ~ l n ( 1 q~o)
( 1 4 )
T h e f i r st t e r m o n t h e r i g h t - h a n d - s i d e o f Eq . ( 1 4 )
a r i s e s f r o m t h e e n e r g y r e q u i r e d t o f o r m t h e m i c e l l e
c o r e s o l v e n t i n t e r f a c e a n d o p p o s e s t h e f o r m a t i o n
o f m i c e l le s , w h e r e a s t h e s e c o n d t e r m , w h i c h o r i g i -
n a t e s f r o m t h e c o n f i g u r a t i o n a l e n t r o p y o f m i x i n g
s o l v e n t m o l e c u l e s w i t h A s e g m e n t s i n t h e c o r o n a ,
f a v o r s m i c e l li z a t i o n . A s Z As w a s a s s u m e d t o b e
z e r o , n o e f f e c t o f b l o c k A o n m i c e l l i z a t i o n c o u l d
b e o b s e r v e d i n t h e m o d e l o f M u n c h a n d G a s t ,
o t h e r t h a n t h e c o n f i g u r a t io n a l e n t r o p y o f m i x i n g
o f s o l v e n t m o l e c u l e s w i t h A m o n o m e r s i n th e
c o r o n a . T h e m i c e ll i z a li o n m o d e l o f N a g a r a j a n a n d
G a n e s h [ 9 1 ] t o o k i n t o a c c o u n t A s o lv e n t in t e r-
a c t i o n s : h o w e v e r , t h is w a s a t t h e e x p e n s e o f a n
a n a l y t i c a l e x p r e s s i o n f o r ( ~c Mc .
I n t h e m i c e l l i z a t i o n t h e o r y d e v e l o p e d b y
N a g a r a j a n f in d G a n e s h [ 9 1 ] t h e m i c e ll e s w e r e a l s o
a s s u m e d t o h a v e a c o m p l e t e l , , , s e g r e g a t e d c o r e
r e g i o n , c o n s i s t i n g o f o n l y t h e B b lo c k , a n d a c o r o n a
r e g i o n c o n s i s l i n g o f th e s o l v e n t S a n d t h e s o l v e n t -
c o m p a t i b l e A b l o c k . T h e f i'e e e n e r g y o f m i c e l li z a -
t i o n w a s e x p r e s s e d i n t e r m s o f c h a n g e s in t h e s t a t e
o f d i l u t i o n o f b l o c k s A a n d B , c h a n g e s i n th e s t a t e
o f d e f o r m a t i o n o f b l o c k s A a n d B . l o c a l i z a t i o n o f
t h e c o p o l y l n e r mo l e c u l e s f i t t h e mi c e l l e i n t e r f a c e ,
a n d f o r m a t i o n o f t h e m i c e l l a r c o r e s o l v e nt
i n t e r f a c e . T h e h y d r o p h o b i c i n t e r a c t i o n s , w h i c h
s t r o n g l y i n f l u e n c e t h e m i c e l l i z a t i o n i n w a t e r a s
d i s c u s s e d i n S e c t i o n 4 . w e r e i m p l i c i t l y a c c o u n t e d
f o r in t h e e x p e r i m e n t a l l y d e t e r m i n e d Z v a l u e s u s e d
f o r t h e P E O w a t e r a n d P P O w a t e r in t e r a c t io n
f re e e n e r g y t e r m s . T h e m i c e l l iz a t io n p a r a m e t e r s
w e r e o b t a i n e d f i 'o m m i n i m i z a t i o n o f t h e f re e e n e r g y
e x p r e s s i o n w i t h r e s p e c t t o t h e r a d i u s o f t h e m i c e l le
c o r e a n d t h e c o r o n a t h i c k n c s s . T h e m o s t i m p o r t a n t
c o n t r i b u t i o n l a v o r i n g m i c e l l i z a t i o n a r i s e s f r o m t h e
c h a n g e in s t a l e o f d i h it i o n o f b l o c k B a s i t g o e s
f r o m s o l v e n t S in t o t h e m i c e l l e c o r e . N o t e t h a t t h e
m u t u a l i n c o m p a t i b i l i t y o f b l o c k s A a n d B is a l s o
e s s e n t ia l f o r m i c e l li z a t i o n , o t h e r w i s c t h e i n c o m p a t i -
b i l i t y b e t w e e n t h e s o l v e n t a n d p o l y m e r b l o c k s w i l l
g i v e r i s e t o m a c r o s c o p i c p h a s e s e p a r a t i o n : t h e
i n c o m p a t i b i l i t y o f b l o c k s A a n d B w a s i m p l i c i t ly
t a k e n i n to a c c o u n t b y th e c o m p l e t e l y s e g r e g a t e d
c o r e s t r u c t u r e a s s u m e d f o r t h e m i c e l l e . T h e m o s t
i m p o r t a n t
c o n t r i b u t i o n
o p p o s i n g m i c e l l i z a t i o n
o r i g i n a t e s f r o m t h e f o r m a t i o n o f t h e m i c e ll e c o r e
s o l v e n t i n t e r f a c e . T h e o p t i m a l a g g r e g a t i o n n u m b e r
i s d e t e r m i n e d p r i m a r i l y b y b a l a n c i n g t h e h ' e e
e n e rg y , c o n t r i b u t i o n s d u e t o c h a n g e i n s t a t e o f
d i h i ti o n a n d d e f o r m a t i o n o f t he A ( m o r e i m p o r t a n t
i n t h e c a s e o f t he P E O P P O , w a t c r s y s t e m t a n d B
b l o c k s , fa w ~ r in g l o w a g g r e g a t i o n n u m b e r s , a n d t h e
f re e e n e rg y c o n t r i b u t i o n s d u e t o f o r m a l i o n o f t h e
m i c e l le c o r e s o l v e n t i n t e r t h c e t h a t f a v o r h i g h
a g g r e g a t i o n n u m b c r s . T h e t h e o r y o f N a g a r a j a n
f in d G a n c s h , d e v e l o p e d f o r d i b lo c k c o p o l y m e r s ,
s u c c e e d s m p r e d i c t i n g q u a l i t a t i v e l 5 l h c c o n t r i b u -
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P. Alexandrhl i s . T .A. Ha t ton/Co l lo ids Sur[ aces A: Ph) s icochem. Eng. Aspects 96 (1995) 1 46
tions of the copolymer composition and molecular
weight on the micellization of triblock copolymers.
The trends are the same in both theory and experi-
ment, although the theory overpredicts the experi-
mental ly obtained free energies of micellization by
a factor of eight [76], resulting in unrealistically
low CMC values. Part of the numerical discrepancy
could be eliminated by adding extra terms in the
expression for the free energy of micellization, to
account for the entropy penalty involved in joining
two AB diblock copolymer molecules to create a
ABzA triblock, and in having loops in the micelle
core.
Prochazka et al. [97] modified Nagarajan's
model to treat the association of ~ B A triblock
copolymers in solvents selective for A blocks, by
introducing two additional terms into the free
energy expression discussed above. These terms
accounted for the reduction in entropy due to loop
formation of the middle (B) block, and the localiza-
tion of the two A-B joints of the copolymer at the
interface between the core and the corona of the
micelle. This modification resulted in less negative
AG ' values (in the correct direction with respect
to agreement between experiment and theory), but
the decrease in the absolute value of the predicted
AG was approximately 10%, not enough to
account for the eight-fold overprediction of the
experimental AG-' values. A comparison of the
model calculations by Nagarajan and Ganesh [91]
for a diblock copolymer with the results of
Prochazka et al. [97] obtained for a triblock
copolymer (of the same molecular weight and
composition as the diblock) showed the aggrega-
tion number of the diblock copolymer micelles to
be three to five times higher than that for the
triblock micelles. Linse [50,51] concluded, using a
mean-field lattice micellization model, that there is
a five-fold increase in CMC and a four-fold
decrease in aggregation number on going from a
di- to a triblock PEO-PPO copolymer. Prochazka
et al. [97] also made an interesting observation
regarding the effect of the deformation energy
contribution on the model AG =expression: model
calculations using the deformation energy terms of
Nagarajan and Leibler et al. [88] resulted in
different aggregation numbers, although both
terms were based on Flory's [98] theory.
Recently, Izzo and Marques [99] studied theo-
retically, using scaling concepts, the formation of
dilute phases of spherical, cylindrical, and planar
aggregates of diblock A-B and triblock ~B-A
copolymer chains in solvents selective to B. They
treated the micelle formation of the triblock copol-
ymers in the same way as that of the diblocks, the
only difference being that the core was composed
of twice as many chains of half the length of B.
This resulted in AG triblock ~2AG:~diblock, which is
counterintuitive and opposite to the micellization
theories examined above. It thus appears that the
entropy loss associated with the formation of
loops is essential for describing the micellization
of triblock copolymers. The loop (backfolding)
entropy term was first introduced by ten Brinke
and Hadziioannou [ 100] in their model of B A-B
copolymer micelle formation in A-homopolymer,
and modified by Balsara et al. [101] in a similar
model. Balsara et al. [101] commented on the
applicability of the theory of Leibler et al. [88],
originally proposed for micellization of A-B block
copolymer in A-homopolymer, in the theories of
Munch and Gast [90] and Nagarajan and Ganesh
[91], that treat micellization in selective solvents.
They noted that, in the presence of solvents, the
description of the core corona interface is compli-
cated owing to the three-component (owing to the
presence of solvent at the interface) nature of the
problem [101]. These points, in addition to the
hydrogen bonding interactions between both PEO
and PPO, and water, may account for the quantita-
tive discrepancy between predictions of phenome-
nological block copolymer micellization theories
and the experimental results of Ref. [28] for the
system PEO- PPO PEO/water.
6.2. Mean f ie ld lat t i ce mod els
Another approach to modeling block copolymer
micelles has used the Scheutjens-Fleer theory
[ 102], an extension of the Flory-Huggins analysis
of homogeneous polymer solutions [98], in which
the polymer chains are allowed to assume different
conformations on a lattice. A first-order Markov
approximat ion (i.e. the position of a segment
depends only on that of the preceding segment in
the chain) is used in the Flory-Huggins analysis
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P. . 4 h ' . va mh ' id is , T . A . H a t to n / ( ' o l l o id s S u Ja ~ e s A . Ph r , v i co ch em. EHg . . 4. V~ec l. v 9 6 r 1 ~ )9 5 , I 4r~ 2 3
a n d , a s a r e s t l l t , t h e c h a i n c o n f o r m a t i o n f o l l o w s
t h e p a t h o f a r a n d o m w a l k . I n a d d i t i o n , a m e a n -
f i e l d a s s u m p t i o n i s i n v o k e d t o d e s c r i b e t h e i n t e r -
a c t i o n s b e t w e e n d i f f e r e n t p o l y m e r s e g m e n t s . T h e
f r e e e n e r g y o f th e s y s t e m is m i n i m i z e d i n o r d e r t o
c a l c u l a t e t h e e q u i l i b r i u m t h e r m o d y n a m i c p r o p e r -
t ie s o f t h e p o l y m e r s o l u t io n . F o r s p a t i a l l y i n h o m -
o g e n e o u s s y s t e m s s u c h a s t h o s e c o n t a i n i n g i n t e r -
f a c e s , S c h e u t j e n s a n d c o - w o r k e r s [ 9 5 , 1 0 2 , 1 0 3 ]
r e s t r i c t e d t h e m e a n - f i e l d a p p r o x i m a t i o n t o t w o
d i m e n s i o n s l i. e. w i t h in p a r a l l e l o r c o n c e n t r i c l a t t i c e
l a y e rs l a n d a p p l i e d a s t e p - w e i g h t e d r a n d o m w a l k
t o a c c o u n t f o r t h e in h o m o g e n e i t i e s n o r m a l t o t h e
l a y e r s . T h e p o l y m e r a n d s o l v e n t m o l e c u l e s a r e
a s s u m e d t o b e d i s t r i b u t e d o v e r a l a t ti c e , s u c h t h a t
s o l v e n t m o l e c u l e s a n d p o l y m e r s e g m e n t s o c c u p y
o n e l a t t ic e s it e e a c h . E a c h p o l y m e r c h a i n c a n
a s s u m e a l a rg e n u m b e r o f p o s s ib l e c o n f o r m a t i o n s ,
d e f i n e d b y t h e l a y e r n u m b e r s i n w h i c h s u c c e s s i v e
s e g m e n t s a r e f o u n d . T h e r e c a n b e m a n y d i f f e r e n t
a r r a n g e m e n t s f or e ac h c o n f o r m a t i o n : if t he n u m b e r
o f p o l y m e r c h a i n s i n e a c h c o n f o r m a t i o n is sp e c if ie d ,
t h e c o n f i g u r a t i o n a l e n t r o p y c o n t r i b u t i o n t o t h e
s y s t e m f r ee e n e r g y c a n b e e v a l u a t e d . T h e o t h e r
c o n t r i b u t i o n s t o t h is f re e e n e r g y , d u e t o t h e i n t e r -
a c t i o n s b e t w e e n t h e p o l y m e r m o l ec u l e s, s o h e n t
m o l e c u l e s a n d t h e s u r f a c e , a r e c h a r a c t e r i z e d b y
F I o r y H u g g i n s 7 - p a r a m e t e r s . I f t h e f re e e n e r g y o f
t h e s y s t e m is m i n i m i z e d w i t h r e s p e c t to t h e n u m b e r
o f p o l y m e r c h a i n s i n e a c h c o n f o r m a t i o n , it is
p o s s i b l e t o c a l c u l a t e t h e e q u i l i b r i u m s e g m e n t d e n -
s i t y p r o f i l e s . Se l f - c o n s i s t e n t f i e l d t h e o r y c a n b e u s e d
t o c a l c u l a t e t h e s e g m e n t d e n s i t y p r o f i l e s in a m i c e l l e
o n c e t h e a g g r e g a t i o n n u m b e r o f t h e m i c e ll e is
k n o w n [ 9 5 , 1 0 3 , 1 0 4 ] . T o fi n d t h is a g g r e g a t i o n
n u m b e r , s m a l l s y s te m t h e r m o d y n a m i c s c a n b e u s ed
[ 1 0 5 1 0 7 ] , a c c o r d i n g t o w h i c h th e c h a n g e i n f r ee
e n e r g y d u e t o t h e c h a n g e i n t h e n u m b e r o f m i c e ll e s
( a t c o n s t a n t t e m p e r a t u r e , p r e s s u r e a n d n u m b e r o f
m o l e c u l e s l m u s t b e z e r o a t e q u i l i b r i u m . T h i s
' e xc e ss f r ee e n e r g y is t h e s u m o f t h e e n e r g y
r e q u i r e d t o c r e a t e m i c e l le s a n d t h e e n e r g y d u e t o
t h e t r a n s l a t i o n a l e n t r o p y o f t h e m i c e ll e s. S i nc e t h e
s e g m e n t d e n s i t y p r o f l e s a r e r e q u i r e d i n o r d e r t o
c a l c u l a t e t h e e n e r g y o f m i c e l l e f o r m a t i o n , a n i t e r a -
t i r e p r o c e s s i s f o l l o w e d . T h e s e l f - c o n s i s t e n t m e a n -
f ie l d l a t ti c e t h e o r y o f S c h e u t j e n s a n d F l e e r [ 1 0 2 ]
h a s b e e n u s e d t o s t u d y m a n y c o l l o i d a l sy s t e m s ,
i n cl u di n g a d s o r p t i o n o f h o m o p o l y m e r s a n d b lo c k
c o p o l y m e r s o n s u rf a c e s [ 1 0 8 ] , i n t e r a c ti o n s b e t -
w e e n a d s o r b e d p o l y m e r la y e r s [ 1 0 9 ] , a n d t h e
f o r m a t i o n o f m i c e l le s [ 1 0 3 ] , v e s ic l e s [ 1 0 4 ] a n d
m e m b r a n e s [ 1 1 0 ] .
R e c e n t l y , t h e S c h e u t j e n s a n d F l e e r t h e o r y h a s
b e e n e x t e n d e d b y H u r t e r e t a l . [ 4 8 , 4 9 ] t o s t u d y
t h e f o r m a t i o n o f m i c e ll e s b y l in e a r P E O
P P O P E O a n d b r a n c h e d , s t a r- l ik e , a m p h i p h i l i c
b l o c k c o p o l 5 m e r s : t h e s o l u b i l iz a t io n o f n a p h t h a -
l e n e i n t h e s e mi c e l l e s h a s a l s o b e e n a n a l y z e d a s a
f u n c t io n o f p o l y m e r s t r u ct u r e , c o n l p o s i t i o n a n d
m o l e c u l a r w e i g h t [ 4 8 , 4 9 ] . T h e d e t a i l e d d e n s i t y
p r o f i le s o f t h e p o l y m e r s e g m e n t i n t h e m i c e l l e s
w e r e o b t a in e d , a n d m a c r o s c o p i c q u a n t it i e s s u c h a s
c r i t i c a l m i c e l l i z a t i o n c o n c e n t r a t i o n , a g g r e g a t i o n
n u m b e r , a n d m i c e l l e s i z e w e r e c a l c u l a t e d . T h e
c a l c u l a t i o n s s h o w t h a t h i g h e r - m o l e c u l a r - w e i g h t
p o l y m e r s f o r m l a r g e r m i c e ll e s. T h e a d d i t i o n o f t h e
s o l u t e c a u s e s t h e m i c e l l e t o b e c o m e l a r g e r , w i t h a
l o w e r c o n c e n t r a t i o n o f w a t e r a n d a h i g h er c o n c e n -
t r a t i o n o f P P O i n t h e m i c e l le c o r e . T h e s i m p l e
l a t t i c e t h e o r y f o r fl e x i b le c h a i n mo l e c u l e s , h o w e v e r ,
c a n n o t c a p t u r e e f l' ec ts s u c h a s t h e p h a s e b e h a v i o r
o f P E O a n d P P O i n w a t e r , w h e r e a h > w e r c r i t i c a l
s o l u t i o n t e l n p e r a t n r e is o b s e r v e d . T o p r e d i c t s u c h
b e h a v i o r , t h e g a u c h e a n d t r a n s b o n d o r i e n t a t i o n s
o f t h e p o l y l n e r c h a i n m u s t b e a c c o u n t e d f o r.
L e e r m a k e r s [ 1 1 1 ] c o m b i n e d t h e r o t a t i o n a l i so -
m e r i c s t a t e s c h e m e ( w h i c h a c c o u n t s f o r t h e g a u c h e
t r a n s o r i e n t a t i o n s i n a c h a i n a n d e l i m i n a t e s b a c k -
f o l d i n g l w i t h s e l f - c o n s i s t e n t l i e l d t h e o r y t o p r e d i c t
t h e f o r m a t i o n o f l ip id b i l a y e r m e m b r a n e s a n d l ip id
v e si cl es . A n a p p r o a c h w h i c h i s c o m p u t a t i o n a l l y
m o r e s i m p l e t h a n t h e r o t a t i o n a l i s o m e r i c s t a te
s c h e m e , b u t n e v e r t h e l e s s a c c o u n t s f o r t h e t e m p e r -
a t t i re a n d c o m p o s i t i o n d e p e n d e n c e c ,f t h e in t e r -
a c t i o n p a r a m e t e r Z in a p h y s i c a l ly a c c e p t a b l e
m a n n e r , h a s r e c e n t l y b e e n p r e s e n t e d b y K a r l s t r o m
[ 4 6 ] . T h i s m o d e l f o r P E O r e c o g n i z e d t h a t c e r t a i n
s e q u e n c e s o l t h e g a u c h e t r a n s o r i e n t a t i o n s i n a n
E O s e g m e n t w o u l d l e a d
I t L I. p O I L l r
c o n f o r m a t i o n ,
w h i l e o t h e r s w o u l d b e e s s e n t ia l l y n o n - p o h u : t h e
p r o b a b i l i t y o f n o n - p o l a r c o n f o r m a t i o n s i n c r e a se d
w i t h t e m p e r a t u r e , m a k i n g t h e p o l y m e r m o r e
h y d r o p h o b i c . T h e s o l u b i l i t y g a p i n P E O w a t e r
a n d P P O w a t e r p h a se d i a g r a m s c o u ld b e re p r o -
d u c e d u s i n g t h i s m o d e l .
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4 P, Alexandridis , T .A. Hat ton/ 'Col lo ids Sur /aces A. Phys icochem. Eng. Aspects 96 (1995) 1 46
A m o d e l f o r b l o c k c o p o l y m e r m i c e l le s h a s b e e n
d e v e l o p e d [ 4 8 , 5 0 ] w h i c h i n c o r p o r a t e s K a r ls t r om s
i d e a s t o a c c o u n t f o r t h e c o n f o r m a t i o n a l d i s tr i b u -
t i o n i n P E O a n d P P O . A n i n c r e a s e i n t e m p e r a t u r e
c a u s e d t h e m o d e l - p r e d i c t e d m i c e l l e a g g r e g a t i o n
n u m b e r t o i n c r e a s e , a n d t h e C M C c o n c e n t r a t i o n
t o d e c r e a s e . T h e a g g r e g a t i o n n u m b e r i n c r e a s e d
r a p i d l y w i t h p o l y m e r c o n c e n t r a t i o n u n d e r d i l u t e
c o n d i t i o n s , b u t w a s a p p r o x i m a t e l y i n v a r i a n t t o
p o l y m e r c o n c e n t r a t i o n a t c o n c e n t r a t i o n s f a r f r o m
t h e C M C . R a i s i n g t h e P P O c o n t e n t o r t h e m o l e c u -
l a r w e i g h t o f t h e c o p o l y m e r i n c r e a s e d t h e a g g r e g a -
t i o n n u m b e r , a n d d e c r e a s e d t h e C M C . T h e m o d e l
p r e d i c t s t h a t t h e m o s t h y d r o p h o b i c p o l y m e r ,
P l u r o n i c P 1 0 3 , d o e s n o t f o r m s p h e r i c a l m i c e l l e s a t
h i g h e r p o l y m e r c o n c e n t r a t i o n s , o w i n g t o p h a s e
s e p a r a t io n o r f o r m a t i o n o f a gg r e g a te s o f n o n -
s p h er i ca l s t ru c tu re . F i g . 1 5 (a) s h o w s th e e f fec t o f
P E O b l o c k s i z e o n t h e s e g m e n t d e n s i t y p ro f i le a n d
m i c el le si ze f or P E O P P O - P E O c o p o l y m e r s w i th
t h e s a m e P P O b l o c k [ 4 8 ] . A n in c r e a se in P E O
b l o ck s i ze re s u l t ed i n a s m a l l er m i ce l l e co re , w h i l e
t h e c o r o n a b e c a m e m o r e e x t e n d e d . T h i s i s r e f l e c t e d
i n t h e d e c r e a s i n g a g g r e g a t i o n n u m b e r f o r t h e
m i c e l l e s s h o w n i n F ig . 1 5 (b ) . T h e c o n f o r m a t i o n o f
t h e p o l y m e r w a s a f f e c t e d b y b o t h t h e t e m p e r a t u r e
a n d t h e c o m p o s i t i o n o f th e s u r r o u n d i n g s o l u t io n ;
b o t h P E O a n d P P O h a d a lo w e r fr a c ti o n o f p o l a r
s e g m e n t s i n t h e c o r e o f t h e m i c e ll e s, a n d t h e p o l a r
f r a c t io n d e c r e a s e d w i t h a n i n c r e a s e i n t e m p e r a t u r e .
T h e m i c e l l e a g g r e g a t i o n n u m b e r w a s f o u n d t o
i n c r e a s e w i t h a n i n c r e a s e in t h e s o l u t e b u l k c o n c e n -
t r a t i o n . T h i s i s i n a g r e e m e n t w i t h t h e e x p e r i m e n t a l
r e s u lt s o f A 1 - S a d e n e t a l. [ 2 9 ] , w h o f o u n d t h a t t h e
i n c r e a s e in t h e h y d r o d y n a m i c r a d i u s o f L 6 4 a g g r e -
g a t e s u p o n h e x a n e s o l u b i l i z a t i o n w a s g r e a t e r t h a n
c o u l d b e e x p l a in e d b y t h e a d d i t i on a l v o l u m e o f
th e s o l u te . I t w a s p o s tu l a ted th a t th e i n crea s e i n
h y d r o d y n a m i c r a d i u s r e s u l t e d f r o m a c o m b i n a t i o n
o f a n i n c r e a se i n th e a g g r e g a t i o n n u m b e r a n d t h e
i n c o r p o r a t i o n o f t h e s o l u t e ( h e x a n e i n t h e st u d y o f
R ef . [ 2 9 ] ) .
T h e p h a s e b e h a v i o r o f a q u e o u s s o l u t i o n s o f
b l o c k p o l y m e r s co n t a in i n g P E O a n d P P O h a s
b e e n m o d e l e d b y L i n se [ 5 1 ] o n t h e b a s is o f a
m e a n - f i e ld l a t t i c e t h e o r y ( s im i l a r t o t h a t u s e d b y
H u r t e r e t a l. [ 4 8 ] ) d e v e l o p e d f or m u l t i c o m p o n e n t
m i x t u r e s o f c o p o l y m e r s w i t h i n t e r n a l s ta t e s o c c u r -
1 0
0 8
0 6
0.4
0 2
0 0
1 0
O8
0.6
0 4
0 2
0 0
l 0
t
8 ~
0.6
O 2
0.0 a,
0 5
(a)
1 4 0 0
12U 0
11)O U
22 dO 0
b
6 0 0
4 0 0
20.0
0 0
( b )
l r I l ~ -
i l l
1.%
0 f , 10 15 20
r i
~ i i /
/
/
" ' . .
I
. . . . . . , . . .
I , , . ' I I T ' . . , . ,
5 10 15 20
i
P 1 0 4 1
t
25 30
P105
25 3u
T ~ I 4
/
L l [ . . . . . . . . . 1. . . . . . .
10 15 20 25 30
L a y e r Number
, r q i 2 0
i
A~gr~gahCrl~l S Mlcetle --
. . . . . S,ze ~ ~
o &
~ Eg
~ m
8 ~
c o
g - , o
~ i i ; 0
2 0 40 60 80 100
P E O C o n t e n t
(wt%)
Fig. 15. (a) Effect of PEO block siz e on the segm ent density
profiles in micelles form ed by PEO PPO PEO copolyme rs
having the sam e PPO block , as p redicted by a self-consistent
mean-field model. The volum e fraction of PEO in the m icelle
is represented by the dotte d lines, the v olum e fraction of PPO
by the solid lines, and the volume fraction of water by the
broken lines. An increase in PEO block size results in a smaller
micelle core, wh ile the corona becom es mo re extended. This is
reflected in the decreasing aggregation num ber for the m iceles
as shown in (b). (Reprinted with permission from Ref. [49]:
copyright Am erican Chem ical Society, 1993.}
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P . A h x and r i d i s , T . A . Ha t t on / C o l l o i d s Sur /m e .~ A . P i t v s i c oc l t em. E H ~ . A sp e c t s 96 I 1995 J 1 46
25
r i n g i n h e t e r o g e n e o u s s y s t e m s . T h e r e g i o n s f o r
u n i m e r s o l u t i o n , s p h e r i c a l m i c e l l e s , a n d e l o n g a t e d
r o d s h a v e b e e n e x a m i n e d fo r t h re e P E O - P P O -
P E O t r ib l o c k c o p o l y m e r s . A s e m i q u a n t i t a t iv e
d e s c r i p t i o n o f t h e s tr o n g t e m p e r a t u r e d e p e n d e n c e
o f t h e p h a s e b e h a v i o r w a s o b t a i n e d . A t l o w p o l y -
m e r c o n c e n t r a t i o n s a n d t e m p e r a t u r e s a u n i m e r
s o l u t i o n w a s f o u n d , w h e r e a s a t e i t h e r h i g h e r t e m -
p e r a t u r e s o r h i g h e r c o n c e n t r a t i o n s a s o l u t i o n o f
s p h e r i c a l m i c e l l e s w a s p r e s e n t . A t r a n s i t i o n f r o m
s p h e r i c a l t o i n f i n i t e l y l o n g r o d - l i k e a g g r e g a t e s
o c c u r r e d a t e v e n h i g h e r t e m p e r a t u r e s , a n d e v e n t u -
a l l y t h e s y s t e m s e p a r a t e d i n t o t w o p h a s e s . F i g . 1 6
s h o w s t h e p h a s e d i a g r a m o f P l u r o n i c s P I 0 5 , P 9 5 ,
a n d P 1 0 4 a s p r e d i c t e d b y th e m o d e l o f L i n s e [ 5 l ] .
A c o m p a r i s o n o f t h e p h a s e d i a g r a m s i n d i c a t e d t h a t
a r e d u c t i o n o f th e c o p o l y m e r m o l e c u l a r w e i g h t b y
a p p r o x im a t e l y 2 0 a t c o n s ta n t P P O / P E O c o m -
pos i t i on r a t i o ( i . e . f r om P105 to P95) l e d t o a n
i n c re a s e in C M C f o r a g iv e n t e m p e r a t u r e ,
a
s m a l l e r
m i c e l l a r r e g i o n , a n d a r e g i o n o f ro d s o f a s i m i l a r
r a n g e b u t s l i g h t l y s h i f t e d t o l o w e r t e m p e r a t u r e s .
D e c r e a s i n g t h e P E O b l o c k s iz e , w h i l e k e e p i n g t h e
P P O b l o c k c o n s t a n t ( i . e . f r o m P 1 0 5 t o P 1 0 4 1 ,
r e s u l t e d i n a s h if t o f t h e C M C c u r v e t o l o w e r
c o p o l y m e r c o n c e n t r a t i o n a t a g iv e n t e m p e r a t u r e ,
a s m a l l e r m i c e l l a r r e g i o n , a n d a s m a l l e r r e g i o n o f
r o d - l i k e a g g r e g a t e s w h i c h i s s h i f t e d c o n s i d e r a b l y
t o lo w e r te m p e r a t u r e s [ 5 1 ] . C o m p a r i s o n o f t h e
m o d e l i n g p r e d i c t io n s f o r t h e p h a s e d i a g r a m o f
L i n s e [ 5 1 ] w i t h t h e e x p e r i m e n t a l l y d e t e r m i n e d
p h a s e d i a g r a m o f M o r t e n s e n [ 6 1 ] r e v ea l s q u a l i t a -
t iv e a g r e e m e n t w i th r e s p ec t t o t h e n u m b e r o f
p h a s e s o c c u r r i n g a n d t h e i r r e l a t i v e l o c a t i o n .
H o w e v e r , t h e c a l c u l at e d C M C c u r v e o c c u r r e d a t
t e m p e r a t u r e s 2 0 3 0 K h i g h e r t h a n t h o s e e x p e r i-
m e n t a l l y o b s e r v e d , a n d t h e c a l c u l a t e d r e g i o n f o r
t h e h e x a g o n a l p h a s e o c c u r r e d a t a l o w e r t e m p e r -
a t u r e , t h u s l e a d i n g t o a t e m p e r a t u r e r a n g e f o r t h e
m i c e l l a r s o l u t i o n p h a s e w h i c h i s t o o n a r r o w .
S e g m e n t d e n s i t y p r o f i l e s i n d i c a t e d t h a t , a t a g i v e n
c o n c e n t r a t i o n , t h e r a d i a l e x t e n s i o n i n c r e a s e d
w h e r e a s t h e h e a d g r o u p a r e a d e c r e a s e d w i t h
i n c r e a s i n g t e m p e r a t u r e f o r b o t h m i c e l le s a n d r o d s .
A t t h e t r a n s i t i o n f r o m m i c e l l e s t o r o d s t h e r a d i a l
e x t e n s i o n w a s r e d u c e d a b r u p t l y b y a p p r o x i m a t e l y
10 1151
.
I n a d d i t i o n l o p h a s e b e h a v i o r . L i ns e [ 5 0 ] u s e d
3 4 0
" x :
3 2 O
l a )
[
I w o p n a s e r e g ,o , 1
r o d s
rni ~lle~
3 0 0 =
O 0 0 . 1 0 2
t ) t O
0 3
Y
t b )
34 0 ;, , i
3 2 0
i
two
p h a s e
ro j,on
\
-. rods i
m~celres i
3 0 0 ~ • : •
0 . 0 0 . 1 0 . 2
O l c
0 . 3
3 4 0
3 2 0
t w o p h a s e r e g io n
3 0 0 . . . . .
0 . 0 0 . 1
- - : = - r o d s
m l c e l l e s I
m o n o m e r s I
0 2 0 . 3
O o t
F i g . 1 6. P h a s e d i a g r a m s o f l a ) P I 0 5 , I b l P 9 5 , a n d l c) P I 0 4
P l u r o n i c P E O P P O P E O c o p o l y m e r s I p r e d i c t e d b .~ a se lf -
c o n s i s t e n t m e a n - f ie l d m o d e l m o d e l ) i n d i c a t i n g r e g io n s o f
u n i m e r s , m i c e l l e s , r o d s , a n d p h a s e s e p a r a t i o n . ( R e p r i n t e d w i t h
p e r m i s s i o n f r o m R e f . [ 5 1 ] : c o p y r i g h t A m e r i c a n ( h e r o i c a l
S o c i e t y , 1 9 9 3 . t
t h e m o d e l t o p r e d i c t m i c e l l i z a t i o n p r o p e r t i e s l i .e .
C M C , a g g r e g a ti o n n u m b e r , h y d r o d y n a m i c r a d iu s )
a n d t h e i r t e m p e r a t u r e d e p e n d e n c e f o r P l u r o n i c
c o p o l y m e r s o l u t io n s . F i g . 1 7 (a ) s h o w s t h e C M C
a n d t h e c o n c e n t r a t i o n o f t h e f re e p o l y m e r a s a f u n c -
t io n o f t e m p e r a t u r e fo r v a r io u s P E O P P O P E O
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26 P. Alexandridis , T .A. Ha t ton/Co l lo ids Sur lac es A. Phys icochem. Eng. ,4apects 96 (1995) 1 46
( a )
3 3 0
3 2 0
3 1 0
3 0 0
0 . 0
'.~ ~i. -- ' ~ - -
• \ " " " \ ' - . ~ P g 5 P 8 5
" " \ . " ~ P 1 0 4
, . ~ ~ " ~ . . ~ ° ~ , ~ ~ ~
0 , 1 0 . 2 0 , 3 0 . 4 0 . 5
3 0
20
P 1 0 3
oa
Z
/
P 1 0 5
%
P 8 5
0 . 0 0 . 2 0 . 4 0 . 6
(b)
~,n,
F i g . 1 7. (a } C a l c u l a t e d ( b y a s e l f - c o n s i s t e n t m e a n - f i e l d m o d e l
m o d e l } C M C ( ) a n d t h e c o n c e n t r a t i o n o f f re e p o l y m e r ( . )
a s a f u n c t i o n o f t e m p e r a t u r e f o r P l u r o n i c s P 8 5 , P 9 5 , P 1 0 5 ,
P 1 0 4, a n d P 1 03 . ( b ) E ff e ct o f v a r y in g t h e c o p o l y m e r P E O / P P O
c o m p o s i t i o n { a t a g i v e n t e m p e r a t u r e } o n t h e m i c e l l a r a g g r e g a -
t i o n n u m b e r • ( R e p r i n t e d w i t h p e r m i s s i o n f r o m R e f . [ 5 0 ] ;
c o p y r i g h t A m e r i c a n C h e m i c a l S o c i e ty , 1 9 93 •)
mers, and provide detailed information on the
microstructure of the micelles. Copolymers of vary-
ing blockiness and block sequence could be han-
dled, and no a priori assumptions with respect to
the core-corona interface were necessary. The
model calculations showed qualitative agreement
with experimental predictions on the effect of
temperature, concentration, hydrophobicity, and
molecular weight of the polymer; quantitative
agreement was also good, considering that all
the model input parameters were obtained from
independent experiments. The phenomenological
theory of Nagarajan and Ganesh [91], developed
originally for diblock copolymers and extended to
triblocks by Prochazka et al. [97], was also suc-
cessful in predicting qualitatively the contributions
of copolymer composition and molecular weight
to the micellization of triblock PEO PPO-PEO
copolymers (although the predicted CMCs were
unreasonably low). A sharp interface had to be
assumed, however, between the core and the
corona, and variation of the number of blocks or
block sequence was not straightforward. Further,
the effects of temperature on micellization were
not examined. An advantage of the Nagarajan and
Ganesh theory is that the various contributions to
the micellization free energy were easily quantified,
leading to a better appreciation of their magnitude
and influence on the micellization.
copolymers [50]. Both the PPO/PEO ratio and
the copolymer molecular weight influence micelli-
zation, in qualitative agreement with experimental
CMC data [28]. The effect of varying the copoly-
mer PPO/PEO composition (at a given temper-
ature) on the micellar aggregation number is
presented in Fig. 17(b) [50]. For PE O- PP O PEO
polymers with the same PPO/PEO composition,
increasing the molecular weight resulted in increas-
ing the aggregation number. Note also that the
model predicted an
' i n f i n i t e
aggregation number
for Pluronic P103, similar to the predictions of
Hurter et al. [48,49].
The self-consistent mean-field theory used by
Hurter et al. [48,49] and Linse [50,51] was able
to reproduce the anomalous phase behavior of
PEO and PPO homopolymers, predict the micelli-
zation behavior of PEO-PP O PEO block copoly-
7 . S u r face ac ti v ity o f P E O P P O c op o l ym e r s
7 .1 . A d s o r p t i o n a t t h e a i r w a t e r in t er J ~ w e
Surface tension data for various Pluronic P EO-
PPO PEO copolymer aqueous solutions have
been reported by Alexandridis et al. [ 112] at two
temperatures, 25 and 35'~C. Fig. 18 shows surface
tension values for Pluronics P105 and P85, plotted
semilogarithmically with respect to the copolymer
concentration in the solution [112]. At low con-
centrations the surface tension decreased with
increasing concentration for all copolymers, in
accordance with the Gibbs adsorption isotherm
[113]. A change in slope (break) was observed in
the surface tension curve at a characterist ic concen-
tration, after which the surface tension values
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P. A h' .vamtridis, T. A. Ha tton/C olloi & Sm:l~lces ,4. Pit rsicoclw m. /z ) l t~ . A ,v* ec ts 9 ,5 f 19 95 .J 1 4r~ 27
7 O
6 5
Z 6 0
E
5 5
5 0
4 5
~L 4
O 9
35
3O
7
6 5
Z 6
:)
g
5 5
5 )
o~ 45
" ~ 4 0
0 9
3 5
3 3
t ' ~ > 2 5 c
P1
0 5
; L
\ , \
[ - . ~ , j, ,
, , @ 5
5 . 5 " C " X " 1 .
II
. . . . . . . . r . . . . . . . . i . . . . . . . . i . . . . . . . . i . . . . . . . . i . . . . . .
I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
f - , - t . i
5 " C \ . • o . . o ]
~ : ~
A . ' e ' o
• • o . •
" A " • ¥
I ' " A Y A , t 1 . . . l . . z T l ~ l
/
. . . . . . . . i . . . . . . . . t . . . . . . . . i . . . . . . . . i . . . . . . . . i . . , , , , I
~ 0 0 0 0 1 0 . 1 1 0
C o n c e n t ra tio n % w / v )
f -i g. I S. S u r fa c e t e n s i o n f o r P l u r o n i c P 1 0 5 a n d P 8 5 a q u e o u s
s o l u t i o n s , p l o t t ed a s a f u n c t i o n o f c o p o l y m e r c o n c e n t r a t i o n a t
2 5 a n d 3 5 C . ( A d a p t e d w i t h p e r m i s s i o n f r o m R e f . [ l l 2 ] :
c o p y r i g h t A m e r i c a n C h e m i c a l S o c i e t y , 1 99 4 .)
c o n t i n u e d t o d e c r e a s e u n t i l a p l a t e a u w a s r e a c h e d
( s e c o n d b r e a k ) : t h e s u r f a c e t e n s i o n v a l u e s r e m a i n e d
c o n s t a n t w i th f u r t h e r i n c r e a se in c o p o l y m e r c o n -
c e n t r a t i o n [ 1 1 2 ] . T h i s b e h a v i o r (t w o b r e a k s ) w a s
o b s e r v e d f o r m o s t c o p o l y m e r s st u d i e d a n d w a s
v e r y r e p r o d u c i b l e . T h e f i r s t ( l o w - c o n c e n t r a t i o n l
b r e a k o c c u r r e d a t a c o p o l y m e r c o n c e n t r a t io n o f
a p p r o x i m a t e l y 0 .0 0 1% , w h i c h w a s r o u g h l y t h e
s a m e f o r b o t h t e m p e r a t u r e s s t u d ie d a n d f o r a
n u m b e r o f c o p o l y m e r s . T h e s u r f a c e t e n s io n v a l u e
a t t h e o n s e t o f t h i s b r e a k v a r i e d f r o m 4 0 t o 5 0 m N
m ~. T h e p r e s e n c e o f t w o b r e a k s h a s l e d t o s o m e
c o n f u s i o n i n t h e l i t e r a t u r e o n P E O P P O P E O
c o p o l y m e r s r e g a r d i n g t h e i n t e r p r e t a t i o n o f t h e
s u r f a c e t e n s i o n d e p e n d e n c e o n c o n c e n t r a t i o n , a n d
t h e e x t r a c t io n o f t h e C M C f r o m s u c h a s e t o f d a t a
[ 3 0 . 1 1 4 , 1 1 5 ] . I n m o s t c a s e s i t h a s b e e n a s s u m e d
t h a t t h e f ir s t c h a n g e i n s l o p e s ig n i f ie d t h e C M C
[ 1 1 5 , 1 1 6 ] , a l t h o u g h o t h e r i n t e r p r e t a t i o n s , s u ch a s
f o r m a t i o n o f ' u n im o l e c u l a r' m i c e ll e s [ 1 1 4 ] , h a v e
b e e n g i v en . r h e r e g i o n o f d e c r e a s i n g s u r f a c e t e n -
s i o n t h a t f o l l o w s t h e b r e a k a t 0 . 0 0 1 % , a n d t h e
s e c o n d b r e a k h a v e o f t e n b e e n i g n o r e d [ 1 1 4 , 1 1 5 ] .
b u t i t h a s b e e n s u g g e s t e d t h a t t h e y w e r e d u e t o
b r o a d c o p o l y m e r m o l e c u l a r w e ig h t d i s t r ib u t i o n
a n d t o t h e p r e s e n c e o f i m p u r i t ie s [ 3 0 t .
G o o d a g r e e m e n t b e t w e e n t h e C M C s d e t e r m i n e d
f r o m d y e s o l u b il i z a ti o n [ 2 8 ] a n d t h e c o n c e n t r a t i o n
a t t h e s e c o n d b r e a k o f t h e s u r f a c e t e n s i o n c u r v e
h a s b e e n s h o w n [ 1 1 2 ] , l e a d i n g t o t h e c o n c l u s i o n
t h a t th e h i g h e r - c o n c e n t r a t i o n b r e a k c o r r e s p o n d s
t o th e C M C a n d s i g ni fi es th e f o r m a t i o n o f p o l y m o -
l e c u l a r m i c e ll e s w i t h a w e l l - d e f in e d h y d r o p h o b i c
i n t e r i o r ( i . e . a b l e t o s o l u b i l i z e o r g a n i c s o l u t e s ) . I t
s h o u l d b e n o t e d t h a t th e c o p o l y m e r c o n c e n t r a t i o n s
a t t h e s e c o n d b r e a k , e x t r a c t e d f r o m t h e s u r f a c e
t e n s i o n d a t a o f W a n k a e t al . [ 3 0 ] ( 0 ,3 % f o r F 1 2 7 .
a n d 0 .0 1 e2 f o r P1 0 4 , b o t h a t 25 C) a n d P r a s a d
e t a l. [ 1 1 4 ] . a r e al s o c o m p a r a b l e w i t h t he C M C
v a l u e s f o u n d i n t h e d y e s o l u b i l i z a t i o n s t u d y o f
A l e x a n d r i d i s e t a l . [ 2 8 ] . T w o o t h e r o b s e r v a t i o n s
s u p p o r t i n g t h e c o n c l u s i o n t h a t : h e h i g h e r -
c o n c e n t r a t i o n b r e a k i s t h e C M C a r e (i ) t h e p l a te a u
o b s e r v e d i n t h e s u r f a c e t e n s i o n v a l u e s a f t e r t h e
h i g h - c o n c e n t r a t i o n b r e a k , a n d l i ll t h e e f fe c t o f
t e m p e r a t u r e o n t h e c o p o b m e r c o n c e n t r a t i o n a t
w h i c h t h e s e c o n d b r e a k o c c u r s. A t t a i n m e n t o f a
c o n s t a n t s u r f a c e t e n s i o n v a l u e i s a n i n d i c a t i o n o f
m i c e ll e f o r m a t i o n ( C M C ) in s o l u t io n s o f t y p ic a l
s u r f a c t a n t s 1 7 1 1 3] . W h i l e t h e s u r f a c e t e n s i o n o f
P l u r o n i c c o p o l y m e r s o l u t i o n s k ee p,,, d e c r e a s i n g
w i t h i n c r e a si n g c o p o l y m e r c o n c e n t r a t i o n a f te r t h e
f i r s t l o w - c o n c e n t r a t i o n b r e a k , i t r e m a i n s c o n s t a n t
a f t e r t h e s e c o n d b r e a k ( a d i p i n t h e s u r f a c e t e n s i o n
a f t e r t h e b r e a k o b s e r v e d f o r P l u r o n i c P 8 5 is m o s t
l i k e l y d u e t o i m p u r i t i e s [ 2 6 ,4 0 , 11 7 1 1 . R e g a r d i n g
t h e e ff ec t o l t e m p e r a t u r e , t h e c o p o l y m e r c o n c e n -
t r a t i o n a t w h i c h t h e s e c o n d c h a n g e i n s l o p e o c c u r s
d e c r e a s e d s i g n i f i c a n t l y w i t h i n c r e a s i n g t e m p e r -
a t u r e , i n g o o d a g r e e m e n t w i t h t h e C M C s o b t a i n e d
f r o m d y e s o lu b i l iz a t i o n [ 2 8 ] a n d o b s e r v a t i o n s i n
o t h e r s t u d i e s o f P l u r o n i c c o p o l y m e r s [ 2 7 , 6 4 ] ,
w h e r e a s t h e c o n c e n t r a t i o n a t t h e l i r s t b r e a k d i d
n o t w t r y s i g n i f i c a n t l y [ 1 1 2 ] .
H a v i n g a t t r i b u t e d t h e h i g h - c o n c e n t r a t i o n b r e a k
{ c h a n g e i n t h e s l o p e ) o f t h e s u r f a c e t e n s i o n c u r v e
t o t h e f o r m a t i o n o f m i c e ll e s in t h e b u l k I C M C ) ,
w e e x a m i n e t h e r e g i o n b e l o w t h e o r i g in o f t h e f ir s t
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28 P. Alexandridis, T.A . Hatton/Colloids Surjaces A: Physicochem. En g. Aspects 96 1995) 1 46
( l o w - c o n c e n t r a t i o n ) b r e a k . T h e w i d e m o l e c u l a r
w e i g h t d i s t r ib u t i o n o f t h e c o p o l y m e r s s tu d i e d , a n d
t h e p r e s e n c e o f h y d r o p h o b i c i m p u r i ti e s , h a v e b e e n
s i n g l e d o u t i n t h e p a s t a s p o s s i b l e f a c t o r s c a u s i n g
d i ff e re n c e s i n t h e C M C v a l u e s o b t a i n e d b y s u r f a c e
t e n s i o n a n d d y e s o l u b i l i za t i o n m e t h o d s . T h e e f f ec t
o f m o l e c u l a r w e i g h t d i s t r ib u t i o n o n t h e s u r fa c e
t e n s i o n o f n o n - i o n i c s u r f a c t a n t s h a s b e e n n i c e l y
d e m o n s t r a t e d b y C r o o k e t al. [ ,1 1 8 ] , w h o s t u d i ed
t h e s u r fa c e t e n s io n o f n o r m a l ( P o i s s o n ) d i s t r i b u t i o n
a n d s i n g l e s p e c i e s o c t y l p h e n o x y - e t h o x y e t h a n o l s
( O P E l l o, w h e r e 1 - 1 0 i s t h e n u m b e r o f e t h y l e n e
o x i d e s e g m e n t s ) , a n d m i x t u r e s o f s i n g le s p e c ie s
a n d n o r m a l d i s tr i b u ti o n O P E 4 a n d O P E l o . T h e
a u t h o r s f o u n d p r e f e re n t i a l a d s o r p t i o n o f m o l e c u l e s
o f s h o r t e r P E O - c h a i n l e n g t h (O P E 3 _ 5) at t h e a i r
w a t e r i n t e r f a c e ; t h i s w a s d e m o n s t r a t e d b y a l e s s
s h a r p ( b u t s t i l l , o n l y s i n g l e l b r e a k i n t h e s u r f a c e
t e n s i o n c u r v e , a n d a d i p i n t h e s u r f a c e t e n s i o n
v a l u e s i n t h e v i c in i t y o f th e b r e a k . S u c h b e h a v i o r
w a s a t t r i b u t e d t o t h e r e p l a c e m e n t o f h i g h l y s u r fa c e
a c ti v e , s h o r t P E O - c h a i n - l e n g t h m o l e c u l e s ( c o n t r ib -
u t i n g t o a l o w s u r f a c e t e n s i o n v a l u e ) o n t h e i n t e r -
f a ce b y t h e m a j o r c o m p o n e n t , w h e n t h e se h i g h ly
s u r f a c e a c t i v e m o l e c u l e s w e r e s o l u b i l i z e d i n m i c e l l e s
f o r m e d b y t h e m a j o r c o m p o n e n t . N o m a n i f e s t a -
t i o n s o f th e a b o v e w e r e s e e n i n t h e s u r f a c e t e n s i o n
c u r v e s f o r P l u r o n i c s [ 1 1 2 ] . A s f o r t h e i n f lu e n c e o f
i m p u r i t i e s , a d e c r e a s e i n s u r f a c e t e n s i o n w i t h t i m e ,
o b s e r v e d i n s o m e e x p e r i m e n t s a t l o w c o p o l y m e r
c o n c e n t r a t i o n s , w o u l d i n d i c a t e t h e p r e s e n c e o f
h y d r o p h o b i c i m p u r i t ie s t h a t a d s o r b s l ow l y o n t h e
s u r f a c e [, 1 1 7 ] . H o w e v e r , t h e p r e s e n c e o f s u c h i m p u -
r i ti e s s h o u l d r e s u l t i n a l o w e r i n g o f t h e s u r f a c e
t e n s i o n ( d e s c r i b e d a b o v e a s a d i p ) b e l o w t h e
s t e a d y v a l u e o b t a i n e d a f te r t h e C M C . S u c h a d i p
w a s n o t o b s e r v e d i n t h e v i c i n it y o f t h e l o w -
c o n c e n t r a t i o n b r e a k i n t h e s u r f a c e t e n s i o n c u r v e ,
b u t c a n b e s e e n in s o m e o f o u r e x p e r i m e n t s ( e.g .
P l u r o n i c P 8 5 ) a t t h e h i g h - c o n c e n t r a t i o n b r e a k . I t
c a n t h u s b e c o n c l u d e d t h a t n e i t h e r p o l y d i s p e r s i t y
o f th e c o p o l y m e r s iz e n o r h y d r o p h o b i c i m p u r i ti e s
c a u s e t h e l o w - c o n c e n t r a t i o n b r e a k o b s e r v e d i n
t h e su r fa c e t e n si o n c u rv e s o f P E O P P O - P E O
s o l u t i o n s .
I t h as b e e n p r o p o s e d t h a t t h e lo w - c o n c e n t r a ti o n
b r e a k i n t h e s u r fa c e t e n s i o n v s. c o p o l y m e r c o n c e n -
t r a t i o n c u r v e s i s d u e t o a c h a n g e i n c o n f i g u r a t i o n
( s t ru c t u r a l t r a n s i t io n ) o f th e c o p o l y m e r m o l e c u l e s
a t t h e a i r - w a t e r i n t e r f a c e [ , 1 1 2 ] . C o n s t a n t s u r f a c e
c o v e r a g e h a s b e e n a t t a i n e d a t b u l k c o p o l y m e r
c o n c e n t r a t i o n s r a n g i n g f r o m 10 -6 to 10 3 ,
w i th t he P E O - P P O - P E O c o p o l y m e r m o l ec u le s
a d s o r b e d a t t h e i n t e r f a c e p o s s i b l y a s a n i n v e r t e d
U ' ( t h is is o n e o f t h e p r o p o s e d o r i e n t a t i o n s f o r
h o m o p o l y m e r P g O a t t h e i n te r fa c e [, 1 1 9 ] ) a n d t h e
P E O c h a i n s l o c a t e d a t t h e a i ~ w a t e r i n t e r f a c e . A t
a b u l k c o n c e n t r a t i o n o f a p p r o x i m a t e l y 1 0 3 % , a
s t r u c t u r a l t r a n s i t i o n o c c u r s a n d t h e c o p o l y m e r
l a y e r b e c o m e s m o r e c o m p a c t ; w a t e r i s e x p e l l e d
[ 1 2 0 ] a n d P E O s e g m e n t s p r o t r u d e i n t o t h e a q u e -
o u s s o l u ti o n [ , 1 21 ] o r f o ld a r o u n d P P O . M o r e
c o p o l y m e r m o l e c u l e s c a n f it a t t h e i n t e r f a c e (c a u s -
i n g i t t o b e c o m e t h i c k e r [ , 1 2 2 ] ) a n d t h e s u r f a c e
t e n s i o n c o n t i n u e s t o d e c r e a s e w i t h i n c r e a s i n g b u l k
c o p o l y m e r c o n c e n t r a t i o n , b u t a t a s l o w e r r a t e (n o t e
a l so t h a t a b u l k c o n c e n t r a t i o n o f 10 3 % is c o m p a -
r a b l e t o t h e b u l k P E O c o n c e n t r a t i o n r e q u i r e d f o r
f ul l c o v e r a g e o f th e a i r - w a t e r i n t e rf a c e a c c o r d i n g
t o G l a s s [ 1 2 3 ] a n d t h e b u l k P P O c o n c e n t r a t i o n
a t w h i c h p h a s e s e p a r a t i o n o c c u r s [ 1 2 4 ] ) . O n f o r -
m a t i o n o f m i c e ll e s i n t h e b u l k , a f u r t h e r i n c r e a s e
i n th e n u m b e r o f c o p o l y m e r m o l e c u le s in t h e b u l k
i s a c c o m m o d a t e d b y a n i n c r e a s e i n t h e n u m b e r o f
m i c e ll e s, t h e a c t i v i t y o f t h e c o p o l y m e r i n t h e b u l k
r e m a i n s a p p r o x i m a t e l y c o n s t a n t , a n d t h e s u r f ac e
t e n s i o n a t t a in s a s t e a d y v a l u e t h a t d o e s n o t c h a n g e
w i t h f u r t h e r i n c r e a s e i n t h e b u l k c o p o l y m e r
c o n c e n t r a t i o n .
T h e v a l u e s fo r s u rf a c e a re a p e r c o p o l y m e r m o l e -
c u le , c a l c u l a te d t h r o u g h t h e G i b b s a d s o r p t i o n i so -
t h e r m [ 1 1 2 , 1 1 4 ] , a r e g e n e r a l l y s m a l l c o m p a r e d t o
t h o s e o f n o n - i o n i c s u r f a c ta n t s w i t h a n a l ip h a t i c
c h a in a n d P E O h e a d g r o u p ( o f c o m p a r a b l e s iz e t o
t h e P E O b l o c k o f P l u r o n i c s ) [ 1 2 5 , 1 2 6 ] , i n d i c a t in g
t h a t t h e r e is c o n s i d e r a b l e f o l d i n g o f th e p o l y m e r s
a t t h e a i r - w a t e r i n t e rf a c e [ 1 1 4 ] a n d / o r d e s o r p t i o n
o f P E O s e g m e n t s in t h e w a t e r p h a s e [ 1 2 7 , 1 2 8 ] .
T h e a r e a p e r c o p o l y m e r m o l e c u l e i n c r ea s e d w i t h
t h e n u m b e r o f E O s e g m e n t s i n t h e m o l e c u l e f o r a
f am i ly o f P l u r o n i c c o p o l y m e r s w i th t h e s a m e P P O
a n d v a r y i n g P E O b l o c k l e n g t h , a s se e n i n F i g. 19
[ - 1 1 2 ] . A l s o s h o w n o n t h e s a m e p l o t a r e a r e a s p e r
m o l e c u l e f o r a s e ri es o f o c t y l p h e n o x y - e t h o x y e t h a -
n o l s u r f a c t a n t s ( a r e a v a l u e s w e r e e x t r a c t e d f r o m
C r o o k e t a l . [ 1 1 8 1 1 . A n i n c r e a s e i n t h e l e n g t h o f
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P. A l e x a n d r i d i s T . 4 . Ha t t o n / C o l l o i d s S u t J a c e s A : P i t v s i c o c h e m . EH ~ . A v w c t s 9 6 i l ?95 , 1 4 t ~ 29
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a s a f u n c t io n o f t h e n u m b e r o f E O s e g m e n t s . A l s o s h o w n o n
t h e s a m e p l o t a r e a r e a s p e r m o l e c u l e f o r a s e r i e s o f
o c t y l p h e n o x y ~ t h o x y e t h a n o l s u r f a c t a n t s ( a r ea v a l u e s w e r e
e x t r a c t e d f ro m R e f. [ l l 8 ] k I R e p r i n t e d w i t h p e r m i s s i o n f ro m
R c f. [ 1 1 2 ] : c o p y r i g h t A m e r i c a n C h e m i c a l S o c i e ty . 1 9 9 4 3
t h e h y d r o p h o b i c ( P P O ) b l oc k , f o r a g r o u p o f
c o p o l y m e r s t h a t h a v e t h e s a m e P E O b l o c k s i z e ,
r e s u l t e d i n a d e c r e a s e i n t h e a r e a t h a t e a c h m o l e c u l e
o c c u p i e s a t t h e a i r w a t e r i n t e r f a c e , i n a g r e e m e n t
w i t h P r a s a d e t a l . [ 1 1 4 ] . T h i s f i n d i n g s u g g e s t s a
m o r e c o m p a c t i n te r f a ci a l l a y e r f o r l a r g e r P P O
s e g m e n t s i z e , w i t h t h e c o p o l y m e r m o l e c u l e s o r i -
e n t e d o n t h e s u r f a c e i n a c o i l e d m a n n e r h a v i n g
P P O o u t o f t h e a q u e o u s p h a s e , a n d t h e h y d r o p h i l ic
P E O s e g m e n t s at t h e e x t r e m i t ie s o f t h e c o p o l y m e r
p a r t l y a n c h o r i n g t h e p o l y m e r in t h e a q u e o u s p h a s e
a n d p a r t l y f o l d in g a r o u n d P P O .
T h e a r e a s p e r m o l e c u l e r e p o r t e d b y Y e a t e s e t a l .
[ 1 2 9 ] w e r e h ig h e r f o r C I E s C i t h a n f o r C i E 1 5 C i
( C = m e t h y l g r o u p , i = 5 12, E = e t h y l e n e o x id e
g r o u p ) , i n d i c a t i n g a g a i n t h a t t h e h y d r o p h o b i c
m o i e t y m a k e s t h e i n te r r ac i a l la y e r m o r e c o m p a c t .
P h i p p s e t a l. [ 1 2 1 ] s t u d ie d t h e c o n f o r m a t i o n o f
P l u r o n i c F 1 2 7 a t t h e h e x a n e - w a t e r i n t e r f a c e
e m p l o y i n g a n e u t r o n r e f l e c ti v i ty t e c h n i q u e . U s e o f
h e x a n e w a s n e c e s s a r y t o a c h i e v e c o n t r a s t v a r i a t io n ;
h o w e v e r , t h e a u t h o r s r e p o r t i n d i c a t i o n s t h a t t h e
p r e s e n c e o f h e x a n e h a d l it tl e e f fe c t o n e i t h e r t h e
a d s o r b e d a m o u n t o r th e c o n f o r m a t i o n o f t he c o p o l -
y m e r o n t h e w a t e r s i de o f th e i n t e r f a c e : in t h i s
r e s p e c t , t h e i r f i n d i n g s s h o u l d a l s o b e a p p l i c a b l e
t o c o p o l y m e r s a d s o r b e d a t a i r w a t e r i n t e rf a c es .
P l u r o n i c 17 12 7 w a s f o u n d t o a d o p t a c o n f o r m a t i o n
t h a t e x t e n d e d b e y o n d t h e m i c e l l a r r a d i u s i n w a t e r
a n d , t h u s , a p p e a r e d q u i t e s t r e t c h e d . T h e e s t i m a t e d
( u n p e r t u r b e d ) r a d i u s o f g y r a t i o n o f a P E O b l o c k
is 2.0 n m a n d t h a t o f t h e P P O b l o c k 1 .6 n m : t h e
v o l u m e f r a c t i o n p r o f i l e o f 17 12 7 a t t h e i n t e r f a c e
{ f i t t e d f r o m t h e n e u t r o n s c a t t e r i n g d a t a ) s h o w e d
t h e P E O s e g m e n t s t o p r o t r u d e 9 n m i n to t h e w a t e r
s u b p h a s e a n d P P O t o e x t e n d 4 n m i n to h e x a n e
[ 1 2 1 ] . O n t h e co n t r a r y , a r a n d o m c o p o l y m e r of
p o l y ( v i n y l a l c o h o l - c o - a c e t a t e ) w a s f o u n d t o a d o p t
a v e r y f i a t c o n f o r m a t i o n a t t h e in t e r f a c e , f o r m i n g
a d e n s e l a y e r o f a p p r o x i m a t e l y 2 n m t h ic k n e s s. T h e
e x p e r i n a e n t a l f i n d i n g s r e g a r d i n g t h e c o n f o r m a t i o n
o f t h e P l u r o n i c c o p o l y m e r a t ti le i n t e r fa c e w e r e
b o r n e o u t b y a S c h e u t j e n s F l e e r s im u l a t i o n ( s im -
i l a r t o t h e o n e u s e d f o r m o d e l i n g b l o c k c o p o l y m e r
m i c e ll e s [4 8 51 ] ) t h a t s h o w e d t h e c o p o l y m e r
a t t a c h e d t o t h e i n te r f a ce b y t h e h y d r o p h o b i c P P O
b l o c k a n d t h e P E O b l o c k s h a v i n g a - p a r a b o l i c
s e g m e n t d e n s i t y p r o f i le , s i m i l a r t o a p o l y m e r c h a i n
g r a f t e d t i t a s o l i d s u r f a c e . Bo t h t h e p r o t r u s i o n o f
P E O i n t h e w a t e r s u b p h a s e a n d t h e r e s e m b l a n c e
o f P E O t o a g r a f t e d c h a i n a g r e e w i t h t h e f i n d i n g s
o f th e s u r f a c e t e n s i o n s t u d y o f A l e x a n d r i d i s e t a l.
[ l i e ]
7 .2 . A d s o r p t i o n a t s o l i d w a t e r i n t er l i tc e s
T h e a d s o r p t i o n o f s e v e ra l P l u r o n i c P E O
P P O P E O c o p o l y m e r s (L 6 1 , L 6 2 , L 6 4 , 1 -'3 8. F 6 8 ,
F 8 8 , a n d F I 0 8 ) o n t o p o l y s t y r e n e l a t e x p a r t i c l e s
w a s d e s cr i b e d b y K a y e s a n d R a w l i n s [ 1 3 0 ] .
L a n g m u r i a n i s o t h e r m s w e r e o b t a i n e d a n d , in g e n e -
r a l, m a x i m u m a d s o r p t i o n o c c u r r e d a f t e r t h e m e a -
s u re d a p p a r e n t C M C . T h e a r e a s o c c u p ie d b y a
c o p o l y m e r m o l e c u l e a t t h e i n t e r fa c e in t h e p l a t e a u
r e g i o n o f t he a d s o r p t i o n i s o t h e r m w e r e f o u n d t o
b e g r e a t e r t h a n t h o s e o b s e r v e d a t t h e a i r w a t e r
i n t e r f a c e [ 1 1 2 , 1 1 4 ] , a n d c o n s i d e r a b l y l es s t h a n
t h o s e a t a q u a r t z w a t e r i n t e r f a c e { r e p o r t e d i n a n
e a r l y s t u d y b y H e y d e g g e r a n d D u n i n g [ 1 3 1 ] ) ,
i n d i c at i n g t h e i m p o r t a n c e o f th e n a l u r e o f t h e
i n t e r f a c e o n a d s o r p t i o n . T h e a r e a - p e r - m o l e c u l e
vah tes m ea su re d we re 2 .85 , 3 .20 , 5 .90 , 6.51 , 15 .10 ,
1 7 .5 2 , a n d 2 4 . 2 6 n m 2 f o r P l u r o n i c s L 6 1 , L 6 2 , L 6 4 .
F 3 8 , F 6 8 , F 8 8 , a n d F 1 0 8, r e s p e c t i v e ly . E x a m i n a t i o n
o f t h e m o l e c u l a r a r e a s a t t h e i n t e r D c e s h o w e d t h a t
t h e a d s o r b e d P P O b l o c k s f o r m e d s l n al l l o o p s o r
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30 P. Alexandridis . T .A. H at ton/C ol lo ids Sur laces A. Phys icochem. Eng. Aspe cts 96 (1995) 1 46
were tightly coiled on the surface; this was con-
firmed by determination of the adsorbed layer
thickness from light scattering and electrophoresis
experiments [- 130].
Baker and Berg [132] investigated the adsorp-
tion configuration of PEO and its copolymers with
PPO (Pluronic triblock copolymers P75, P85, F68,
F98, F108, and random PEO-PPO copolymers
50-HB-260, 50-HB-2000, 50-HB-5100} on model
polystyrene latex dispersions. Adsorption iso-
therms and adsorbed layer thickness (determined
from dynamic light scattering) for a number of
Pluronics on latex particles are presented in Fig. 20
[132]. The adsorption isotherms exhibited the
expected behavior, both adlayer thickness and
1 6
0 t 4
x
1 2
v 1 0
c-
O
~ . 8
o 6
t~
4
t--, 2
0 3
( a }
1 6
1 4
c
1 2
> . ,
6
...
. 0
4
o
" o 2
. <
0
( b )
_ 4 3 - . . . . . . . . . . 0
~X
Z , . . . - / ~ - % - / - - / / / -
• - X - - . . . . . -X
. , , ; F 1 8
, ~ " , - - ~ - F 9 8
[ ; 4 , F 6 8
- - ', P 8 5
c
- -~ - P 7 5
I I I I i
0 1 2 3 4 5
E q u i l l i b r iu m p o l y m e r c o n c e n t r a t i o n ( g / m L x 1 0 4 )
i 1 i i i
. . . . o
_ A - - - - -o - F 1 0 8
o - o- -£-- . . . . - -~ - F98
o F68
. . . . • ~ P 8 5
'2 . 8 - - - - - o - . P 7 5
/ @ - o "
(3 is
e . . . . ; 2 ~
I l
0 1 2 3 4 5
E q u i t l ib r i u m p o l y m e r c o n c e n t r a t i o n ( g / m L x 1 0 4 )
Fig. 20. (al Adsorp tion isotherms and (b) ad sorbed layer
thickness (determined from dynamic light scattering) for a
numb er of Pluronic copolymers on latex particles: 3, P75;
X, P85; O, F68: A, F98; ) , FI08. (Adapt ed with permi ssion
from Ref. [ 132]; copyright American Chemical Society, 1988.)
specific adsorption increasing with the PEO block
size and the bulk polymer concentration. The
thickness isotherms have essentially the same shape
as conventional high-affinity specific adsorption
isotherms, rising rapidly at low concentrations and
leveling off at bulk polymer concentrations corre-
sponding to the plateau adsorption coverage. The
specific adsorption values for Pluronics reach their
plateau values at bulk copolymer concentrations
slightly higher than the copolymer CMCs. In
addition, the concentration corresponding to the
adsorption plateau is well above the concentration
required to create a monolayer of polymer seg-
ments at the particle surface (assuming all segments
are in contact with the surface). These data suggest
that the polymers adsorb in the extended configu-
ration, with thin PPO loops and trains attached
to the surface and longer PEO tails extending
away from the surface [ 132]. Berg and co-workers
[133] also studied the effect of particle radius on
the hydrodynamic thickness of adsorbed PEO
homopolymers and PEO PPO PEO copolymers.
In the latter case, the adlayer thickness increased
with particle radius, but the dependence was more
complex than that of PEO adsorption.
The effect of surface modification of polystyrene
latex particles on subsequent protein adsorption
was studied by gee et al. [134]; the surface modi-
fiers used were Pluronic PEO PPO-PEO block
copolymers. The hydrodynamic thickness 5h of the
adsorbed (on the particles) copolymers is plotted
against the number of EO segments in Fig. 21
[134]. Shown for comparison as curves A and B
are the dimensions for a rod-like chain and a coil-
like chain. The scaling exponent of the fit to the
experimental points suggested a coil-like chain
conformation for the PEO blocks of the adsorbed
copolymers. Fig. 21 shows that the dimensions of
the adsorbed Pluronic copolymers are smaller than
those of the extended rod-like chain (curve A), but
larger than those of the adsorbed PEO homo-
polymer (curve B). Those 6h values are also larger
than the corresponding free surfactant unimer
dimensions, suggesting that the picture of qoops
and trains for the hydrophobic PPO block and
'loops and tails for the hydrophilic PEO blocks
is a suitable description for the adsorbed copoly-
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P . A l c x a n d r i d is , T . A . H a n o n , C o l l o i d s S u r l u c e s A: P hrsic<,chem. Er ie. ,4.V~ect.v96 r 1995,, 1 4r, 31
1 0 0 0
<.%
03 1 0 0
o
~ . 1 0 '
R
T
1
/
/
A (rod-like chain, D- a ~ o)/ /
h
/
• • Exp. data
/ • (Pluronics /
..... , / PS O.08B m)
. /
. /
• . / ' i
..~ q
B(adsorbed PEO coil, o~ J
- a ) j
,
. . . . .
L , , , , , , , d , ,
I
10 100 1000
a ( n u m b e r o f E O s e g m e n t s )
F i g . 2 1. H y d r o d y n a m i c t h i c k n e s s ' h o f a d s o r b e d P h t r o n i c
P E O P P O P E O c o p o l y m e r s p lo t t ed a g a i n st t h e n u m b e r o f
E ( ) s e g t n e n t s . C u r v e A i s t h e e x t e n d e d c h a i n d i i n e n s i o n
a c c ord ing to D i , a I3 .7 ~ i ) . Cu rv e B re p re se nt s ~ t~ vs . de g re e
o f p o l y m e r i z a t io n l \) r a d s o r b e d P E O h o m o p o l y m e r . [ A d a p t e d
w i t h p e r m i s s i o n f r o m R e f. [ - 1 3 4 ]: c o p y r i g h t A c a d e m i c P r e s s .
1989.1
m e r s [ 1 3 4 ], i n a g r e e m e n t w i th t h e s t u d y o f B a k e r
a n d B e r g [ 1 3 2 ] .
K i l l m a n n e t a l. [ 1 3 5 ] r e p o r t e d o n t h e h y d r o d y -
n a m i c l a y e r t h ic k n e ss o f P E O h o m o p o l y m e r a n d
P E O P P O P E O c o p o l y m e r s a d s o r b e d o n p o ly -
s t y r e n e l a t e x a n d p r e c i p i t a t e d s i l i c a p a r t i c l e s . T h e
c o n f o r m a t i o n o f t he a d s o r b e d p o l y m e r w a s d e t e r-
m i n e d b y t h e s u rf a c e p o l y m e r i nt e r a c ti o n s : h y d r o -
p h o b i c i n t e r a c t i o n s e x i s t o n t h e a p o l a r l a t e x
s u r fa c e , w h e r e a s h y d r o g e n b o n d s d o m i n a t e o n t h e
p o l a r s ili ca s u rf a ce . P E O P P O P E O c o p o l y m e r s
a d h e r e d t o t h e la t ex v ia t h e h y d r o p h o b i c P P O
s e g m e n t s : t h e P E O b l o c k s e x t e n d e d i n t o t h e s o l u -
t i o n a s ta il s, a n d t h e l a y e r t h i c k n e s s d e p e n d e d o n
t h e m o l a r m a s s o f t h e P E O b l oc k s. T h e a d s o r p t i o n
m e c h a n i sm o f P E O P P O - P E O c o p o ly m e r s o n
s il ic a r e s e m b l e d th a t o f P E O h o m o p o l y m e r : t h e
a d l a y e r t h ic k n e s s e s o f t h e b l o c k c o p o l y m e r s w e r e
v e r y s m a l l a n d c o m p a r a b l e t o t h e t h i c k n e s s o f
P E O a d l a y e r s o n s i l i c a [ 1 3 5 ] .
T h e a d so r p t i o n o f P E O P P O - P E O b lo c k
c o p o l y m e r s o n s i l i c a , a s w e l l a s t h e a d s o r p t i o n o f
P E O h o m o p o l y m e r , h a s b e e n st u di e d b y T i b e r g
e t a l. [ 1 3 6 ] a n d M a l m s t e n e t a l . [ 1 3 7 ] . F o r a
n u m b e r o f p o l y m e r s w it h a to t a l m o l e c u l a r w e i g h t
o f a p p r o x i m a t e l y 1 5 00 0 , it w a s f o u n d t h a t t h e
a d s o r b e d a m o u n t is r a t h e r l o w ( 0 .3 5 0 . 40 m g m e )
a n d i n d e p e n d e n t o f t h e P P O c o n t e n t i n t h e r a n g e
0 3 0 % P P O . F o r a c o p o l y m e r w i th a t o t a l m o l e c u -
l a r w e i g h t o f 4 0 0 0 a n d 5 0 % P P O c o n t e n t , t h e
a d s o r b e d a m o u n t w a s a p p r o x i m a t e l y 0 .2 0 m g m e.
T h i n a d s o r b e d l a ye r s , w i t h h y d r o d y n a m i c t h ic k -
n e ss e s o f a p p r o x i m a t e l y 2 5 a m , w e r e f o r m e d b y
a l l p o l y m e r s i n v e s t i g a t e d . T h e p H d e p e n d e n c e o f
t he a d s o r b e d a m o u n t a n d t h e h y d r o d y n a m i c t h ic k -
n e s s w e r e s i m i h t r t o t h a t d i s p l a y e d b v P E O h o m o -
p o l y m e r s . E l l i p s o m e t r y e x p e r i m e n t s p r o v i d e d
i n f o r m a t i o n o n b o t h a d s o r p t i o n a n d d e s o r p t i o n
k i n e t i c s , b o t h o f w h i c h a r c f a s t p r o c e s s e s i n t h e s e
s y s t e m s a n d o c c u r o % rer a n u m b e r o f m i n u t e s .
F i n a ll y , e l l ip s o m e t r y e x p e r i m e n t s s h o w e d t h a i a n
a b r u p t i n c r e a s e i n t h e a d s o r b e d a m o u n t o c c u r r e d
p r i o r t o m i c e l l e f o r m a t i o n i n t i l e s o l u t i o n . A t
t e m p e r a t u r e s a b o v e t he C M T . t he a d s o r b e d
a m o u n t r e m a i n e d i n d e p e n d e n t o f t e m p e r a t u r e ( in
t h e t e m p e r a t u r e r a n g e s t u d i e d 1. T h e h y d r o d y n a n a i c
t h i c k n e s s w a s l n t l c h s m a l l e r t h a n t h e h y d r o d y -
n a m i c d i a m e t e r o f t h e s o l u t i o n m i c e l le s f o r a ll
t e m p e r a t u r e s [ 1 3 7 ] . N o a b r u p t i n c re a s e in t h e
a d s o r b e d a m o u n t w a s o b s e r v ed o n h y d r o p h o b i c
s u r f a c e s, a l t h o u g h a s t r o n g i n c r e a s e i ll t h e a d s o r b e d
a m o u n t w a s o b s e r v e d a s t i l e s y s t e m a p p r o a c h e d
t h e p h a s e b o u n d a r y . T h e q u a l i t a t i v e a s p e c t s o f l h is
f i n d i n g w e r e p r e d i c t e d t h e o r e t i c a l h r ; is b e i n g a
c o n s e q u e n c e o f a p a r ti a l p h a s e s e p a r a t i o n p h e l l o m -
e n o n d u e t o e l e v a te d c o p o l y m e r c o n c e n t r a t i o n in
t h e s u r f a c e z t, n e [ 1 3 6 ]. T h e e x p e r i m e n t a l t i n d m g s
w e r e i n t e r p r e t e d u s i n g a r o o d • l i e d m e a n - l i e l d
t h e o r y w h i c h t a k e s t h e r e x e r s e t e m p e r a t u r e p h a s e
b e h a v i o r o f th e s v s t e m i n t o a c c o u n t [ 1 3 6.1 3 7 1-
8. G e l s f o rm e d b y P E O - P P O - P E O b lo ck
c o p o l y m e r s
P E O P P O P E O c o p o l y m c r s o lu t io n s of h ig h
c o p o l y m e r c o n c e n t r a t i o n e x h i b it a d r a m a t i c
c h a n g e i n x i s c o s it y a t t e m p e r a t u r e s c l o s e t o
a m b i e n t , r e v e a l i n g a ' t h e r m o r e v e r s i b l e g e l a t i o n
[ 2 7 , 3 0 ,4 0 , 1 3 8 , 1 3 9 ]. S e v e r a l m e c h a n i s m s h a v e b e e n
p r o p o s e d a s d r i v i n g f o r c e s f o r t h i s t h e r n - ml g e h t t i o n :
R a s s i n g a n d A t t w o o d [ 1 4 0 ] r e l a te d t h e g e l t r a n s -
i t i o n t o i n t r i n s i c c h a n g e s i n t h e m i c e l l a r p r o p e r t i e s ,
V a d n e r e e t a l , [ 1 4 1 ] d i s c u s s e d t h e g e l a t i o n i n t e r ms
o f e n t r o p i c c h a n g e s i n v o l v i n g lo c a ll y o r d e r e d w a t e r
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32
P. Alexandridis. T.A . Hatton/Colloids Sur/ itces A: Physicochem. Eng . Aspe cts 96 1995) 1 46
m o l e c u le s cl o se t o t h e h y d r o p h o b i c P P O s e gm e n ts ,
w h e r e a s W a n k a e t a l. [ 3 0 ] a n d W a n g a n d J o h n s t o n
[ 1 3 8 ] s p e c u l a t e d o n t h e p o s s i b i li t y o f a n o r d e r e d
t h r e e - d i m e n s i o n a l s t r u c t u r e d s t a t e o r n e t w o r k .
R e c e n t l y , n e u t r o n s c a t t e r i n g s t u d i e s s h o w e d t h a t
t h e o b s e r v e d c h a n g e i n v i s c o s i t y is d u e t o a h a r d -
s p h e r e c r y s ta l l i z a ti o n a s t h e m i ce l le c o n c e n t r a t i o n
a p p r o a c h e s t h e c r it i ca l v o l u m e f r a c t i o n o f 0 .5 3
( m i c e l l e s c l o s e - p a c k e d ) [ 6 1 ~ 6 4 ] . A t e v e n h i g h e r
t e m p e r a t u r e s t h e g e l ' d is s o lv e s a g a in . T h e P E O
c h a i n s i n t h e m i c e ll a r m a n t l e i n t e r p e n e t r a t e e x t e n s -
i v e l y i n t h e g e l , y i e l d i n g a d y n a m i c c o r r e l a t i o n
l e n g t h o f m a g n i t u d e 0 .8 n m a t t h e h i g h e s t c o n c e n -
t r a t i o n u s e d ( a b o u t 0 .3 5 g m 1 - 1 ) [ 5 7 ] . O s c i l l a t o r y
s h e a r m e a s u r e m e n t s s h o w t h a t t h e g e l a t i o n o n s e t
t e m p e r a t u r e a n d t h e t h e r m a l s t a b i l it y r a n g e o f t h e
g e l i n c r e a s e w i t h i n c r e a s i n g P E O b l o c k l e n g t h
[ 5 7 3 .
U l t r a s o n i c r e l a x a t i o n a n d 1 3C N M R s p e c t r a
w e r e r e p o r t e d f o r P l u r o n i c F 1 2 7 d i s s o lv e d i n w a t e r
a n d
D 2 0
r e s p e c t i v e ly , b y R a s s i n g e t a l. [ 1 4 2 ] .
T h e p o l y m e r s o l u t i o n e x h i b i t e d r e v e r s e t h e r m a l
b e h a v i o r : a 2 0 % w / w s o l u t i o n f o r m e d a g e l a t
r o o m t e m p e r a t u r e . T h e u l t r a s o n i c r e l a x a t i o n , a s
w e l l a s th e m e t h y l c a r b o n r e s o n a n c e , o f t h e P P O
b l o c k s h o w e d d i s t in c t a n o m a l i e s o v e r t h e t e m p e r -
a t u r e r a n g e i n w h i c h t h e g e l f o r m a t i o n t a k e s p l a c e .
S u c h a n o m a l i e s , h o w e v e r , w e r e a l s o o b s e r v e d i n
d i l u t e s o l u t i o n s w h e r e a g e l i s n o t b e i n g f o r m e d . I t
w a s d e d u c e d , o n t h e b a s i s o f t h e r e s u l ts o b t a i n e d ,
t h a t o b s e r v e d r e l a x a t i o n s a r o s e p r e d o m i n a n t l y
f r o m c o n f o r m a t i o n a l c h a n g e s r e l a t e d t o a l t e r -
n a t i o n s i n t h e o r i e n t a t i o n o f th e m e t h y l g r o u p s id e
c h a i n s in t h e P P O b l o c k o f t h e p o l y m e r . T h e s e
c o n f o r m a t i o n a l c h a n g e s w e r e t h o u g h t t o b e
i n d u c e d b y e x t ru s i o n o f h y d r a t e d w a t e r w i t h
i n c r e a s i n g t e m p e r a t u r e f r o m t h e e x i s t i n g m i c e l l e s ,
t h e i n te r i o r s o f w h i c h c o n s i s t o f t h e P P O s e g m e n ts .
I t w a s f u r t h e r p r o p o s e d t h a t t h e d e h y d r a t i o n ,
w h i c h c a u s e s a h i g h e r f r i c t i o n b e t w e e n t h e e n d
g r o u p s i n t h e p o l y m e r c h a in , w a s r e s p o n s i b l e f o r
t h e r e v e r s e t h e r m a l b e h a v i o r o f t h e v i s c o s it y a n d ,
h e n c e , f o r th e g e l fo r m a t i o n i n c o n c e n t r a t e d c o p o l -
y m e r s o l u t i o n s [ 1 4 2 ] .
T h e t e m p e r a t u r e - i n d u c e d g e l a t i o n i n P l u r o n i c
F 1 2 7 c o p o l y m e r s o l u t i o n s w a s r e f l ec t e d i n t h e
r a p i d i n c r e a se o f t h e s t o r a g e m o d u l u s b y t w o o r
m o r e o r d e r s o f m a g n i t u d e [ 3 0 ] . T h e s t o ra g e m o d u -
l u s G ' a p p r o a c h e d a s t e a d y v a l u e a f t e r g e l f o r m a -
t i o n , w h i c h i n c r e a s e d w i t h i n c r e a s i n g c o p o l y m e r
c o n c e n t r a t i o n [ 3 0 ] . T h e g e l f o r m a t i o n t e m p e r a t u r e
d e c r e a s e d w i t h i n c r e a s i n g p o l y m e r c o n c e n t r a t i o n .
T h e c o p o l y m e r g e ls e x h i b i t e d a c h a r a c t e ri s t ic y i e ld
s t re s s a t w h i c h t h e y f l o w w h e n s t re s s is a p p l i e d .
T h e y i e l d v a l u e w a s z e r o f o r s o l u t i o n s b e l o w t h e
g e l t e m p e r a t u r e , a n d i n c r e a s e d a b o v e t h e g e l t e m -
p e r a t u r e w i t h f u r t h e r i n c r e a s e i n t h e t e m p e r a t u r e .
T h e y i e ld v a l u e in c r e a s e d w i t h p o l y m e r c o n c e n -
t r a t i o n ; t h e h i g h e s t y i e l d v a l u e s w e r e a b o v e 1 0 0 0
P a . W a n k a e t a l . [ 3 0 ] a l s o n o t e t h a t t h e g e l s w e r e
n o t b i r e f r in g e n t , in d i c a t i n g t h a t t h e y w e r e n o t
a n i s o t r o p i c l i q u i d c r y s t a l l i n e p h a s e s . B r o w n a n d
c o - w o r k e r s [ 2 7 , 5 7 ] m e a s u r e d t h e v i s c o el a st i c p r o p -
e r t ie s o f P l u r o n i c s P 8 5 , P 8 7 , a n d F 8 8 a s a f u n c t i o n
o f t e m p e r a t u r e . T h e y d e f i n e d t h e g e l a t io n t e m p e r -
a t u r e a s t h e t e m p e r a t u r e a t w h i c h s t o r a g e m o l u l u s ,
G , e q u a l s t h e l o s s s h e a r m o d u l u s , G . T h e g e l a t i o n
t e m p e r a t u r e f o r P l u r o n i c P 8 5 i s p r e s e n t e d a s a
f u n c t i o n o f c o n c e n t r a t i o n i n F ig . 2 2 ( a ) [ 2 7 ] . P 8 5 ,
P 8 7 , a n d F 8 8 s h o w e d q u a l i t a t i v e l y s i m i l a r g e l l i n g
b e h a v i o r , b u t w i t h i m p o r t a n t q u a n t i t a t i v e d i f f e r -
e n c e s a s s h o w n i n F ig . 2 2 ( b l [ 5 7 ] . P 8 5 f o r m e d a
g e l a t 3 5 C a t 2 0 % c o p o l y m e r c o n c e n t r a t i o n . I t
c a n a l s o b e s e e n in F i g . 2 2 ( b ) t h a t P 8 5 f o r m e d a
g el o v e r a r a t h e r n a r r o w t e m p e r a t u r e i n t e r v a l (u p
t o 4 8 : : C t , w h e r e a s P 8 7 a n d F 8 8 r e m a i n e d a s g e l s
u p t o 8 0 - 'C . T h e m a x i m u m v a l u e s o f G ' a re s i m i l ar
f o r F 8 7 a n d F 8 8 , a n d s o m e w h a t s m a l l e r f o r P 85 .
B r o w n e t a l . [ 5 7 ] c o n c l u d e d t h a t t h e g e l a t i o n
o n s e t t e m p e r a t u r e a n d t h e t h e r m a l s t a b i l i t y r a n g e
o f t h e g e l i n c r ea s e d w i t h i n c r e a s in g P E O b l o c k
le ng th .
T h e s o l - g e l t r a n s i t i o n t e m p e r a t u r e w a s m e a -
s u r e d a s a fu n c t i o n o f c o p o l y m e r c o n c e n t r a t i o n f o r
n in e P l ur on ic P E O - P P O - P E O c o p ol ym e r s b y
V a d n e r e e t a l. [ 1 4 1 ] . F i g . 2 3 ( a ) s h o w s a p l o t o f t h e
l o g a r i t h m o f t h e c o p o l y m e r c o n c e n t r a t i o n v s.
l I T
f o r s o m e o f t h e p o l y m e r s s t u d i e d i n R e f. [ 1 4 1 ] .
T h e e n t h a l p y o f g e l a ti o n , A H ~ ge l, w a s e s t i m a t e d
f r o m a r e l a t i o n s h i p s i m i l a r t o t h a t u s e d f o r o b t a i n -
i n g A H ° o f m i c e l li z a ti o n f r o m C M C v s. t e m p e r -
a t u r e d a t a ( s e e S e c t i o n 4 . 1 ) . A p o s i t i v e e n t h a l p y
c h a n g e b e t w e e n 5 a n d 1 0 k c a l m o l . 1 o f c o p o l y m e r
w a s o b s e r v e d i n a l l c a s e s . U n l i k e t h e g e l a t i o n o f
g e l a t i n e , w h e r e t h e l a r g e e n t h a l p y c h a n g e ( - 6 7
k c a l m o 1 - 1 ) f a v o r s t h e g e l a t i o n p r o c e s s , t h e
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s h e a r m e a s u r e m e n t s } i t s
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t r a t i o n a d a p t e d w i t h p e r m i s s i o n f r o m R e f . [ 2 7 ] : c o p y r i g h t
A m e r i c a n C h e m i c a l S o c ie t y , 1 99 1.1 b ) C o m p a r i s o n o f t e m p e r -
a t u r e - d e p c n d e n t g e l a t io n
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P 8 5 . P 8 7 . a n d F 8 8 .
A d a p t e d w i t h p e r m i s s i o n
f r o m R e f. [ 5 7 ] :
c o p y r i g h l A m e r i c a n C h e m i c a l S o c i e t y , 1 9 9 2 . t
e n t h a lp y c h a n ge in th e c as e o f P E O - P P O - P E O
g e l a t i o n i s u n f a v o r a b l e . T h e c o p o l y m e r c o n c e n -
t r a t i o n r e q u i r e d t o g i v e a g e l t r a n s i t i o n t e m p e r a t u r e
o f 2 5 C i s p l o t t e d i n F i g . 2 3 b 1 a s a f u n c t i o n o f
t h e P P O / P E O c o m p o s i t io n r a tio [ 1 4 1 ] . T h e
f o l l o w i n g t r e n d s w e r e n o t e d : t h e c o n c e n t r a t i o n
r e q u ir e d f o r g e l f o r m a t i o n w a s a p p r o x i m a t e l y t h e
s a m e f o r c o p o l y m e r s w i t h t h e s a m e s i ze P P O
b l o ck : f o r a g i v e n P P O / P E O r a ti o , t h e p o l y m e r
c o n c e n t r a t i o n r e q u ir e d fo r f o r m a t i o n o f a g e l a t
a g i v e n t e m p e r a t u r e ) d e c r e a s e d w i t h i n c r e a s i n g
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g e h U i o n t e m p e r a t u r e f o r v a r i o u s P l u ro n i c P E O P P O P E O
c o p o l y m e r s . b ) C o n c e n t r a t i o n o f c o p o l y m e r
r e q u i r e d t o g i v e a
g e l t r a n s i t i o n t e l n p e r a t u r e o f 2 5 C , p l o t t e d a s a f u n c t i o n o f t h e
c o p o l y m e r P P ) / P E O c o m p o s i t i o n r a ti o . I A d a p t e d f ro m
Ref . [ 141] . )
o b s e r v e d b e t w e e n l o g M w a n d 1 T tk~r c o p o l y m e r s
w i t h t h e s a m e P P O / P E O r a ti o. T h e p r e s e n ce o f
N a C 1 , K C 1 , a n d N a S O 4 d e c r e a s e d t h e g e l t r a n s i t i o n
t e m p e r a t u r e , w h e r e a s t h e o p p o s i t e e f fe c t w a s
o b s e r v e d w i th u r e a , a l c o h o l a n d s o d i u m d o d e c y l -
s u l fa t e . T h e e n t h a l p y o f g e l f o r m a t i o n w a s n o t
s i g n i f ic a n t l y a l t e r ed b y t h e a d d e d s u b s t a n c e s , s u g -
g e s t i n g t h a t e n t r o p y p l a y s t h e m a j o r r o l e i n t h c
g e l a t i o n p r o c e s s [ 1 41
]
T h e p o t e n t i a l o f u s i n g P l u r o n i c F 1 2 7 g e l s f o r
c o n t r o l l e d d r u g d e l i v e r y w a s s t u d i e d b y G i l b e r t
e t a l. [ 1 4 3 ] : t h e e ff e c t o f s o l u t e s a n d p o l y m e r s o i l
t h e g e l a t i o n p r o p e r t i e s w a s a l s o r e p o r t e d . T h e
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g e l a t i o n t e m p e r a t u r e w a s d e t e r m i n e d f o r a r a n g e
o f F 12 7 c o n c e n t r a t i o n s ( 2 4 - 3 4 % ) w i t h b e n z o i c a c i d
l o a d i n g i n c r e a s i n g f r o m 0 t o 2 % w / v . T h e g e l s o l
t r a n s i t i o n t e m p e r a t u r e d e c r e a s e d a s t h e b e n z o i c
a c i d c o n c e n t r a t i o n i n c r e a s e d [ 1 4 3 ] . T h e h o m o l o -
gou s se r i e s
o f p a r a h y d r o x y b e n z o a t e
e s t e rs , me t h y l ,
e t hy l , p ropyl , a nd bu t y l , we re a l so c ons i de re d : t he
m o r e l i p o p h i l i c c o m p o u n d s r e s u l t e d i n a g r e a t e r
d e c r e a s e i n t h e g e l - s o l t e m p e r a t u r e f o r a l l c o p o l y -
m e r c o n c e n t r a t i o n s s t u d i e d . A d d i t i o n o f P E O
h o m o p o l y m e r i n cr e a se d t h e g e l a ti o n t e m p e r a t u r e ;
t h e e x t e n t o f t h i s i n c r e as e d e p e n d e d o n t h e a d d e d
P E O c h a i n l e n g t h a n d c o n c e n t r a t i o n . T h e e f f ec t o f
a d d i t i v e s o n g e l f o r m a t i o n i n a q u e o u s F 1 2 7 s o l u -
t i o n s h a s a l s o b e e n r e p o r t e d b y M a l m s t e n a n d
L i n d m a n [ 5 9 , 6 0 , 1 4 4 ] . T h e s t a b i l it y ra n g e o f t h e
g e l p h a s e d e p e n d e d s t r o n g l y o n t h e p r e s e n c e o f
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re g i on , a s we l l a s t he c l oud po i n t , t o l owe r t e mpe r -
a t u r e s , a s s h o w n i n F i g . 2 4 [ 5 9 ] . N a S C N , a t y p ic a l
sa l t i ng i n c oso l u t e , d i sp l a c e s t he whol e gel r e g i on
a n d t h e c l o u d p o i n t u p t o h i g h e r t e m p e r a t u r e s ( s e e
F i g. 2 5 ) [ 5 9 ] . T h e e ff ec ts o f a d d e d P E O a n d P P O
h o m o p o l y m e r s w e r e al s o i n v e s t ig a t e d . I t w a s f o u n d
t h a t P E O o f i n t e rm e d i a t e m o l e c u l a r w e ig h t c au s e d
t h e g el t o m e l t ' a t a c e r t a in h o m o p o l y m e r a m o u n t
w h i c h d e p e n d e d o n t h e c o p o l y m e r c o n c e n t ra t i o n .
T h e e f fi c ie n c y o f P E O i n i n d u c i n g m e l t i n g o f t h e
g e l i n c r e a s e d w i t h P E O m o l e c u l a r w e i g h t , b u t a t
v e r y h i g h P E O m o l e c u l a r w e ig h t s p h a se s e p a r a t i o n
( r a t h e r t h a n g e l m e l t i n g ) o c c u r r e d . T h e g e l m e l t i n g
b e h a v i o r w a s a l s o o b s e rv e d u p o n a d d i t i o n o f a
c a t i o n i c p o l y e l e c t r o l y t e , p o l y ( d i a l l y l d i m e t h y l a m -
m o n i u m c h l o r i d e )( P D A D M A C ) . A d d i ti o n of P P O
h o m o p o l y m e r , h o w e v e r , t e n d e d t o i n c r e a s e t h e
s t a b i l i t y r e g i o n o f t h e g e l , a l t h o u g h t h i s d e p e n d e d
o n t h e P P O m o l e c u l a r w e i g h t [ 1 4 4 ].
9 . C o m p a r is o n o f P E O - P P O c o p o ly m e r s w i th
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a f u n c t i o n o f N a C I c o n c e n t r a t i o n . [ A d a p t e d w i t h p e r m i s s i o n
f r o m R e f. [ 5 9 ] : c o p y r i g h t A m e r i c a n C h e m i c a l S o c i e t y , 1 99 2.1
( P B O ) h a s b e e n r e p o r t e d i n t h e l a s t f e w y e a r s b y
t h e g r o u p s o f A t t w o o d , P r ic e , a n d B o o t h f r o m t h e
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P E O - P B O P E O c o p o l y m e r s h a v e t he a d v a n t a g e
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o v e r c o m p a r a b l e P E O P P O P E O c o p o ly m e r s.
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d i f u n c t i o n a l P P O , f o l l o w e d b y p o l y m e r i z a t i o n o f
e t h y l e n e o x i d e to f o r m t h e o u t e r b l o c k s : t h e a n i o n i c
p o l y m e r i z a t i o n o f P O is c o m p l i c a t e d b y a s id e
r e a c t i o n , i n v o l v i n g h y d r o g e n a b s t r a c t i o n f r o m t h e
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P . .4 h , x a m l r i d i s . T . A . H a t t o n / C o l h ) i d s S m / / h c c s . 4. P h y .~ i c o c la ' m . E H q ,. 4 . V w ( ' tx 9 6 ( l ug ) 5 1 4 4 35
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c5
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5
F i g . 2 5 . { a) P h a s e b e h a v i o r o f t h e F 1 2 7 / w a t e r s y s t e m m t h e
p r e s e n c e o f 2.4 M N a S C N . A l s o s h o w n i s th e p h a s e d i a g r a m
f o r t h e p o l y m e r w a t e r s y s t e m . ( b ) G e l a t i o n p o i n t ( G P ) , m e l t i n g
p o r e I M P ) , a n d c l o u d p o i n t IC P I o f a 3 0 % F 1 2 7 s o l u t i o n a s
a f u n c t io n o f N a S C N c o n c e n t r a t i o n . (A d a p t e d w i t h p e r m i s s i o n
f r o m R c f . [ 5 9 ] : c o p y r i g h t A m e r i c a n C h e m i c a l S o c i e t y , 1 9 9 2 . )
w i t h a d i ff e re n t P P O c o n t e n t . T h e a n i o n i c p o l y m e r -
i z a t i o n o f b u t y l e n e o x i d e , h o w e v e r , is m o r e
s t r a i g h t f i ~ r w a r d , s i n c e t r a n s f e r d o e s n o t o c c u r a n d
d i f u n c t i o n a l P B O c a n b e e x c lu s i v el y p r o d u c e d .
L uo e t a l . [ 1145] p r e pa r e d , us ing se que n t i a l
a n i o n i c c o p o l y m e r i z a t i o n , a p o l y ( e t h y l e n e o x id e }
p o l y ( b u t y l e n e o x i d e j - p o l y ( e t h y l e n e o x i d et t r i b lo c k
c o p o l y m e r , d e n o t e d E O s s B O t , E O s s , w h e r e E O
r e p r e se n t s e t h y l e n e o x id e a n d B O r e p r e s e n ts b u t y l -
e n e o x i d e , a n d d e t e r m i n e d i t s m i c e l l a r a n d g e l a t i o n
p r o p e r t i e s in a q u e o u s s o l u t i o n . S u r f a c e t e n s i o n ,
l i g h t s c a t t e r i n g i n t e n s i t y , p h o t o n c o r r e l a t i o n
s p e c t r o s c o p y , a n d g e l a ti o n m e a s u r e m e n t s w e r e
m a d e a t v a r io u s t e m p e r a t u r e s o v e r c o p o l y m e r
c o n c e n t r a t i o n s r a n g i n g f r o m d i l u t e s o l u t i o n t o
t h e ge l ( > 2 0 % ) . S u r f a c e t e n s io n m e a s u r e m e n t s a t
3 0 C o v e r t h e 1 0 3 l o g I ~ c o n c e n t r a t i o n r a n g e
sho we d a s i ng l e b r e a k a t 0 .3 g 1 ~ , i nd i c a t i v e o f
t h e C M C . T h e C M C o f t h e E O s sB O 1 7 E ( )s ~ c o p o l y -
m e r w a s l o w e r th a t t h e C M C o f P l u r o n i c F 6 8 , a
P E O P P O P E O c o p o l y m e r o f c o m p a r a b l e m o le c-
u l a r w ei g h t a n d P E O c o m p o s i t i o n . T h e m i ce ll e
r a d i u s w a s a p p r o x i m a t e l y 6 n m . w h i l e t h e m i c e ll e
' m o l e c u l a r w e i g h t i n c r e a s e d w i t h t e m p e r a t u r e :
t h e a g g r e g a t i o n n u m b e r w a s 1 3 a t 30 C a n d 2 0 at
5 5 C . T h e m i c e l l iz a t i o n e n t h a l p y . AH was es t i -
m a t e d t o b e 3 4 k J m o l - 1 f i' o m a n a l y z i n g C M ( ' v s.
t e m p e r a t n r e d a t a . T h e l o w e r a n d u p p e r t e m p e r -
a t u r e b o u n d a r i e s f o r t h e g e l r e g i o n w e r e i d e n t i t i e d .
a n d t h e o c c u r r e n c e o f s y n e r e s i s ( s e p a r a t i o n o f
wa te r f r om the ge l ) wa s no t e d . N ic h ,~ l a s e t a l .
[ 1 4 6 ] c o m p a r e d t h e m ic e l la r a n d g e l a t i o n p r o p e r -
t ie s o f E O s ~ B O t ; E O s s , E O ~ t B O 2 s E O 7 1 , an d
E OI 32BOs .~E Ol .~2 c opo lym e r s i n a que ous so lu -
t io n s : t h e c o p o l y m e r s h a d s i m il a r P E O c o m p o s i -
t i ons [ 75 go wt .Ck PE O) bu t d i f f e r e n t c ha in l e ng ths
( t h e c o p o l y m e r m o l e c u l a r w e i g h t s w e r e 7 0 0 0. 8 8 0 (/.
a n d 1 7 5 00 , , e s p e c t i v e l y l. M c a s u r e m e n t s w e r e m a d e
a t 3 0 C o v e r a w i d e c o n c e n t r a t i o n r a n g e . C r i t i c a l
m i c e l l i z a t i o n a n d g e l c o n c e n t r a t i o n s d e c r e a s e d
m a r k e d l y a s t h e c o p o l y m e r c h a in l e n g th i n c re a s e d .
whi l e m ic e l l a r we igh t s a nd s i z e s i nc r e a se d . T he
C M C s f o r E O = t B O 2 s E O = t a n d t ~ () l~ e B ( )5 3 E O t 3 e
w e r e 0 .1 6 a n d < 0 . 0 2 g 1 ~ r c s p e c l iv e l y : t h e
a g g r e g a t i o n n u m b e r s w e r e 4 0 a n d 6 0 0 t\ ~ r
E O - 1 B O e s E O - t a n d E O t _ L ~B O 5 3 [ X )I 3 ., , r e s p e c -
t i v e l y . P h a s e s e p a r a t i o n w a s o b s e r v e d i n d i l u t e
s o l u t i o n s o f th e c o p o l y m e r o f h i g h e r m o l e c u l a r
w e i g h t ( E O ] 3 , B O s 3 E O I 3 2 1 . T h e m i c e l l a r , g e l a t i o n
a n d d r u g r e le a se p r o p e r t i e s o f a q u e o u s
E O a u B O l s E O a u s o l u t i o n s w e r e i n x . c s t i g a l e d b y L u o
e t al. [ 1 4 7 ] . . \ b r i e f i n v e s t ig a t i o n n f t h e r a te o f
r e l e a se o f s a li c y l i c a c id f r om the , ,e l s e r ve d t o
i l l us t r a t e use f u l sus t a ine d r e l e a se . T he ge l a t i on
b e h a v i o r , i n t e r p r e t e d v i a t h e h a r d - s p h e r e m o d e l .
wa s c ons i s t e n t w i th t he m e a sur e d m ic e l l a r s i z e s .
D i b lo c k P E O P B O c o p o l y m e r s ha v e a ls o be e n
s y n t h e s iz e d a n d c h a r a c t e ri z e d b y t h e M a n c h e s t e r
g r o u p . S u n e t a l. [ 1 4 8 ] r e p o r t e d p r e l i m i n a r y r e s u lt s
o n t h e p r e p a r a t i o n o f
E O I 3 5 B ( ) 3 3
a nd i t s m ic e l l i z a -
t i o n a n d s u r l a c e p r o p e r t i e s i n d i h t t e a q u e o u s
s o l u ti o n s . S ix P E O P B O d i b l o c k c n p o l y m e r s
( EO~,eB O : ,
I { O s o B O 1 3, l 7,O 4 ,~ B O s , | { O 5 n B O 4 ,
E O _~ 4 BO to , a n d E O _ , : B O s ) h a v e b e e n p r e p a r e d b y
Be d de l s e t a l. [ 14 9] . a nd t he i r m ic e l l i z a t i on i n
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6 P. Alexandridis, T.A. Hatton/Colloids Surfaces A: Physicochem. Eng. Aspects 96 1995) 1-46
aqueous solution investigated. Surface tension,
static and dynamic light scattering and gel permea-
tion chromatography techniques were used to
study solutions at temperatures in the range
20-50 :C over a wide concentration range (up to
100 g 1- x . CMCs, micellar molar masses and radii
have also been reported [ 149]. The the rmodynam-
ics of micellization of EOmBO, diblock copolymers
was discussed in relation to that of related triblock
copolymers (EOmBO,EO,, and EOmPO,EO,,).
Tanodekaew et al. [150] prepared a series of
diblock copolymers EO3oBO, (with n in the range
3-16) by sequential anionic polymerization, and
investigated the association behavior of the copoly-
mers in aqueous solution and the gelation of their
concentrated micellar solutions. At temperatures
in the range 30-50°C, a minimum PBO block
length of 4 to 5 units was required for micellization.
Gelation of micellar solutions of the copolymers
was observed for copolymers with PBO block
lengths of 6 to 7 units or more. The relationships
between (i) standard Gibbs energy of micellization
and molecular characteristics and (ii) critical gela-
tion concentration and micellar characteristics
were explored. Diblock PEO-PBO copolymers
(allyl-BOlzEO15, allyl-BO12EO25, and allyl-
BO~zEO35) were investigated by Hatton and
co-workers [151]. The formation of micelles was
studied using fluorescence methods and the
thermodynamic parameters of micellization esti-
mated from a closed association model; the micelle
hydrodyn amic radii (Rh) were also determined; a
small decrease in Rh was observed with an increase
in temperature.
9.2. PEO PS block copolymers
Evidence for the formation of micelles in aque-
ous solutions of poly(styrene)-poly(ethyleneoxide)
(PS-PEO) block copolymers has been reported by
Bahadur and Sastry [152], and Riess and Rogez
[153]. The polymers were synthesized with ethy-
lene oxide as the major component in order to
make the copolymers water soluble, the polysty-
rene being extremely hydrophobic. The apparent
molecu lar weight of PS PEO micelles was found
by Riess and Rogez [ 153] and Xu and co-workers
[72,154] to increase with increasing copolymer
molecular weight and decreasing PEO content.
Diblock copolymers formed larger micelles than
triblock copolymers. Bahadur and Sastry [152]
found the micelle size to decrease with an increase
in PS content, which shows that the micelle size
also depends on the insoluble block. Increasing the
temperature resulted in the formation of micelles
with a smaller hydrodynamic radius. This is proba-
bly because water becomes a poorer solvent for
ethylene oxide at higher temperatures, so the
corona becomes less swollen with water, resulting
in a decrease in the hydrodynamic radius; similar
temperature effects have been observed in PEO
PPO-PEO copolymer micelles (see Section 5.1).
Critical micellization concentrations of these
PS-PEO block copolymers were estimated from
the increase in the fluorescence intensity of pyrene
with an increase in polymer concentration, and
found to be in the order of 1-5 mg 1 ~ [155]. The
micelle-water partition coefficient of pyrene was
found to be in the order of 3 × 105 , based on the
polystyrene volume [155]. Gallot et al. [156]
investigated micelle formation in methanol-water
mixtures for several series of PEO- PS block copol-
ymers, each having a constant PS chain length and
varying lengths of the PEO block. Light scattering
and small angle neutron scattering measurements
carried out in methanol showed that the CMCs
were primarily dependent on the molecular weight
of the PS block and that, for a given molecular
weight of the PS block, the aggregation number
decreased with an increase in the PEO chain
length. Despite the decrease in aggregation
number, the hydrodynamic radius of the micelles
was found to increase with PEO chain length. The
SANS results showed the polystyrene core to be
compact and small compared to the overall dimen-
sions of the micelles, and the micelles to be
spherical.
9.3. CiE surfactants
It is interesting to compare the micellization of
PEO PP O- PE O block copolymers to that of CiEj
(j-ethyleneglycol i-alkyl ethers) low molecular-
weight non-ionic surfactants [157,158], where
both amphiphiles have a PEO headgroup. In the
latter systems, it was found that the CMC
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P. Alexandridis, T .A . Hatton/Colloids Sur/}wes A. Ptu'sicochem. En g.
spects
96 (1995) l 4V~ 37
d e c r e a s e d a n d t h e m i c e l l a r s i z e i n c r e a s e d s t r o n g l y ,
b o t h w i t h i n c r e a s i n g t e m p e r a t u r e . T h u s , i n t h e s e
r es p ec ts , t h e a n a l o g y b e t w e e n P E O - P P O c o p o l y -
m e r s a n d C ~ E,i s e e m s v a l id . H o w e v e r , t h e p r o -
n o u n c e d s e n s it iv i ty o f t h e C M C t o t e m p e r a t u r e ,
o r o f t h e C M T t o c o n c e n t r a t i o n o b s e r v ed f o r m a n y
P E O P P O P E O c o p o l y m e r s ( se e S e c t io n s 4 .1 , 5 .1
a n d 5 . 2 ) i s n o t e v i d e n t w i t h t h e n - a l k y l p o l y ( e t h y l -
e n e g l y c o l ) e t h e r s , C i E j [ 1 5 9 ] . T h e d e c r e a s e i n
h y d r o p h i l i c i t y o f t h e P E O b l o c k s w i t h in c r e a s in g
t e m p e r a t u r e h a s c o m p a r a t i v e l y l it tl e e f fe c t o n t h e
C M C f o r C ~ E j [ 1 5 9 ] : i n th e c a s e o f P E O -
P P O P E O c o p o l y m e r s o l u t io n s , a 1 0 K d e c r e a se
i n t e m p e r a t u r e r e s u l t e d i n c h a n g e s i n t h e C M C o f
m o r e t h a n a n o r d e r o f m a g n i t u d e [ 2 0 , 2 8 ]. T h e
n u m b e r o f E O s e g m e n t s a l s o h a s a s m a l l e f fe c t o n
t h e C M C o f C i E j s u r fa c t a n t s [ 1 5 7 ,1 5 9 ] ; th e c h a n g e
in C M C p e r E O s e g m e n t f o r P l u r o n i c c o p o l y m e r s
w a s s m a l le r t h a n a n e q u i v a l e n t c h a n g e f o r t h e
C ~E i ,
i n d i c a ti n g t h a t t h e i n f lu e n c e o f t h e P E O
b l o c k o n C M C d i m i n i s h es a s t h e s iz e o f t h e
s u r f a c t a n t i n cr e as e s. T h e m a i n d e t e r m i n a n t o f t h e
C M C f o r C /E .i w a s f o u n d t o b e t h e l e n g t h o f t h e
a lk y l ( h y d r o p h o b i c ) c h a in . T h e l o g a r it h m o f C M C
f o r t h e
C i E s , i = l O
15 . s e r i e s de c r e a se d l i ne a r ly
w i t h i n c r e a s i n g c a r b o n n u m b e r i n t h e a l k y l c h a i n
( C M C v a r ie d b y a p p r o x i m a t e l y 2 .5 o r d e r s o f m a g -
n i t u d e o v e r t h e c a r b o n n u m b e r r a n g e s t u d i e d )
[ 1 5 S ] .
T h e f re e e n e r g y o f m i c e l li z a t io n ,
A G
, d e c r e a s e d
w i t h i n c re a s i n g n u m b e r o f c a r b o n a t o m s in t h e
a l k y l c h a i n , a n d w i t h i n c r e a s i n g t e m p e r a t u r e f o r
. / - e t h y l e n e g l y c o l - i - a l k y l e t h e r s w i t h t h e s a m e
h e a d g r o u p ( P E O ) a n d v a r y i n g h y d r o p h o b i c t ai l
[ 1 5 8 , 1 6 0 ] . F r e e e n e r g y c h a n g e s f o r m i c e ll e f o r m a -
t i o n p e r m e t h y l e n e g r o u p w e r e e q u a l t o 0 .6 9
k c a l m o l ~, o r - 2 . 8 8 7 k J m o l ~, t o b e c o m p a r e d
w i t h a A G p o- o f a p p r o x i m a t e l y - 0 . 4 5 k J m o l - l
[ 3 0 ,6 9 , 76 ] f o r t h e P E O - - P P O P E O c o p o l y m e r s .
A H d e c r e a s e d w i t h i n c r ea s i n g c a r b o n n u m b e r ,
whi l e
T A S ='
i n c r e a s e d , i n d i c a t i n g t h a t A S c o n t r i b -
u t e s m a i n l y t o m i c e l l i z a ti o n , w h i le A H
c o u n t e r a c t s m i c e ll i za t io n . T h e h e a d g r o u p c o n t r i b u -
t i o n t o A G ~ d e c r e a s e d w i t h i n c r e a s i n g t e m p e r a t u r e .
a t t r i b u t e d t o d e h y d r a t i o n o f t h e P E O c h a in . F o r
. / -e t h y l e n e g l y c o l i - al k y l e t h e r s w i t h t h e s a m e
h y d r o p h o b i c t a i l a n d v a r y i n g P E O h e a d g r o u p { e . g .
f o r t he sys t e m C~¢ ,E i . . j= 17 , 32 , 44 , 63 [ 157] ) A G '
i n c r e a s e d ( b e c a m e l e s s n e g a t i v e t w i t h i n c r e a s i n g
n u m b e r o f E O s e g m e n t s in t h e h y d r o p h i l i c h e a d -
g r o u p , a n d d e c r e a s e d w i t h i n c r e a s i n g t e m p e r a t u r e .
A H a n d A S d e c r e a s e d w i t h i n c r e as i n g n u m b e r
o f E O s e g m e n ts , w h il e t h e h e a d g r o u p c o n t r i b u t i o n
t o AG i n c re a s e d w it h i n c r e as i n g n u m b e r o f E O
s e g m e n t s .
1 0 Se l ec t a pp l ica t i o ns o f b lo ck co po l y m er s o l u t i o ns
I t ) .l . S o l u h i l i z a t i o n q l ' o r ~ a n i c s
A n i n t e r e s t i n g p r o p e r t y o f a q u e o u s m i c e l la r sy s -
t e m s i s t h e i r a b i l i t y t o e n h a n c e t h e s o l u b i l i t y i n
w a t e r o f o t h e r w i s e w a t e r- i n s o l u b l e h y d r o p h o b i c
c o m p o u n d s . T h i s o c c u r s b e c a u s e th e c o r e o f t h e
m i c e l l e p r o v i d e s a h y d r o p h o b i c m i c r o e n v i r o n m e n t ,
s u i t a b l e f o r s o l u b i l i z i n g s u c h m o l e c u l e s . T h e p h e -
n o m e n o n o f s o l u b i li z a ti o n f o r m s t h e b a s is f o r m a n y
p r a c t ic a l a p p l i c a ti o n s o f a m p h i p h i l e s . T h e e n h a n c e -
m e n t i n t h e s o l u b i l i t y o f l y o p h o b i c s o l u t e s i n
s o l v e n t s a l f o r d e d b y a m p h i p h i l i c c o p o l y m e r
m i c e l l e s h a s s h o w n p r o m i s e i n m a n y i n d u s t r i a l a n d
b i o m e d i c a l a p p l i c a t i o n s [ 1 7 ] .
T h e s o l u b i l iz a t i o n o f p a r a - s u b s t i tu t e d a c e t a n i-
l i d e s i n a s e r i e s o f P l u r o n i c P E O P P O P E O
t r i b lo c k c o p o l y m e r s d e p e n d e d o n b o t h t h e c o p o l y -
m e r c o m p o s i t i o n a n d t h e n a t u r e o f t h e s o l u b il i za t e
[ 1 6 1 ] . T h e m o r e h y d r o p h o b i c h a l o g e n a t e d a c e t an -
i l i de s e xh ib i t e d a de c r e a se i n so lub i l i t y w i th a n
i n c r e a s e i n t h e c o p o l y m e r P E O c o n t e n t , w h e r e a s
t h e o p p o s i t c t r e n d w a s n o t e d w i t h le ss h y d r o p h o b i c
d r u g s s u c h a s a c e t a n i l i d e a n d t h e h y d r o x y - ,
m e t h o x y - , a n d e t h o x y - s u b s t i tu t e d c o m p o u n d s . T h e
s o l u b i l i ty o f i n d o m e t h a c i n i n a q u e o u , ; s o l u t i o n s o f
P E O P P O P E O c o p o l y m e r s w a s s tu d ie d b y k i n
a n d K a w a s h i m a [ 7 ] . T h i s h y d r o p h o b i c a n ti -
i n f l a m m a t o r y d r u g w a s s o l u b i l i z e d i n s i g n i f i c a n t
a m o u n t s o n l y a b o v e a c e r ta i n t h r es h o l d c o p o l y m e r
c o n c e n t r a t i o n , m o s t l i k e l y r e l a t e d t o t h e C M C .
T h e s o l u b i l i z i n g c a p a c i t y i n c r e a s e d w i t h p o l y m e r
m o l e c u l a r w e i g h t a n d s o h l t i o n t e m p e r a t u r e .
A p p r o x i m a t e l y 0 .5 m o l i n d o m e t h a c i n p e r t o o l o f
c o p o l y m e r c o u l d b e s o lu b i li z e d in t h e c o p o l y m e r
s o l u t i o n s [ 7 ] .
N a g a r a j a n e t a l . [ 1 5 ] s t u d i e d t h e s o l u b i l i z a t i o n
o f a r o m a t i c a n d a l ip h a t i c h y d r o c a r b o n s i n s o lu -
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P. ,41exandrMis , T . A . Ha t to n /C ol l o i& Su l ju ces A . Phys icoch em. E l l g . ,4~spec ts 96 ( 1995i 1 46
tl
r a t h e r t h a n m i c e l l a r g r o w t h [ 1 7 ] . T h e s o l u b i l i -
z a t i o n o f 1 , 6 - d i p h e n y l - l ,3 , 5 - h e x a t ri e n e { D P H ) , a
f l u o r es c e n c e d y e , in P l u r o n i c c o p o l y m e r m i c e l la r
s o l u t i o n s w a s i n v e s t i g a t e d b y A l e x a n d r i d i s a n d
c o - w o r k e r s [7 6 , 1 6 2 ] . D P H p a r t i ti o n e d f a v o r a b ly
i n t h e m i c e l l a r p h a s e , w i t h a p a r t i t i o n c o e f f i c i e n t
t h a t w a s h i g h e s t f o r P 1 2 3 ( a P l u r o n i c c o p o l y m e r
w i t h 7 0 % P P O ) a n d d e c r e a s e d i n t h e o r d e r
P 1 2 3 > P 1 0 3 > P 1 0 4 = P I 0 5 > F 1 2 7 > P 8 4 > P 8 5
= F 1 0 8 > F 8 8 > F 6 8 > P 6 5 . I t c a n b e c o n c l u d e d
f r o m t h e s e d a t a t h a t t h e s o l u b i l i z a t i o n w a s i n f l u -
e n c e d b y b o t h t h e r e l a ti v e ( w i th r e s p e c t to P E O )
a n d a b s o l u t e s iz e o f t h e h y d r o p h o b i c P P O b l o c k .
10.2. Protection ql microorganisms
I n s e ct a n d m a m m a l i a n c el ls a r e b e c o m i n g
i m p o r t a n t i n th e b i o t e c h n o l o g y i n d u s t r y f o r t h e
p r o d u c t i o n o f c o m p l e x p r o t e i n s . T h e s e c el ls a r e
s e n s i t iv e t o s h e a r , a n d a r e t h u s d i ff i cu l t t o g r o w t o
h i g h c e l l c o n c e n t r a t i o n s s u c h a s t h o s e f o u n d i n
f e r m e n t a t i o n f o r th e p r o d u c t i o n o f an t i b io t i cs . C e l l
d e a t h o c c u r s d u r i n g s p a r g i n g b e c a u s e o f h i g h s h e a r.
P E O P P O P E O c o p o l y m e r s h a v e b e en s h o w n t o
p r o t e c t m a m m a l i a n c el ls f r o m s u c h d a m a g e .
M u r h a m m e r a n d G o o c h e e [ 1 2 ,1 6 3 ] e x a m in e d
t h e s t r u c t u r a l f e a tu r e s o f n o n - i o n i c p o l y g l y c o l
c o p o l y m e r m o l e c u l e s r e s p o n s i b le f o r t h e p r o t e c t iv e
e f fe c t i n s p a r g e d a n i m a l c e ll b i o r e a c t o r s . I t w a s
s h o w n t h a t t h e s a m e P l u r o n i c c o p o l y m e r s t h a t d i d
n o t i n h i b i t c e l l g r o w t h w e r e t h e o n e s t h a t p r o v i d e d
p r o t e c t i o n f r o m s p a r g i n g i n t h e b i o r e a c t o r s u s e d
i n t h i s s t u d y . T h e p r o t e c t i v e a b i l i t y o f P l u r o n i c s
w a s f o u n d t o c o r r e l a t e w i t h t h e h y d r o p h i l i c - l i p o -
p h i l i c b a l a n c e ( H L B ) ; t h e c o p o l y m e r s w i t h t h e
l a rg e s t H L B ( h i g h e r P E O c o n t e n t ) w e r e t h e p r o t e c -
t iv e a ge nts . T h e P E O P P O - P E O c o p o ly m e r s
s t u d i ed w e r e m o r e e ff e ct iv e t h a n h o m o p o l y m e r
P E O i n p r o t e c t i n g t h e m i c r o o r g a n i s m s . S u p p l e -
m e n t i n g t h e c e ll c u l t u r e m e d i u m w i t h 0 .2 % P l u r o -
n i c 1 _3 5 p r o v i d e d s i g n i f ic a n t l y m o r e p r o t e c t i o n f o r
c e l l s g r o w i n g i n a K o n t e s a i r l i f t b i o r e a c t o r t h a n
a d d i t i o n o f 0 .2 % P l u r o n i c F 6 8 [ 1 2 ] . I t w a s d e m o n -
s t r a t e d , h o w e v e r , t h a t i n c r e a s i n g t h e F 6 8 c o n -
c e n t r a t i o n t o 0 . 5 % , w h i c h r e s u l t s i n a m o l a r
c o n c e n t r a t i o n c o m p a r a b l e t o t h a t o f 0 .2 % L 3 5 ,
p r o v i d e d s i g n i f i c a n t c e l l p r o t e c t i o n ; L 3 5 a n d F 6 8
a p p e a r t h u s t o b e c o m p a r a b l e p r o t e c t i v e a g e n t s o n
a m o l a r b a si s [ 1 6 3 ] . Z h a n g e t a l. [ 1 3 ] m e a s u r e d
t h e m e c h a n i c a l p r o p e r t i e s o f h y b r i d o m a c e ll s t a k e n
f r o m a c o n t i n u o u s c u l t u r e i n t h e p r e s e n c e o r
a b s e n c e o f P l u r o n i c F 6 8 i n t h e c u l t u r e m e d i u m .
T h e m e a n b u r s t i n g m e m b r a n e t e n s i o n a n d t h e
m e a n e l a st i c a re a c o m p r e s s i b i li t y m o d u l u s o f th e
c e l l s w e r e s i g n i f i c a n t l y g r e a t e r i n a m e d i u m c o n -
t a i n i n g 0 . 0 5 ( ' } F 6 8 , c o m p a r e d t o t h a t w i t h o u t
P l u r o n i c c o p o l y m e r . T h e s h o r t - t e r n 1 e f f e c t o f
P l u r o n i c c o p o l y m e r w a s a ls o t e s t ed b y i ts a d d i t i o n
a t v a r i o u s l e v e l s u p t o 0 . 2 % i m m e d i a t e l y b e f o r e
t h e m e c h a n i c a l p r o p e r t y m e a s u r e m e n t s : a s i g n i f i -
c a n t s h o r t - t e r m e ff ec t c o u l d o n l y b e d e t e c t e d a b o v e
a t h r e s h o l d c o p o l y m e r c o n c e n t r a t i o n o f 0 .1 % [ 1 3 ].
O r t o n [ 1 4 ] f o u n d P l u r o n i c F 6 8 t o p r o v i d e s ig n if i-
c a n t p r o t e c t i o n i n m a m m a l i a n c e l l s i n a s p a r g e d
b u b b l e c o l u m n r e a c to r . I t w a s o b s e r v e d t h a t , w h e n
F 6 8 w a s p r e s e n t i n t h e m e d i u m , c e l l s n o l o n g e r
a t t a c h e d t o b u b b l e s a n d t e n d e d t o a g g r e g a t e l e s s
a m o n g t h e m s e l v e s . A s h e d d i n g e f f ec t w a s a l s o
v i s u a l i z e d [ 1 4 ] : o n a b u b b l e f o r m e d i n a m e d i u m
w i t h o u t F 6 8 , t h e c e l l s w e r e f o u n d t o b e a s s o c i a t e d
w i t h t h e b u b b l e s u r f a c e , w h e r e a s c e ll s w e r e q u i c k l y
s h e d f r o m t h e g a s l i q u i d i n t e r f a c e i n t h e p r e s e n c e
of P lu r on i c I - ' 68 .
10.3. Medical applications o[ PEO PPO PEO
copolymers
' M i c r o c o n t a i n e r s f o r d r u g t a r g e t i n g u si n g
P l u r o n i c P E O P P O P E O b l o c k c o p o l y m e r s w e r e
p r e p a r e d b y K a b a n o v a n d c o - w o r k e r s [ 1 0 , 1 6 4 ] .
I l l o r d e r t o t a r g e t s u c h m i c r o c o n t a i n e r s t o a c e r t a i n
c el l, th e c o p o l y m e r m o l e c u l es w e r e c o n j u g a t e d w i t h
a n t i b o d i e s a g a i n s t a t a r g e t - s p e c if i c a n t i g e n o r w i t h
p r o t e i n l i g a n d s s e l e ct i v e ly i n t e r a c t i n g w i t h t a r g e t
c e l l r e c e p t o r s : p r o t e i n s w e r e a t t a c h e d t o a l d e h y d e
g r o u p c o n t a i n i n g P l u r o n i c m o l e c u l e s u s i n g
t h e r e d u c t i v e a l k y l a t i o n r e a c t i o n . T h e o b t a i n e d
c o n j u g a t e s w e r e t h e n i n c o r p o r a t e d i n t o m i c e l l e s
c o n t a i n i n g s o l u b i l i z e d d r u g s ( s u c h a s f l u o r e s c e i n
i s o t h i o c y a n a t e ( F I T C ) a n d h a l o p e r i d o l ) b y si m p l e
m i x i n g o f t h e c o r r e s p o n d i n g c o m p o n e n t s . I t w a s
f o u n d t h a t s o l u b i li z a ti o n o f F I T C in P l u r o n i c
c o p o l y m e r m i ce l le s c o n s i d e r a b l y i n f l u e n c e d t h e di s -
t r i b u t i o n o f F I T C i n a n i m a l ( m o u s e ) ti s su e s , a n d
r e s u l t e d in a d r a s t i c i n c r e a s e o f F I T C f l u o r e s c e n c e
i n th e l u n g . F h e p e n e t r a t i o n e f fi c ie n c y o f F I T C i n
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4 P. Alexandridis, T. A. H atton/Colloids Sur]aces A: Physicochent . Eng. Aspects 96 1995) 1-4 6
l u n g t i ss u e s i n c r e a s ed w i t h i n c r e a s in g P E O -
P P O - P E O c o p o l y m e r h y d r o p h o b i c i ty in t he s er ie s
F 6 8 , P 8 5 , L 6 4 . S u c h b e h a v i o r w a s a t t r ib u t e d t o
e i t h e r d i ff e re n t p a r t i ti o n i n g p r o p e r t i e s o f P l u r o n i c
c o p o l y m e r s i n t i s s u e m e m b r a n e s o r d i f f e r e n c e s
i n m i ce ll e s iz e. C o n j u g a t i o n o f F I T C - c o n t a i n i n g
m i c e l l e s w i t h i n s u l i n v e c t o r r e s u l t e d i n i n c r e a s e d
F I T C p e n e t r a t i o n i n a l l t i s s u e s , i n c l u d i n g t h e b r a i n .
S p e c i fi c t a r g e t i n g o f t h e s o l u b i l i z e d F I T C i n t h e
b r a i n w a s o b s e r v e d w h e n a P l u r o n i c c o n j u g a t e
w i t h a n t i b o d i e s t o t h e a n t i g e n o f b r a i n g l i a l c e ll a 2-
g l y c o p r o t e i n ) w a s i n c o r p o r a t e d i n t o t h e m i c el le s .
A c o n s i d e r a b l e i n c r e as e o f F I T C f l u o r es c e n c e i n
t h e b r a i n a n d d e c r e a s e o f it s f lu o r e s c e n c e i n t h e
l u n g s h a d b e e n r e g i s t e r e d u n d e r t h e s e c o n d i t i o n s .
T h e p o s s i b il i ty o f u s in g m i c e l la r m i c r o c o n t a i n e r s
f o r t a r g e t i n g n e u r o l e p t i c s h a l o p e r i d o l ) i n t h e b r a i n
w a s a l so s t u d i e d . I n c o r p o r a t i o n o f a n t i b o d i e s
t o a z - g l y c o p r o t e i n i n t o h a l o p e r i d o l - c o n t a i n i n g
m i c e l le s r e s u l t e d i n a d r a s t i c i n c r e a s e o f t h e
e f f e ct i v e n e ss o f t h e d r u g , i n d i c a t i n g t h a t v e c t o r -
c o n t a i n i n g P l u r o n i c c o p o l y m e r m i ce l le s c a n e f fe c-
t i v e l y t r a n s p o r t s o l u b i l i z e d n e u r o l e p t i c s a c r o s s t h e
b l o o d b r a i n b a r ri e r. T h e s a m e r e s e a rc h g r o u p a l s o
d e m o n s t r a t e d t h a t a l o w - m o l e c u l a r - w e i g h t c o m -
p o u n d A T P ] , s o l u b i l i z e d i n P l u r o n i c m i c e ll e s ,
a c q u i r e s t h e a b i l i t y t o p e n e t r a t e w i t h i n a n i n t a c t
c e l l i n v i t r o [ 1 6 5 ] .
G e ls f or m e d by P l u ro n ic P E O - P P O P E O
c o p o l y m e r s w e r e e v a l u a t e d b y G u z m a n e t al. [ 9 ]
a s m e d i a f o r t h e s u b c u t a n e o u s a d m i n i s t r a t i o n o f
d r u g s ; p h e n o l s u l f o p h t h a l e i n P R ) w a s u s e d a s a
t r ac e r . T h e t y p e o f P l u r o n i c c o p o l y m e r F 1 0 8 o r
F 1 2 7) , t h e i r c o n c e n t r a t i o n s , a n d t h e e f f e ct o f s o l u te s
N a O H , N a C 1 o r P R ) o n g e l a t i o n p r o p e r t i e s w e r e
s t u d ie d ; s o d i u m h y d r o x i d e a n d s o d i u m c h l o r i d e
d e c r e a s e d t h e g e l s o l t r a n s i t i o n t e m p e r a t u r e ,
w h e r e a s t h e o p p o s i t e e f fe c t w a s o b s e r v e d w i t h P R .
T h e i n v i t r o r e l e a s e r a t e s o b t a i n e d f o r P R w e r e
i n v e r se l y p r o p o r t i o n a l t o t h e c o n c e n t r a t i o n o f
c o p o l y m e r u s e d a n d a z e r o - o r d e r r e le a s e r a t e w a s
o b s e r v e d i n a l l p r e p a r a t i o n s a s s a y e d , T h e a m o u n t
r e l e a s e d a t a g i v e n t i m e w a s s m a l l e r a t h i g h p o l y -
m e r c o n c e n t r a t i o n s ; t h i s w a s a t t r i b u t e d t o t h e g e l
m a t r i x b e i n g m o r e r ig i d. P l u r o n i c F 1 2 7 / P R p r e p a -
r a t i o n s w e r e a d m i n i s t e r e d s u b c u t a n e o u s l y t o
W i s t a r r at s; P R p l a s m a l ev e ls w e r e c o m p a r e d w i th
t h o s e r e a c h e d a ft e r s u b c u t a n e o u s o r i n t r a v e n o u s
a d m i n i s t r a t i o n o f a P R a q u e o u s s o l u t i o n T h e g el
f o r m u l a t i o n p r o d u c e d a s u s t a i n e d p l a t e a u l ev e l
w i t h i n 1 5 r a i n t h a t l a s t e d 8 9 h . G o o d f i ts f o r
e x p e r i m e n t a l i n v i v o d a t a f r o m P l u r o n i c g el s w e re
o b t a i n e d u s i n g a z e r o - o r d e r i n p u t a n d f i rs t - o rd e r
o u t p u t t w o - c o m p a r t m e n t p h a r m a c o k i n e t ic s m o d e l;
t h e f it te d p a r a m e t e r s o f th e m o d e l i n d i c a t e t h a t
r e le a s e o f P R f r o m t h e g e l c o n t r o l l e d t h e P R
a b s o r p t i o n p r o c e s s . T h e r e s u l t s o b t a i n e d s u g g e s t
t h a t F 1 2 7 a q u e o u s g el s m a y b e o f p r a c t ic a l u s e a s
m e d i a f o r t h e s u b c u t a n e o u s a d m i n i s t r a t io n o f
d r u g s .
T h e a d s o r p ti o n o f P P O P E O c o p o ly m e r s a t
s u r fa c e s a s a m e a n s o f p r e v e n t in g p r o t e i n a d s o r p -
t i o n a t t h e s e s u r f a c e s h a s b e e n e x p l o r e d b y L e e
e t a l. [ 1 6 6 ] a n d A m i j i a n d P a r k [ 1 6 7 ] . P r o t e i n s
a d s o r b t o a l m o s t a l l s u r f a c e s d u r i n g t h e f i r s t f e w
m i n u t e s o f b l o o d e x p o s u r e . T h e r e h a s b e e n m u c h
e f f o r t i n m i n i m i z i n g o r e l i m i n a t i n g p r o t e i n a d s o r p -
t i o n , a s s u r f a c e s t h a t e x h i b i t m i n i m a l p r o t e i n
a d s o r p t i o n a r e i m p o r t a n t i n m a n y a p p l i c a t i o n s ,
i n c l u d i n g d e v i c e s t h a t c o m e i n t o c o n t a c t w i t h
b l o o d , m e m b r a n e s f o r s e p a r a t i o n p r o c e s s e s , c o n -
t a c t le n s e s, et c. [ 1 6 6 ] . L e e e t a l. [ 1 6 6 ] f o u n d t h e
a d s o r p t i o n o f h u m a n a l b u m i n o n l o w d en s i ty
p o l y e t h y l e n e L D P E ) su r fa c e s t r e a t e d w i t h c o p o l y -
m e r t o b e s i g n i fi c a n t ly le ss t h a n o n t h e u n t r e a t e d
s u r f a c e . T h e p r o t e i n r e s i s t a n c e t o t h e c o p o l y m e r -
t r e a t e d s u r f a c e w a s h i g h l y d e p e n d e n t o n t h e
a m o u n t o f c o p o l y m e r a d s o r b e d a n d o n t h e m o b i l-
i ty o f t h e P E O s e gm e n ts . S ta r -l ik e P E O - P P O
b l o c k c o p o l y m e r s w e r e f o u n d t o h a v e b e t t e r
a d s o r p t i o n p r o p e r t i e s t h a n t r i b l o c k s . D e s p i t e t h e
f a v o r a b l e r e s ul ts , t h e a u t h o r s v o i c e c o n c e r n o v e r
t h e s t a b il i ty o f th e a d s o r b e d c o p o l y m e r l a y e r a n d
t h e p o s s ib l e d e s o r p t i o n o f t h e c o p o l y m e r s i n t o t h e
t r e a t e d m e d i u m [ 1 6 6 ] . N o t e a l so t h a t , b e c a u s e o f
c o m p e t i t i v e a d s o r p t i o n b e t w e e n c o p o l y m e r s a n d
p r o t e i n s , a n d d e s o r p t i o n o f th e c o p o l y m e r b lo c k s ,
a m b i g u i t i es m a y b e i n t r o d u c e d i n p r o t e i n a d s o r p -
t i on s tud i e s .
A d s o r p t i o n o f f i b r in o g e n a n d p l a te l e t a d h e s i o n
o n t o d i m e t h y l d i c h l o r o s i l a n e - t r e a t e d g l a s s a n d
L D P E s u rf ac e s t r e at e d w i th P E O h o m o p o l y m e r
a n d v ar io u s P E O P P O P E O b l oc k c o p o l y m e rs
w a s e x a m i n e d b y A m i ji a n d P a r k [ 1 6 7 ] . P E O
c o u l d n o t p r e v e n t p l a t e le t a d h e s i o n a n d a c t i v a t i o n ,
e v e n w h e n t h e b u l k P E O c o n c e n t r a t i o n fo r a d s o r p -
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P . A h 'x and r i d i s , T . A . Ha t t on / C o l l oM s Sur / ~ t c e s A . P h r s i c o c h e m E n ~ . . 4 ,spe ct s 96 ( I c )95 J 1 46 41
t i o n w a s i n c re a s e d t o 1 0 m g m l - ~ . P l u r o n i c c o p o l y -
m e r s c o n t a i n i n g 3 0 P O s e g m e n t s c o u l d n o t p r e v e n t
p l a t e l e t a d h e s i o n a n d a c t i v a t i o n , a l t h o u g h t h e
n u m b e r o f E O s e g m e n t s v a r ie d u p t o 7 6 . P l u r o n i c s
c o n t a i n i n g 5 6 P O s e g m e n t s i n h i b i te d p l a t e le t a d h e -
s io n a n d a c t iv a t i o n , a l th o u g h t h e n u m b e r o f E O
s e g m e n t s w a s a s s m a ll a s 1 9. F i b r i n o g e n a d s o r p t i o n
o n t h e P l u r o n i c - c o a t e d s u r f a c e s w a s r e d u c e d b y
m o r e t h a n 9 5 c o m p a r e d to a d s o r p t i o n o n c o n t r o l
s u r f a c e s . T h e a b i l i t y o f P l u r o n i c c o p o l y m e r s t o
p r e v e n t p l a t e l e t a d h e s i o n a n d a c t i v a t i o n w a s
m a i n ly d e p e n d e n t o n t h e n u m b e r o f P O s e gm e n ts ,
r a t h e r th a n t h e n u m b e r o f E O s e g m e nt s [ 1 6 7 ] .
T h i s w a s c o n t r a r y t o t h e f i n d i n g s o f L u o e t a l.
[ 14 71 ] w h o s u g g e s t e d t h a t c o p o l y m e r s w i t h l a r g e
P P O b l o c k s d o n o t a d s o r b w e l l b e c a u s e th e y fo r m
m i c e l l e s i n s o l u t i o n ; i t s h o u l d b e n o t e d , h o w e v e r ,
t h a t t h e n u m b e r o f P l u r o n i c c o p o l y m e r s s tu d i e d
b y L u o e t a l . w a s r a t h e r l i m i t e d . A c c o r d i n g t o
A m i j i a n d P a r k [ 1 6 7 ] , t h e P O s e g m e n t s w e r e
r e s p o n s i b l e f o r a t t a c h i n g t h e c o p o l y m e r t o t h e
s u r f a ce , a n d t h e E O s e g m e n t s f o r r e p e l l in g f i b r in o -
g e n a n d p l a t e l e t s b y a s t e r i c r e p u l s i o n m e c h a n i s m .
1 1 D y na m i cs o f m i ce l l e a nd g e l f o rm a t i o n
T h e k i n e ti c s o f m ic e ll e f o r m a t i o n i n a q u e o u s
s o l u t i o n s o f l o w - m o l e c u l a r - w e i g h t s u r f a c t a n ts h a v e
a t t r a c t e d c o n s i d e r a b l e a t t e n t i o n . I n g e n e r a l , t w o
r e l a x a t i o n p r o c e s s e s h a v e b e e n o b s e r v e d : a f a s t
p r o c e s s / H s m s t i m e r a n g e ) a t t r i b u t e d t o a s u r f a c -
r a n t m o l e c u l e e n t e r i n g o r e x i t i n g a m i c e ll e , a n d a
s l o w e r p r o c e s s { m s - s t i m e r a n g e ) d u e t o m i c e l l e
a s s e m b l y o r d i s s o c i a t i o n [ 1 6 8 ] . T h e l a r g e m o l e c u -
l a r w e ig h t a n d c h a i n - li k e s t r u c t u r e o f b l o c k c o p o l y -
m e t m o l e c u l e s a r e e x p e c t e d t o c o m p l i c a t e t h e
p r o c e ss e s o f m ic e ll e f o r m a t i o n a n d e x c h a n g e o f
c o p o l y m e r m o l e c u l e s b e t w e e n m i c e l l e s a n d t h e
b u l k s o l u t i o n : d i s e n t a n g l e m e n t o f c o p o l y m e r m o l e -
c u l e s f r o ln t he m ic e l l e s shou ld r e su l t i n t im e - sc a l e s
s l o w e r t h a n t h o s e o b s e r v e d i n c o n v e n t i o n a l s u r f ac -
t al lt s. L i n sc a n d M a l m s t e n [ 2 5 ] a n d M a l m s t e n
a n d L i n d m a n [ 5 9 ] f o ll o w e d t h e t e m p e r a t u r e
d e p e n d e n c e o f th e P l u r o n i c F 1 2 7 m i c e l li z a ti o n
p r o c c s s b y p e r f o r m i n g g e l - p e r m e a t i o n c h r o m a t o g -
r a p h y ( G P C ) e x p e r i m e n t s a t d i f f e r e n t t e m p e r -
a t u r e s . T h e m i c e l l e f r a c t i o n a p p e a r e d a s a s e p a r a t e
p e a k i n t h e c h r o m a t o g r a m : t h i s l e d t h e a u t h o r s t o
t h e c o n c l u s i o n t h a t t h e r e s i d e n c e ti m e o f t h e p o l y -
m e r m o le c ul es in th e F 1 2 7 P E O - P P O P E O
m i c e l l e s i s e x t r e m e l y l o n g ( h o u r s ) . H o w e v e r .
F l e i s c h e r [ 6 5 ] r e p o r t e d t h a t t h e l if e ti m e o f a
c o p o l y m e r m o l e c u l e w i th i n t h e m i ce ll e w as s h o r t e r
t h a n t h e m i n i m u m o b s e r w l t i o n t im e o f t h e e x p e ri -
m e n t s ( ~ 3 m s ) . H e r e a c h e d t h i s c o n c l u s i o n f r o m
P F G - N M R m e a s u r e m e n t s o f F 1 27 c o p o l y m e r se lf-
d i f fu s i o n . S i n c e u n i m e r s a n d m i c e ll e s c o e x i s t i n t h e
s o l u t i o n . F l e i s c h e r e x p e c t e d t o o b s e r v e t w o s e l f -
d i f f u s io n c o e f fi c ie n t s , o n e f o r t h e u n i m e r s a n d o n e
f o r t h e m i c e l le s , i f t h e r e w e r e n o e x c h a n g e o f t h e
m o l e c u l e s b e t w e e n t h e t w o s t a t e s w i t h i n t h e d i f f u -
s ion t im e . A s ing l e d i f f us ion c oe f f i c i e n t wa s , ne ve r -
t h e le s s , o b s e r v e d , a n d t h u s F l e i s c h e r [ 6 5 ] i n f e r re d
t h a t t h e c o p o l y m e r s o lu t i o n w a s m t h e d y n a m i c
r a n g e o f f a st e x c h a n g e .
T h e k i n e ti c s o f g e l a t io n f o r a q u e o u s s o l u t i o n s o f
P l u r o n ic F 9 8 a n d F 12 7 P E O P P O P E O c o p o l y -
m e r s w e r e i n v e s t i g a t e d b y W a n g a n d J o h n s t o n
[ 1 3 8 ] w i t h l h e h e l p o f p ul se s h e a r o m e t r y . T h e
p u l s e s h e a r o m e t e r w a s d e s i g n e d t o m e a s u r e t h e
p r o p a g a t i o n v e l o c it y ( a n d f r o m t h is t h e d y n a m i c
s h e a r m o d u l u s ) o f a s h e a r w a v e t h r o u g h a v i s c o -
e l a s t i c m a t e r i a l t h a t h a s b e e n s u b j e c t e d t o c o n d i -
t i o n s o f n o n - s t e a d y s h e a r . T h r e e d i s t i n c t l i n e a r
p h a s e s w e r e o b s e r v e d f o r t h e l o g ( d y n a m i c s h e a r
m o d u l u s , G ' ) v s . t i m e p r o f i l e s , a s c o p o l y m e r s o l u -
t i o n s o f v a r s i n g c o n c e n t r a t i o n s w e r e a l lo w e d t o
p a s s iv e l y w a r m a t r o o m t e m p e r a t u r e t o a t e m p e r -
a t t i r e e x c e e d i n g t h e s o l - t o - g e l t r a n s i t i o n t e m p e r -
a t t i r e , T ,,~ . T h e i n i t i a l e xp on e n t i a l pha se wa s d ue
t o w a r m i n g , w h i le t h e b e g i n n i n g o f th e s e c o n d
e x p o n e n t i a l p h a s e c o i n c i d e d w i t h t h e o n s e t o f t h e
g e l a t i o n p r o c e s s a s d e t e r m i n e d b y v i s u a l o b s e r v a -
t i o n . T h e t h i r d e x p o n e n t i a l p h a s e w a s a t t r i b u t e d
t o t h e r a t e o f f o r m a t i o n o f t h e p o l y m e r n e t w o r k
( g e l a ti o n a p p e a r e d t o b e c o m p l e t e a t t h e b e g i n n i n g
o f t h is p h a s e ). A c o m p a r i s o n o f F 1 2 7 1 3 0 , 7 ~ , =
10 .9 C) a nd V981 37 , Tm = 11 .1 C ) wo uld sugge s t
t h a t t h e c o n c e n t r a t i o n o f c o p o l y m e r re q u i r e d t o
a c h i e v e a c e r t a i n g e l a t i o n t e m p e r a t u r e d e c r e a s e s
w i t h in c r e a s in g m o l e c u l a r w e ig h t o f t h e P P O b l o c k
o f t h e c o p o l y m e r . C h a r a c t e r i s t ic t im e s f o r g e l at i o n
we re 17 an d 9 min for 20 an d 3() (~; F '127, respec-
t i v el y , a n d 4 3 a n d 3 0 m i n f o r 2 0 a n d 3 0 F 9 8 ,
r e s p e c ti v e l y . T h e g e l a t i o n p r o c e s s w a s m o r e r a p i d
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4 2 P. Alexandridis, T. A. Hatton/Colloids SurJktces A: Physicochem. Eng. Aspects 96 1995) 1-46
for F127 (at a given concentration) and the charac-
teristic times decreased with increasing concen-
tration for a given copolymer. Wang and Johnston
[-138] suggested that the rate of gelation for the
Pluronic solutions studied was dependent on the
rate of heat transfer through the polymer solution
(copolymer solutions were allowed to passively
warm at room temperature). If this statement is
true then the attribution by the authors of the
time-dependent results to ~kinetics of gelat ion is
erroneous, and the time-scales they reported were
actually kinetics of heat transfer. It becomes obvi-
ous from the above that the dynamics of block
copolymer micelle and gel formation, although
important from both a theoretical and a practical
point of view, are still unexplored and not well
understood.
1 2 C o n c l u s i o n s
The phase behavior and aggregation proper-
ties of poly(ethylene oxide) -b lock-po ly p ropylene
oxide)-block-poly ethylene oxide) (PEO-PPO-
PEO) copolymers in solution, as affected by the
copolymer molecular composition and concen-
tration, additives, and solution temperature, are
important in understanding the mechanism of
copolymer action underlying the various applica-
tions. While the PEO PPO -P EO copolymers exist
in solution as individual coils (unimers) at low
temperatures and/or concentrations, micelles are
formed on increasing the copolymer concentra tion
and/or solution temperature, as revealed by surface
tension, light scattering, and dye solubilization
experiments. The unimer-to-micelle transition is
not sharp, but spans a concentration decade or
10K. The critical micellization concentration
(CMC) and temperature (CMT) decrease with
increase in the copolymer PPO content or molecu-
lar weight, and in the presence of salts. The depen-
dence of C M C on temperature together with
differential scanning calorimetry measurements
indicate that the micellization process of PEO -
PPO-PEO copolymers in water is endothermic,
and is driven by the entropy gain in water when
unimers aggregate to form micelles (hydrophobic
effect) and the decrease in polarity of EO and PO
as temperature increases. The free energy and
enthalpy of micellization can be correlated to the
number of EO and PO segments in the copolymer
and the copolymer molecular weight. The PEO-
PPO PEO copolymers form micelles with hydro-
dynamic radii of approximately 10 nm and aggre-
gation numbers in the order of 50. The aggregation
number is thought to be independent of concen-
tration and to increase with temperature. Despite
the latter, the micelle hydrodynamic radii are
generally independent of temperature, a result of
dehydration of the PEO segments at elevated
temperatures. Fluorescence molecules have been
used to probe the microenvironment in block
copolymer micelles. Phenomenological and mean-
field lattice models for the formation of micelles
can capture qualitatively the trends observed
experimentally. A self-consistent mean-field theory
was able to reproduce the anomalous phase beha-
vior of PEO and PPO homopolymers, predict the
micellization behavior of PE O- PP O- PE O block
copolymers, and provide detailed information on
the distribution of the copolymer segments in the
micelles. The model calculations showed qualita-
tive agreement with experimental predictions on
the effect of temperature, concentration, hydropho-
bicity, and molecular weight of the polymer: quan-
titative agreement was also good, considering that
all the model input parameters were obtained from
independent experiments. The PEO-PPO PEO
copolymers adsorb on air-water and solid-water
interfaces; for copolymers adsorbed at hydrophobic
interfaces, the PPO block is located at the interface
while the PEO block extends into the solution.
Gels are formed by certain PEO -P PO -P EO block
copolymers at high concentrations, while the
micelles remain apparently intact in the form of a
crystal . The gelation onset temperature and the
thermal stability range of the gel increase with
increasing PEO block length.
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