Indian Joul11al of Pure & Appli ed Physics Vol. 42. February 2004. pp. 142-148

Study of solvent effect on microwave dielectric relaxation and molecular ' dynamics in PVP-PEG mixtures

R J Sengwa & Abhilasha

Microwave Research Lahoratory. Department of Physics JNV University, Jodhpur 342005

Received 17 Jllly 2003; revised 17 Septelllber 2003; accepted 2 Decelllber 2003

The dielectric relaxation or poly(vinyl pyrrolidone) (PVP) (Mw=40000 g mol" ) and poly(ethylene glycol)s (PEG) (Mw = 200 and 400 g mol" ) and their binary mixtures in dilute solutions of dioxane has been studi ed performGd at 10.1

Gl-lz and 35°C. The average relaxation time ' .. , relaxation time corresponding to segmental mot ion " and grouf) rotations ,~ . distribution parameter a and free energy of activation f'>,F, of these mixtures has been determined. These results have heen compared with the values of re laxation times obtained earlier in benzene solutions to explore the affect ,)1' solvent environment on molecular dynamics. The va lues of a for PVP-PEG200 and PVP-PEG400 were found in the range of '" 0.18 - 0.75. The anomalous varia ti on in the val ues OfT .. and 'I, of these hinary systems has been studi ed by considering the stretching effect in PEG molecular chain length in dilute solu tions and the number of hydrogen handing between carbonyl groups of PVP monomer units and terminal hydroxyl groups of PEGs in complex heterogeneous network. Further. in hath benzene and dioxane so lvents the T~ values of PVP-PEGs mixtures are found to be nearly equal. This shows th at group rotation is independent of the nature of solvation med ium and attributed to the rotation of PEG chain ends -01-1 groups about the C-O bond in dynamic equilibrium. The relaxation times ofPVP-PEGs in dilute solutions are found to be independent of the mi xtures and solvent viscosity. The variations in dipole moment!! values of PEGs molecules in difTercl;t solvents were explored by incorporating the solute-solvent interactions in hydrogcn-bonding non-polar solvents.

[Keywords: Diclectric re laxatioll, Molecular dynamics, Poly(vinyl pyrrolidonc), Poly(ethylene glycol)] I PC Code : GO I R 22/00

1 Introduction The behaviour of bin:lry mi xtures of polar polymers

under varying co nditions of composition attracted a

considerab le interest because it helped in formulating

adeq uate models of liquid relaxation. The synthetic polymers

such as, po ly(vinyl pyrrolidone) (PVP), poly(e thylene

glycol) (PEG), poly(propylene glycol) (PPG), poly(viny l

alcohol) (PV A) etc. have a wide range of pharmaceutical,

biological and technological applications, ·l . The PVP-PEGs

binary mixtures exhibit properties not inherent in the parent

polymers in particular high elasticity and high stickiness

with the ability to form strong adhesive j o ints with a variety

of substances i.e. glasses, metals, rocks, polymeric films,

rubbers. human ski n and other biological ti ssues'. Due to

adhes ive, transport, reservoir and biological properties of

PVP-PEG mi xtures; at present these are found increas ing

applications in medicine as universal diffusion matrices for

the contro lled transdennal delivery of drugs havi ng various

struct ures, physico-chem ica l properties, and therapeuti c

aClivities3.4 . The main functional properties of these matrices

are the cont rolled rate of release of the dissolved drugs,

which are determined by the diffusion mobil i ty o f macro­

mo lecular segments and the adhes ion to human sk in". The

physical properties of polymeric mater ials are eventuall y

determined by th e ir mo lecular and superm o lecu lar

structures. The microwave dielectric relaxation studies of

the associating molecules and their mi xtures ) · 12 in non-polar

solvents confirm the steric hindrance to the internal rotation

due to hydrogen bonding and give informat ion about the

effect of so lvent environment on these rotations. Molecular

dynamics at microwave frequencies is characterized by the

observed values of 'to' 't1, 't2, a and ~FT. I n the present work ,

the experiments were conducted at I O. ~ Gl-lz, because the

dielectric loss peak of the polymers in the dilute solutions

of non-polar solvent IJ.

15 occurs around 10 GHz.

PVP at room temperature readily disso lves in an excess

amount of liquid PEG and form true transparent sol utions.

The solubility of PVP in PEG is related to the formation of

hydrogen bonds between carbony l groups o f the PVP units

and terminal hydroxy l groups of the short chains PEGs

molecules as sketched in Fig.!. Both the term inal hydroxyl

groups of PEG mo lecul es are capab le of forming the

SENGWA & AB HILAS H: DI ELECTRIC RELAXATION AND MOLECULAR DYNAMICS IN PVP-PEG MIXTURES 143

hydrogen bonds with PVP units. The PEG behaves as a cross linking agent in the PVP-PEGs super molecular matrix. In

case of binary mixtures of associating polymers. both theoreti ca l and experimental efforts have been done in the

last few years to confirm the so lute-solvent interactions. Several theoretical equations 16· 19 were also developed to

predict the competition between inter· and intramolecular hydrogen bond ing in the polymers mi xtures and the ability

of the solute to bond the polymer intermolecularly.

Earlier, an ex tensive microwave relaxa tion study of poly(ethylene glyco ls) with increas ing molecular chain length had been carried out in dilute solutions of non·polar so l vents2o

.12 and also in pure liquid stateD. Further

microwave dielectric relaxation study of PVP·ethanol in pure liquid state and in benzene soluti ons ll and binary

mi xtures of PVP·PEGs in dilute so lution of benzene24 was

also explored to confirm the formation of homogeneous and

d

PVP

heterogeneous cooperative domains and their dynamics in

the complex molecular network . In this work . the dielectric

relaxation processes in PVP·PEGs mi xtures in dilute solutions of dioxane at 10.1 GHz have been examined to

understand the effect of the so lvent environment on the

molecular dynamics. A comparative study of PVP·PEGs mi xtures in two different so lvents i.e. , benzene and dioxone, has been performed in order to explore the influence of

so lvat ion medium on their relaxation times.

2 Experimental Details

2.1 Materials

Poly(vi ny l pyrrolidone) (PVP) of molecular weigh t 40,000 g mol' l of LR grade was purchased from Loba­

Chemie, India. Poly(ethylene glycol)s of molecular weight 200 g mol' l and 400 g mol' l (PEG200 and PEG400) of LR

Fig. 1- Schemati c diagram of the proposed st ructure of a complex between hi gh·molecular-mass PVP and short-chain PEG

144 INDIAN J PURE & APPL PHYS. VOL. 42. FEBRUARY 2004

grade were obtained from S.D. Fine Chem., India. Dioxane

of spectroscopic grade was obtained from Merck India and

used as a solvent.

The binary mixtures of PYP-PEG200 and PYP-PEG400

of varying concentrations were prepared by mixing 5, 10,

15 ,20.30, 50 and 70 wt% PYP and 5, 10, 15. 20.30 and 50

wt% PYP in the weight of liquid PEG200 and PEG400.

respectively, at room temperature. The binary mixtures were

a llowed approximately for 2 da ys to di sso lv e PYP

completely in the liquid PEGs. For dielectric measurements.

five dilute solutions of varying concentratio ns of each

mixture as a solute were made in dioxane solvent.

2.2 Dielectric measutements

The dielectric constant £' and dielectric loss lO" at 10.1

GHz were determined using the short-circuited waveguide

method proposed by Heston el al.25. This method is the

highly acc urate for the measurement of £' and lO" of po lar

mixtures in dilute solutio ns of non-po lar solvent at very low

concentrations. The sta tic dielectric constant £0 was

determined by heterodyne beat method at L MHz. High

frequency limiting va lue of permittivity £ ~ was taken as the

square of the refractive index li D' which was measured with

Abbe ' s refractometer. All measurements were made at 35

± 0.5°C.

2.3 Data analysis

It has been observed that £0' £', lO" and Eoe are linear

functions of weight fractions of the PYP-PEGs binary

mixtures in dioxane solvent. The linear slopes ao' a' , a" and

a~ corresponding to £0' £', lO" and £~ versus weight fracti o ns

of the solute mi xtures in dilute solutions of dioxane are

recorded in Table I . The linear slopes were used to evaluate

the average relaxation time To' re laxation time corresponding

to segmental m o tion TI and group rotations T2 and di stribution parameter ex using Higasi 's26.~7 equations of

dilute solutions described in detai l e isewherezi . The dielectric

free energy of activation LlFT was evaluated using the Eyring28 formula. The ratios R of the hydroxyl groups of

PEG to the carbonyl groups of PYP monomer units were

also determined using the weights of PYP and PEG in the

PYP- PEG mixtures. prepared for the dielectric

measurements . The evaluated values of R, To' ex, "C I, "C2 and

LlF"C are recorded in Table 2. The molecular dipo le moment

Il of PYP and PEGs have been calculated using Higasi' s29

equation . The values of Il in dioxane and earlier evaluated

va lues of Il in benzene and carbon tetrachioridezi are

recorded in Table 3 for the comparison. Further, the values

of Kirkwood correlation factor g, which are the measure of

the extent of intermolecu lar hydroge n bonding and average

re laxat ion time "Co of PEGs molecules in their pure liquid

states2J and in dilute solutions are also recorded in Table 4 for their comparison.

Table 1- Values of the slopes or Eo' E', E" and E~ versus wt fractions of PYP-PEG mixture in dilute solutions of benzene

and dioxane at 10.1 GHz and 35°C

PVP wt% au a I a " a«

PVP-PEG200 mixturcs in bcnzene* PEG200 8.15 3. 13 1.1 5 0.00

5 7.69 3.95 2.10 -0.08 10 7.37 3.23 2.00 -0.08 20 5.81 2.46 1.41 -0.07

30 5.24 1.64 1.19 -0.07 PVP-PEG200 mixtures in dioxar.e

PEG200 7.27 4.67 2.00 0.00 5 7.27 4.44 1.70 0.20 10 6.88 4.09 1.65 0. 12 15 7.06 4.57 1.64 0.27 20 6.88 4.27 IA4 0.08 30 7.10 3.80 1.31 0.23 50 6.80 3.58 lAO 0.20 70 7A3 3.82 1.33 0. 15 100 10.00 2.63 0.72 0.28

PVP-PEG400 mixtures in benzcnc*

PEG400 5.71 3.33 1.20 -0. 15 5 5.92 2.43 1.2 1 -0.04 10 5.54 2.83 1.41 -0.06 20 5.32 2.58 1.20 -0.07 30 5.00 2.51 1.12 -0.08

PVP-PEG400 mixtures in dioxare

PEG400 4.58 3.06 1.25 0.07 5 5.09 3.69 1.45 0. 14 10 4.71 3.50 1.48 0.10 15 5.14 3.33 1.44 0.2 1 20 5.94 3.39 1.33 0.15 30 5.93 3.20 1.22 0.17 50 6.61 2.55 1.05 0.24 100 10.00 2.63 0.72 0.28

*Rcr No. 2 1

3. Results and Discussion

3.1 Molecular dynamics of PEGs in plIre liquid state

To explore the effect of non-polar so lvents o n the

molecu lar dynamics in PYP-PEG mixtures, it is worthy to

discuss the dielectric parameters of individual polymer in

the ir pure liquid state and also in different so lvents . Earlier23

eva luated values of Kirkwood correlation factor greater than

unity (Table 4) of PEG200 and PEG400 molecules in their

pure liquid state confirm that these molecules are associates

to form mu ltimers with para ll el ordering of the dipo les. The

strength of multimerization is higher in PEG200 molecules

in comparison to PEG400 molec ules. Furthet , due to greater

mo lecular s ize ofPEG400 molecules, the average relaxation

time of PEG400 should be much hi gher than the PEG200

molecules. But, it has been concluded23 that the higher chain

co iling and low strength of multimerizatio n of PEG400

SENGWA & AB HILAS HA: DIELECTRIC RELAXAT ION AN D MOLECULAR DY NAM ICS IN PVP-PEG MIXTU RES 145

Table 2- Microwave die lectric re laxa ti on times. d istributi on parameter and free energy o f acti vati on of PEGs and PVP-PEGs

PVP wt%

PEG200 5 10 20 30

PEG200 5 10 15 20 30 50 70 100

PEG400 5 10 20 30

PEG400 5 10 15 20 30 50 100

*Ref No. 2 1

Pol ymcr

PEG200

PEG400

PVP

*Ref No.21

mi xtures in oi lute so luti ons of benze ne and dioxane at 10.1 GHz and 35°C

R a ' " (ps) " (ps) " (ps) PVP-PEG200 mi xturcs in bcnze nc*

0.63 52.0 70.1 6.0 22 .30 0.37 14.4 28 . I 8.2 11 .27 0.37 20.7 32.6 9.5 5.60 0.46 24.2 40.3 8. I 3. 79 0.4 1 43 .5 47.7 10.9

PVP-PEG200 mi xturcs in dioxanc

0.33 8.2 20.7 6.7 22 .30 0.41 9. I 26. 1 6.7 11.27 0.41 9.8 26.7 6.5 7.4 1 0.40 7.7 24. 1 6.0 5.60 0.47 7.5 28.7 5.4 3.79 0.54 13.6 40.1 5.7 2.22 0.49 14.6 36.4 6.5 1. 59 0.55 15.3 42.4 5.8

0.7 5 1270.0 162.0 4.8 PVP-PEG400 mixturcs in bcnzcne*

0.49 8.5 3 1.9 5.6 I 1.17 0.50 28.3 45 .4 7.7 5.39 0.41 14.4 30.3 7.7 2.79 0.47 16.6 35.9 7.1 1. 86 0.47 14.9 35 .0 6.8

PVP-PEG400 mi xturcs in oioxanc

0 .3 1 7.7 19.2 6.6 I I. 17 0.24 6.8 15 .2 6.5 5.39 0. 18 7.1 13.0 6.8 3.63 0.30 9.0 20.0 7.2 2.79 0.45 I 1. 1 30.5 6.4 1.86 0.49 13.3 35.4 6.3 I. I I 0.57 50.4 61.4 7. 1

0.75 1270.0 162.0 4.8

Table 3- Values of degree of po lymeriza tio n 11 and dipole mo ment I..l in different so lvents

1/

4. 1

8.7

358

Bcnzc ne*

3.94

4.72

~l (D)

Dioxanc

3.49

3.89

57. 12

1..l,1' (D)

Carbon tctrachl oridc* Benzenc* Dioxane

3.28 1.95 1.73

4.20 1.60 1.32

3.05

6F, (keal mol")

3.56 2.77 2.99 3.09 3.45

2.42 2.48 2.52 2.37 2.36 2.72 2.76 2.79 5.48

2.45 3. 18 2.77 2.86 2.79

2.37 2.30 2.32 2.47 2.60 2.71 3.52 5.48

at 35°C

Carbon tctrach loride*

1.62

1.45

Table 4- Value of Kirkwood correlation factor g and ave rage re laxation time '0 at 35°C in pure liquid state*

m o lecu les in comparison to th e PEG200 m o lecul es

compensated the hindrance to the rotation of the mo lecules

as a whole and he nce the o bserved values of "Co o f PEG200

and PEG400 were fo und to be nearl y th e same in the ir pure

liquid state (Table 4 ). g '0 Cps) PEG200 1.99 116.5 PEG400 1.82 115. 1 PVP

*Ref No.23

' " (ps) in non-polar solvents

Carbon Benzene Dioxane lclrachl orioc

52.0 82 301.0 8.5 7.7 15 .8

1270

3.2 Dipole moment

Tab le 3 shows the I..l va lues of PEG200 and PEG400

mo lecules in benzene, carbon tetrac hl o ride and di oxane

146 INDI AN J PUR E & APPL PHYS. VOL. 42. FEBRUA RY 2004

so l vents. The dipole moment of PYP is determined in

dioxane onl y because it is inso luble in benzene and carbon

tetrachloride. T he comparative )..I va lues sugges t that the

structure of PEG molecules vari es significantl y in different

sol vents. Carbon tetrach loride is non-H-bonding so lvent

while the TC-el ectrons or benzene and diox ane can interact

with the protons of the so lute. Therefore. it is expec ted that

the dipole momen t o f PEG mo lecules in benze ne and

cl ioxane should be hi gher than th at in carbon tetrachloride.

Table 3 shows that the ~l values of PEG200 are found to be

significantly hi gher in benzene and dioxane in compari son

to the ~l va lue in carbon tetrach loride. But in case of PEG400

molecules. the observed values of ~l in benzene are higher

than the ).l values in carbon tetrachloride. whil e it is lower

in dioxane. which is interest i ng. These compara ti ve ~l values

suggest that the overall molecular structure o f PEGs w ith

increas ing degree of polymeri za ti on in dioxane so l vent

varies significa ntl y in compari son to its structure in benzene

and ca rbon tetrachl oride so l ve nt. Th e high ~l va lue in

benzene i n compar iso n to th e ~l va lue in ca rbo n

tetrach loride30 is also observed for poly(propy lene glyco l).

The speci fic dipole moment )..I' ll = (~t ~/II)I /\) f PEGs molecules

decreases with the increase in degree of polymeri za tion II

in all these solvents Crab le 3). The decrease in ~l s p is because

of the increase in chain coiling of the molec ules in dil ute

so luti ons of th e non-po lar sol ve nt s. T he ~l va lue of

PYP40000 in dioxane is found to be 57. 120. The hi gh ~t

va lue of PYP molecul es i s expec ted due to i ts larger

molecular size. In case of PYP, the va lue of ~t sp=3 .05 suggests

tha t the cha in co iling of PYP molecules is less in compari son

to the chain coi ling in PEG molecules in dioxane so lvent

which may be because of the presence of pyrro l idone ring

in the side of monomer units of PYP molecules.

3.3 Molecular dynamics of individual polymers in non­polar solvents

The di fferent observed val ues of Lo of PEGs in benzene,

dioxane and carbon tetrach loride (Table 4) confirm that the

molecular dynamics in dilute so lutions of non-polar solvents

is highl y influenced by the so lvent environment. It is found

tha t the "[0 va lues of PEGs in dioxane arc small er in

co mpari so n to th e To valu es in benzene an d carbon

tetrachloride so l vent wh ich suggest th at the hindrance to

the reori entat ion of the molec ule as a whole dec reases in

dioxane sol vent. Dioxane itself is a good proton acceptor

and it takes up the proton from the so lute, thereby reducing

the strength of intermolecular hydrogen bonding and hence

there is an increase in the freedom of the molecules in quasi­

iso lated state. The greater increase in chain co iling of

PEG200 in diox ane cannot be ruled out for lower To va lues.

T he hi gher value o f To of th ese mo lecul es in carbon tetrachloride is due to the higher chain ri gidity in bulk y

environment. In benzene. di oxane and carbon tetrachloride.

To va lues of PEGs decrease with increase in molecular size.

which is due to the increase in chain co il ing and also due

to decrease in the intermolecular interactions. T he viscosity

or benzene, dioxane and carbon tetrach loride arc 0.58. 0.987

and 0.89 cp, respecti ve ly at 35 °C. A lthough the viscos it y

o f dioxane is higher but the observed low va lues in this

so l ve nt confirm that th e average rela>.at ion tim e i s

independent of the so lvent viscosity and it is governed by

the so lute-sol vent interact ions and the molec ular chai n

coi ling. The "[0 va lues of PYP molecules were found to be

very hi gh in compari son to the To va lues of PEGs molecules

in dioxane. 1t is expected because of the large size and

random co iling nature of the PYP chai n.

3.4 Molecular d)lnamics in PV P-PEGs mixtures in non­polar solvent

As confirmed by g va lues, the molecules of PEGs form

multimers w ith the parallel ordering o f the dipoles in their

pu re liquid state. When the randoml y co il ed so lid PYP is

added in the liquid PEG. due to forma tion o f comp lexes

between carbonyl groups of PYP monomer un its and chai n

ends hydroxy l group of PEG molecul es. supermolecular

structure is formed as shown in Fig. I , Th is is confirmed

earl ier by FflR and DSC st udi es~ and the same structure is

used for the interpretation of di electri c rel ax ation times in

benzene so lvent24. Although the ratio of hydroxy l groups to

the carbony l groups (R = Ol-I/C=O) decreases w ith the

increase in PYP concentration but all the prepared mix tures

are ri ch from hydroxy l groups (Table 2).

In PYP-PEG mixtures Feldstein el al. " also conf irmeu

that onl y 44 '7(1 ca rbony l groups of PYP chain form s the

complexes with terminal hydroxy l groups of PEG molecu les.

T herefore, in PYP-PEGs mi xture, there is a targe number

0" uncomplexed PEGs molecules and their va lue decreases

w ith th e increase in PYP conce ntration. In PYP-PEGs

mi xtures two types of cooperati ve domains (CDs) take place

due to the transient structure o f PYP cha i n and al so due to

breaking o f homogeneous structure o f PEG molecules.

CDI'EG' wh ich includes the se lf associated PEG molecules.

and CDpVP.PEG which includes th e PEG molecu les those interact directl y w ith the carbony l groups of the monomer

units of PYP chain. In benzene. onl y PEGs i . ~;o lu b l e. while

in dioxane both PYP and PEGs arc so luble. When PYP

dissolved in PEG is added in very small quant ity in benzene

and dioxane to prepare the dilute so lu tions. the non-po lar

so lvent molecules penetrate into the inter-segmental space.

Due to which, the segments of PYP-PEG ~ u permolecular

structure unfold and get so l va ted resu l ting into di f ferent

molecular species (CD s) . The dynami cs of these spec ies al so

vary because of the different behaviour of the molecular

dynamics o f i ndi vidual po l ymer molecules in these so l vents.

SENGWA & ABH ILASHA: DIELECTRIC RELAXATIO A D MOLECULAR DYNAMICS IN PVP-PEG MIXTURES 147

3.4.1 Overall dynamics of the PEG molecules

The To va lues of PYP-PEG200 mixtures in benzene

so lvent increase with an increase in the PYP concentration

but these va lues are lower than the To va lues of PEG200

which suggest that there is a large probability of the rotation

of PEG molecu le as a whole. The increase In Tu va lues

confi rms that there is an increase in complex formation and

also the gradua l elongati on of PYP-PEG200 complex

network in dilute so lutions of ben zene due to which there

may be expansion in the chai n length of PEG200 and hence

there is increase in To va lueslJ. The PYP-PEG200 mi xtures

are studied up to 30 wt% of PYP. Above this concentration.

the phase separa ti on is observed in dilute so luti ons of

benzene.

In dioxane. the observed To va lues of PYP-PEG200

mixtures are found to be nearl y eq ual to the To value of

PEG200 up to 20 wt% PYP concentration. Above 20 wtlk.

PYP concentration. the observed To va lues of these mi xt ures

are significantly higher than the To ofPEG200. Further. there

is a sl ight increase in To at hi gher PYP concentrati on as

depicted in Fig. 2. These To va lue. can be assigned to the

rotation of PEG200 molecules. Further, these values also

suggest that the molecular dynamics o f PEG200 molecules

above 20 wt% of PYP concentration in the PYP-PEG200

binary mi xtures changes abruptly which may be due to the

formation of di fferent types of cooperati ve domai ns of PEGs

complexed w ith PYP.

In case o f PYP-PEG400 mixtures, the To va lues are

higher than the To va lues of PEG400 in benzene (Table 2

and Fig. 2). The anomalous variation in .0 with an increase

in PYP concen tration shows that in benzene. the molecular

dynamics of PEG400 molecules vary in opposite manner in

comparison to the dynamics of PEG200 molecules in PYP­

PEG200 in benzene solven t2J. Fig. 2 also shows that in case

of PYP-PEG400 mi xtures, To values also vary anoma lously

with the concentration of PYP in dioxane, but these va lues

are lower than the To va lues in benzene so lvent. A t SO wt%

concentration of PYP, the observed va lue of To in dioxane

is found to be very high which shows that the hindrance to

molecular reor ienta ti o n increases grea tl y at hi gher

co nce ntrati on. However, th e v iscosity o f PYP-PEGs mi xtures increases heav il y with in creas in g PYP

concentration in the mi xture. but the observed comparati ve

va lues of To confirm that the molecular dynamics in these

mi xtures is independen t of the so lution viscos ity. This is in

agreement w i th the results of Stockhausen and Abd-EI­

Rehim ·l1 who confirmed that the relaxation tim e o f

PYP24000-PEG200 mixtures is independent of th eir

viscosity.

3.4.2 Segmental motion

The values of distribution parameter a obtained for

III a. o

60

40

r-' 20

III a.

80

60

~ 40

20

o 20 40 60 80

PVP wt ",

Fig. '2 - Plots of ' " and " versus PVP wt '7(> PVP-PEG200 mixtu rcs: (0) dioxanc: ( e ) benzcnc PVP-PEG400 mi xtu res: (6 ) dioxane: ( .&. ) benzene

PEGs. PYP and PYP-PEGs binary mixtures in dilute

so luti ons of benzene and dioxane are fou nd in the range

0.18 - 0.75 (Table 2). A large va lue of a suggests that there

is a significant contribution of segmental motion and group

rotation to the relaxat ion processes in these systems. Earl ier.

large va lues of a of associating pol ymers and their mixtures were also observed by seyera l workersIJ.I5.3o in dilute

so lutions of non-polar solvents.

In PYP-PEG200 mi xtures, it is found that the TI va lues corresponding to segmental motion is hi gher for i'ndi vidual

polymer in comparison to the., va lues of binary mixtures in benzene solutions (Table 2). PYP concentration dependent

T, values are also depicted in Fig. 2. These values confirmed

th at the segmental motion increases in PYP-PEG200

mixtures, which may be due to an increase in the flexibility

of the PYP-PEG200 supermolecular network in dynamic

equi librium1J. But in case of PYP-PEG400. the TI va lues

are higher than that OfTI va lues ofPEG400 in benzene which is due to less tlexibility in PEG400 in binary mi xtures2J. In

dioxane, the LI values of PEGs molecules are lower than the.1 va lues of PYP-PEGs binary mi xtures. This suggests

148 INDI A J PURE & APPL PH YS. VOL. 42. FEBR UA RY 2004

that dioxane accepted more protons from the - OH groups

and thereby reducing the fo rmat ion of interm olecular

hydrogen bonding. Due to thi s, there is a decrease in the

hindrance to the segmental motion of PEGs molec ules.

Fi g. 2 shows that at lower concentrati on of PVP, the 1 1

va lues are o f the order of 1 1 of PEG200 but at hi gher

concentration these va lues are larger. The observed va lues

show thatthe fl ex ibi lity of molecular chain in PVP-PEG400

rni xtu res red uces at hi gher concentrations in dioxane. The

1:1 va lues of PVP in dioxane are much hi gher than the 1 1

va lues of PEGs molecul es which al so shows that there is

higher ri gidity in the segmental motion of PVP molecules

which may be due to the presence of side pyrrolidone ring

in the PVP chain monomer units.

3.4.3 Group rotations

The observed 1 2 values correspondi ng to group rotations

of PEGs and PVP-PEGs mi xtures may be ass igned to the

rotation o f PEG chain ends hyd roxy l groups around the

C-O bonds in dynam ic equ ili briu lll and also the ro tation of

pyrro lidone ring of PVP monomer units. The eva luated 1 1

values of PEG200, PEG400 and PVP-PEGs mi xtures in

d ilute so lutions of benze ne and dioxane are found to be

almost equal. This suggests that the group rotations are

unaffec ted by the so lvent v iscos ity and the constituents o f

binary mi xtures. The 1 1 values of PVP in diox ane may be

ass igned to the rotation o f the py rro li done ring of PVP

monomer uni ts.

The dielectr ic re laxation free energy of ac ti va ti on 6.FI

for these binary mi xtures is found in the range '" 2.30 - 3.56

kcalmor l. These va lues are in agreement wi th the ac ti vation

energy needed fo r the breakage of hydrogen bonds in dilute

so lutions of non-polar so lvents. Furthcr, these va lues suggest

that the molec ular dynamics in PVP-PEGs networks is in

dynami c equil ibri um due to con ti nuous breaking and

reforming of hydrogen bonds.

4 Conclusions

From the comparat i ve systemat ic m icrowa ve dielectri c

re laxation study of PVP-PEGs mi xtures with varying chain

length of the PEG molecules, it is inferred tha t the molecular

dynam ics in these systems is governed by the complex

formation between carbonyl groups anc! hyd roxy l groups

in the PVP-PEGs matr ix anc! the molecular chain co iling.

Further. the molecu lar dynamics is also inriuenced by the

concentrat ion of the PVP and chain length of the PEG

molecules. Environment o f the different non-polar so lvents

signi fica ntl y changes the dynami cs of the reori entati on of

the molecule as a whole and segmental motion. In dilute solutions of PVP-PEGs mi xtures, there is reori entation o f

PEG mo lecul es as a w ho le and th ese may be eit her

uncomplexed PEGs molecul es or the complexed PEGs

molecules w ith PV P in dynamic equili brium. The viscosity

of the solution has no effect on the molecular dynamics in

these binary systems.

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