kirsten l. genson et al- interfacial micellar structures from novel amphiphilic star polymers

8/3/2019 Kirsten L. Genson et al- Interfacial Micellar Structures from Novel Amphiphilic Star Polymers

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symmetr y depending upon th eir chemical composition andmolecular weight.11,12

Very recently,Gnanou and Duran 13 described interfacialstru ctu res of core-shell PS-PEO star am phiphile withth ree diblock ar ms deposited on a solid surface. Sph ericalmicellar stru ctur es imaged by atomic force microscopy(AFM) on solid substrates were very sim ilar to thoseobserved in linear an alogues becau se, probably, each PEOblock in that core-shell stru ctu re had two junction pointsconnecting it to th e core a nd th e peripheral P S block. In

general, this intrinsic feature of core-s h e ll s t a r a m -phiphiles imposes a significant limitat ion on th e abilityofincompat ible arms to rearr ange in response to selectiveenvironment. An excellent alternative in that respect isoffered by heteroarm star s where both t ypes of arms a redirectly linked to the core and therefore have only one

junction point.

Because of synthetic challenges, t he heteroarm am -phiphilic stars with alternating dissimilar arms are notreadily available, and only few exam ples have beendescribed t o date with m ain attention paid to t he bulk s t r u ct u r e a n d p h a s e b eh a v ior of t h e s e p o ly m er s a n dsurface films cast from solution.14 AFM studies of singlemolecules of PS-P2VP star polymers demonstrated thehetroarm star polymer showed a pronounced segregationof the polymer arms in selective solvent conditions, andthe inclusion of metal clusters a mong th e polymer ar msallowed for direct calculation of the number of arms percore. The PS-PAA heteroarm amphiphile with a well-defined rigid core was recently reported by Zubarev etal.,4 who observed r ich self-assembling pr operties inselective solvents. Both spherical and cylindrical super-m icelles were found in water a nd m ethanol as well asreverse supermicelles in n onpolar organic media. However,the qu estion oft he interfacial behavior oft his amph iphilicstar polymer with shape-persistent core remains open.

Here we report our studies on th e interfacial behaviorofth is star block copolymer containing 12 alternat ing arm sof PAA and PS att ached to a rigid aromatic core (Figure

1).4

The nu mber of monomeric unit s was m ) n ) 25 forsymmetric star block copolymer and m ) 30 and n ) 40for asymmetric star amphiphile. We examine the struc-tu res formed at th e air-water and th e air-solid interfacesin attempt to reveal the influence ofmolecular ar chitectur eand the volume fraction of different blocks on the self-organization pr ocess at air-water interface and solidsurface in comparison with regular linear PS-P AA block copolymers.

E x p e r i m e n t a l S e c t i o n

The synt hesis, chemical composition, a nd solution pr opertiesof heteroarm star amphiphiles (PAA25)6-s-(PS 25)6 and (PAA30)6-s-(PS40)6 have been described in detail elsewhere.4 Monomolecularfi lm s of t h e s t ar p ol ym er were p rep ared b y t h e L an g m u ir

technique on an RK-1 trough (Riegel& Kirstein,GmbH) according

t o t h e u s u al p ro ced u re ad ap t ed i n ou r l ab .15 T h e t ro u gh wasplaced in a laminar flow hood. The compounds were dissolvedin chloroform (Fisher, reagent grade) to ∼0.01 mmol/L concen-tr ation. The solution was spread over the wat er (NanoPur e,>18MΩ cm) subphase and dried for 30 min before compression.Monomolecular films were deposited u sing th e Langmuir-Blodgett (LB) technique on cleaned silicon substrates (Semi-conductor Processing Co.). Silicon wafers were treat ed in“piran ha” solution (30% h ydrogen peroxide:94% su lfur ic acid,1:3, chemical hazard!) according t o the standar d pr ocedure.16

Ellipsometric m easurements of monolayer thickness were

carried out with a COMPEL automat icellipsometer (InOmTech,Inc.). Contact angle measurements were performed with thesessile droplet met hod.Ima gingoft he monolayers was performedwith AFM microscopes, Dimen sion-3000 an d Multimode (DigitalI n s tr u m e n t s), i n t h e l ig ht t a p pi n g m o d e a ccor d in g t o a nexperimental procedure described elsewhere.17-19 Surfaces of LBmonolayer wer e probed at severa l ran dom locations with widelyvarying scan sizes from 0.3 to30 µm. The geometrical parametersofa ll molecules were estima ted from molecular models built witht h e Mat eri al s S t u d i o 3 .0 s o ft ware p ack ag e an d t h e C eri u s 2

program on a SGI workstation. The combination of moleculard yn am i cs an d en ergy m i n im i zat i on was u s ed t o g en erat emolecular models.

X-ra y reflectivity measu remen ts oft he Langmu ir monolayersdirectly at t he air-water inter face were conducted on the liquid-surface X-ray spectrometer at the 6ID beamline at the Advanced

Photon Source synchrotron at Argonne National Laboratoryaccordingt ot he usual procedure adapt ed in our lab and describedin the literat ure.20-22 Monomolecular films were pr epar ed by theLangmu ir technique on a temper atu re-contr olled, Teflon tr oughsealed within a h elium filled cham ber to reduce the backgroundscattering from air a nd reduce oxidation rea ction that can damagethe monolayer. A downstrea m silicon double-crystal monochr o-mator was used to select the X-ray beam at the desired energy( λ ) 0.0776 n m).

R e s u l t s a n d D i s c u s s i o n

Sur fac e Be havior . Both h eteroarm sta r block copoly-mers studied here possess well-defined star shape withdissim ilar a rm s attached to t he r igid, shape persistentcore as visua lized by molecula r models pres ent ed in Figur e1. In an extended conforma tion an d face-on orient ation,the molecules occupy very large surface areas with thed ia m e t er of 1 7 a n d 2 4 n m for (P AA25)6-s-(PS 25)6 a n d(PAA30)6-s-(PS40)6 molecules,r espectively (Table 1). In th isconformat ion, the r igid core occupies a sma ll surface areaas compar ed to polymer arm s, an d significant free sur faceis available between adjacent arms.

These molecules formed st able Lan gmuir monolayersa t t h e a i r-water interface and showed th e classicalpressure vs molecular area (π - A) behavior without anysignificant plateau at int ermediate pressur es (Figure 2).

(11) Zhan g, W.; Shi, L.; An, Y.; Shen , X.; Guo, Y.; Gao, L.; Liu, Z.;He, B. La n g mu i r 2003, 19 , 6026.

(12) (a) Zhan g, L.; Eisenberg, A. S cience 1995, 26 8, 1728. (b) Li, S.;Clarke, C. J .; Lennox, R. B.; Eisenber g, A. Colloids Surf. A 1998, 13 3,191. (c) Terrea u, O.; Luo, L.; Eisenber g, A. La n g mu i r 2003, 19 , 5601.(d) Li, S.; Clarke, C. J .; Eisenberg, A.; Lennox, R. B. Thin Solid Films1999, 35 4, 136. (e) Schnit ter, M.; Engelking, J .; Heise, A.; Miller, R. B.;Menzel, H. Macromol. Chem. Phys. 2000 , 201 , 1504.

(13) Francis, R.; Skolnik, A. M.; Carino, S. A. R.; Logan, J. L.;Under hi l l , R. S.; Angot , S.; Tat on, D.; Gnanou, Y.; Dur an, R. S.

Macromolecules 2002, 35 , 6483.(14) (a) Kiriy, A.; Gorodyska, G.; Minko, S.; Tsitsilianis, C.; Ja eger,

W.; Stam m, M. J. Am . Chem. S oc. 2003, 125 , 11202. (b) Gorodyska, G.;Kiriy, A.;Minko, S.;Tsitsilianis, C.;Sta mm, M. Na n o Le t t . 2003, 3, 365.(c) Kiriy, A.; Gorodyska, G.; Minko, S.; Stamm, M.; Tsitsilianis, C.

Macromolecules 2003, 36 , 8704.

(15) (a) Peleshanko, S.; Sidorenko, A.; Larson, K.; Villavicencio, O.;Ornatska, M.; McGrath, D. V.; Tsukruk, V. V. Thin Solid Films 2002,406 , 233. (b) Sidorenko, A.; H ouphouet-Boigny, C.; Villavicencio, O.;McGrath , D. V.; Tsukru k, V. V. Thin Solid Films 2002, 410 , 147.

(16) (a) Tsuk ruk , V. V. Adv. Mater. 2001, 13 , 95. (b) Tsukru k, V. V.;Bliznyuk, V. N.; Hazel, J .; Visser, D.; Everson, M. P . La n g mu i r 1996,12 , 4840. (c) Bliznyuk , V. N.; Everson, M. P.; Tsukr uk, V. V. J. Tribol.1998, 120 , 489.

(17) Tsukruk, V. V. Rubber Chem. Technol. 1997, 70 , 430.(18) Tsukruk, V. V.; Reneker, D. H. Polymer 1995, 36 , 1791.(19) Ratner, B., Tsuk ruk , V. V., Eds.; Scanning Probe Microscopy of

Polymers; ACS Sym p. S er. 1998, 69 4.(20) (a) Vakn in, D. In Methods of Materials Research ; Kaufmann, E.

N., Abbaschian, R., Baines, P . A., Bocarsly, A. B., Chien, C. L., Doyle,B. L., Fultz, B., Leibowitz, L., Mason, T., San ches, J. M., Eds.; JohnWiley & Sons: New York, 2001; p 10d.2.1. (b) Vaknin, D.; Kelley, M.S. Biophys. J. 2000, 79 , 2616.

(21) Weissbuch, I.; Leveiller, F.; Ja cquemain, D.; Kjaer, K.; Als-Nielsen, J.; Leiserowitz, L. J. Phys. Chem. 1993, 97 , 12858.

(22) (a) Larson, K.; Vaknin, D.; Villavicencio, O.; McGrath, D. V.;Tsukruk, V. V. J . Ph y s. Ch e m. B 2002, 10 6 , 7246. (b) Genson, K. L.;Vankin, D.; Villavicencio, O.; McGrath , D. V.; Tsukruk , V. V. J. Phys.Chem. B 2002, 106 , 11277.

N ovel A m ph iph ilic S tar Polym ers L an gm u ir, V ol. 20, N o. 21, 20049045



This shape of the isother ms indicated an absence of anysignificant stru ctural rearr angements within compressedmonolayer usually observed for amphiphilic block copoly-mers wit h compara ble composition (content ofh ydrophilicblock of about 30%, Table 1). Detectable increase in thesurface pressure was observed for the surface area permolecule below 20 nm 2 t h a t i s m u c h s m a l l e r t h a n t h esurface area per molecule estimated for extended con-form at ion and face-on orient at ion (Figure 1).Th is indicatessignificant overlap of th e molecules u pon compr ession orreorganization of their conform ation under very lowpressures.

The steady growth of pressure as the cross-sectionalarea decreased with the l im iting value for (PAA25)6-s-(P S25)6 star slightly lower (9 nm 2) than tha t for (PAA30)6-

s-(PS40)6 star (10 nm2

) (Figure 2). These values are closeto th e cross-sectiona l ar ea of the r igid core region (shownin Figure 1 as dashed circle) calculated to be 9.6 nm 2

(excluding the space between adjacent arms) for either12-arm star polymer. The very modest increase in cross-sectional area for th e larger a symmetric star moleculesand very sim ilar shape of the isotherm s indicated themolecular packing is only slight ly affected by th e bulkierPS chains. On the other han d, both values ar e close to th ecore area, indicating th at dense latera l packing of rigidcores is a defining factor in the formation of condensedmonolayer with both types of arm s playing a minor roleat highest packing densities (Figure 1).

This int erfacial behavior und er compression suggeststhat the PAA and PS arm s are disassociated from the

F i g u r e 1 . (a) Chemical formula of (PAAm)6-s-(PSn)6, (b) molecular model of (PAA25)6-s-(PS25)6, (c) molecular model of (PAA30)6-s-(PS40)6, and (d) side view of (PAA30)6-s-(PS40)6 with a spat ial separat ion ofh ydrophobicP S and hydrophilicP AA arm s placed aboveand below the aromatic core, respectively. Dashed circle represents the calculated core area.

9046 L an gm u ir, V ol. 20, N o. 21, 2004 Gen son et al.



plane ofthe core upon compression tothe condensed state(see a simple model with oppositely extended arms inFigure 1). It is clear th at actual state ofthe arms is differentfrom th is simple molecular graphics representat ion a ndshould include different levels of ordering. Consideringthis surface behavior and the architecture of the starmolecule and literature data, it is reasonable to expectthat hydrophilicPAAarm s were submerged into the water

subphase whereas the hydrophobic PS arms are packedabove th e air-water interface,effectively pinn ing th e coreplane. The calculated cross-sectional ar ea on the largerPS arms in coiled state was 2.17 nm 2 per arm while theshort er PS ar ms should occupy 1.63 nm 2 (Table 1).Ta kinginto account the six PS arm s atta ched to each arom aticcore,t he cross-sectional area per molecule with coiled arm sfor (PAA30)6-s-(PS 40)6 was estimated as 13.1 nm 2 and 9.8n m 2 for (PAA25)6-s-(PS 25)6, which is close but still belowthe onset of forma tion of the condensed monolayer.

The decrease in cross-sectiona l area below these valu esshould force the flexible arms to elongat e in the verticaldirection, th ereby decreasin g th e effective cross-sectionalarea per molecule. However, we suggest that the trans-

forma tion to tr uly brus hlike packing regime is not possiblein t his system due t o the presence of rigid disklike core,limiting effective gra fting den sity of flexible chain s t o 1.6n m 2 /chain. It is important t o emphasize th at ar ms are note ma n a t in g fr om on e a n d t h e s a me p oi n t a s u s u a ll yhappens in conventional star polymers but rather fromsixequidistan t peripheral points ofa fairly large and rigidcore (4.2 nm ) th at keeps the junction points of arm sspatially separated. In addition, considering th at t he PSb lock a t r oom t e m pe r a t u r e i s w el l b e low t h e g la s str an sition (even accoun tin g for low molecula r weight a ndpossible presence of tr apped r esidual solvent) a nd them obility and com pressibility of the PS phase will beseverely limited, it is safe to suggest th at th e stat e of PSblocks will stay virtually unchanged. As a result, even at

high pressure the PS arms will stay in a collapsed statecovering th e aromatic core.Th e same limitat ion is imposedby the core on the hydrophilic PAA arms submerged inthe wat er subph ase, but because of their dissolved stat ethey rema in mobile enough to chan ge their stat e (to thatallowed by the limiting grafting density) under compres-sion. We suggest tha t this key difference between the star -shaped molecules with sizable r igid core an d diblock amph iphiles th at do not ha ve the incompressible core andtherefore can undergothe pancake-brush tran sformat iona t h i gh s u r fa ce p r es su r e s d efin e s t h e s h a pe of t h eLangmuir isoth erms observed here. To corroborate t hishypothesis and toobtain insights intothe structure ofthemonolayers, we used X-ray reflectivity measurement s a ndAFM imaging as discussed below.

I n t e rf a c ia l O rg a n i z a t io n o f L a n g m u i r M o n o -l a y e r s . Because t he dat a for st ar molecule with sh orterar ms were not su fficient qu ality, we will concent ra te her eon (PAA30)6-s-(PS 40)6 molecules. The X-ray reflectivitycurves for this molecule displayed mult iple minima at allsurface pressures (Figure 3). X-ray reflectivity measure-ment for the larger 12-arm star polymer before compres-sion showed th ree well-defined minima, signifying anorganized m onolayer forma tion. At h igher su rface pr es-sure, the minima became more defined and the spacingdecreased, indicating an increase in monolayer thickness

and th e chain elongat ion (Figure 3). The overall angu larvariation of intensity decreased much faster at higherpressures which suggested the sharpening of the mono-l a ye r i n t er fa ce s. T h es e r e su l t s d e mon s t r a t e t h a t t h e(PAA30)6-s-(PS40)6 molecules formed dense uniform mono-layers at all pressures with the overall thickness increas-ing as the surface pressure is raised.

The reflectivity data were modeled using a four-boxmodel as presented in Figure 4 along with examples of spread and compressed molecules built with considerationof most critical pa ram eters (overall th ickness, separ atethicknesses of PS and PAA phases, and cross-sectionalarea per molecule) (Table 2). These boxes with constantelectron d ensity represented the submerged PAA chains,th e core region, th e mixed collapsed, an d th e extended PS

T a b l e 1 . M o l e c u l a r D i m e n s i o n s o f t h e 1 2 -A r m S t a r

P o l y m e r s

(PAA30)6-s-(P S40)6

(PAA25)6-s-(P S25)6

cor e dia m et er (n m ) 4.2 4.2t ot a l dia m et er (n m ) 23.9 17.4P S ext en ded len gt h (n m ) 10.0 6.6P AA ext en ded len gt h (n m ) 8.7 6.8P S en d-t o-en d dist a n ce (n m ) 2.3 2.0P AA en d-t o-en d dist ance (n m ) 1.9 1.8

area of core (nm2

) 9.6 9.6 A 0 (n m 2) 10.0 9.1area of coiled PS a rm (nm2) 2.2 1.6area of coiled PAA arm (nm 2) 1.6 1.3volume of PS ar m (nm 3) 6.5 4.1volume of PAA arm (nm 3) 3.1 2.6P AA volu m e fr a ct ion 0.29 0.35

F i g u r e 2 . π - A isotherm of (PAA30)6-s-(PS40)6 (solid line) and(PAA25)6-s-(PS25)6 (dashed line).

F i g u r e 3 . X-ray reflectivity data and model for (PAA30)6-s-

(PS40)6 at different surface pressures. Data represented bysymbols an d m odel repr esented by line. Inten sities offset forclarity.




chains.23 At low sur face pressu re a nd large cross-sectionalarea electron density distr ibution confirmed a clearlyorganized monolayer with defined layered structure. Theslight difference in electr onic densit y at t he water su rfacefor th e first two blocks (0.365× 10 3 and 0.368× 10 3 e/nm 3,respectively)corresponding to PAAarm s and the ar omaticcore indicated a slight shift in density of the polymericchains. The confinement ofth e hydrophobicar omaticcorewithin a plane created a densely packed layer above thehydrophilic PAAchains. Considering the th ickness oft hefirst box of 1.25 nm close to t he PAA chain dimensions

(Tables 1 and 2), we can suggest th at the h ydrated P AAchains a re ran domly coiled underneat h th e aromatic core

(Figure 4a). The densest region of the film was located

near th e air-water int erface associat ed with th e aromaticcores (second box, Table 2). The PS chains (third box)

form ed a dense layer of 1.7 nm thick above the watersurface, which is close to but slight ly lower th an the PS

ran dom coil size an d indicates th e collapsed stat e of thePS chain s with den sity close to but slight ly below th at for

th e bulk PS. The presence ofth e four th boxwit h extr emelylow electron density (0.048 × 10 3 e/nm 3) indicated the

extension ofsome PS chains beyond the collapsed PS layer(Figure 4a).

(23) Gregory, B. W.; Vaknin, D.; Gray, J. D.; Ocko, B. M.; Stroeve,P.; Cotton, T. M.; Struve, W. S. J. Phys. Chem. B 1997, 10 1, 2006.

Figure 4. Boxm odels of(PAA30)6-s-(PS40)6 at (a)low-pressure regime (2mN/m)a nd (b)high-pressure regime (20 mN/m). Correspondingmolecular models shown to the right.




Upon compressing the monolayer to the low pressureregime (from 0 t o 2 mN/m), the four-box model showed anoverall increase in length and electronic density for allboxes, indicating the monolayer became more denselypacked as the chains elongated. The overall t hicknessincreased from 5.2 nm for th e monolayer before compres-sion to 5.7 nm at h igher pressu re of 2 mN/m (Table 2). Thecross-sectional ar ea decrease forces the cha ins to elongat eslightly in the vertical direction to compensate for theoverall decrease in area. More significant, all four boxesincreased in electronic density with a clearer distinctionbetween the first two boxes. These two boxes containingth e PAA chains became slightly denser alth ough the boxeslengthened moderat ely(from 1.05 to 1.60n m for th e secondbox) while the th ird an d fourt h boxes containing t he PSchains remained with similar th ickness while increasingin density.

The overall thickness of the Langmuir monolayer aswell as t he density increased for a ll layers a s th e sur facepressure increased from 2 to<20 mN/m as was indicatedabove (Figure 3, Table 2). At the intermediate pressures,the monolayer was modeled using four box models withthe steadily increased thickness and electronic densityfor t h e fi r st b ox fr om 1 .3 t o 2 .8 n m , i n di ca t i n g t h esignificant extension ofPA A chains submerged in the water

subphase (Figure 4b). The thickness exceeds the PAAchain dimens ions point ing out on initial elonga tion of th esubm erged PAAchain s. The second boxa ssociated mainlywith the aromatic cores remained the densest region of the layer. The third box associated with the PS chainsvery modestly increased in th ickness.

For th e highest sur face pressures corresponding to thecondensed state of the monolayer (g20 mN/m), two- an dth ree-box models were used to an alyze th e dat a. Alth oughfitting of experimental data for complex m ultilayeredstru ctu res is a lways a challenge, the pr esence of severalwell-defined minima and comparison with expected mo-lecular dimen sions an d electron densities make the resu ltsobtained very una mbiguous. The first box was assignedto the submerged PAA chains, and the second and third

boxes were assigned to the collapsed and extended PSar ms, resp ectively (Figur e 4b). The lack of resolution a ndsmall difference in electronic densities at higher surfacepressures did not allow the aromatic core region to bedistin guished from th e hydr at ed and densely packed PAAchains for the highest pr essure stu died here. The higherelectronic density of the first box inferred the molecularcore was located in the PAA box increasing the effectivedensity (Table 2). Unliketh e lower surface pressures wherea dense region contained the aromatic core and parts of t h e P A A a n d P S c h a i n s , t h e m o d e l f o r t h e 2 0 m N / mm onolayer dem onstrat ed a clearly defined transitionbetween the PAA and PS chains. The lower electronicdensity of the second box (0.317 × 10 3 e/nm 3) confirmedth e presen ce of collapsed PS ar ms locat ed above the air-

water int erface, thereby pinning the a romatic core at th einterface. Upon reaching the highest surface pressure th evertical electron densit y distribut ion can be described th ebest with a two-boxm odelwith th e insignifican t differencein density between th e PAA and PS ph ases, except somevery low-density region on a top of the layer indicating,probably, initial stage ofth e monolayer collapse. The totalthickness of the m onolayer increased to 7.9 nm at thehighest su rface pressur e (Table 2). Considering th at thedensity of packing of PAA and PS became indistinguish-able, we cannot estim ate separate PS and PAA layerthicknesses in this state.

M o n o l a y e r O rg a n i z a t i o n a t S o l i d S u r fa c e s . AF Mimaging of LB monolayers deposited on a hydrophilicsilicon surface provided additional informa tion aboutmorphology of monolayers, assuming that the monolayermorphology was not altered by the monolayer transferthat was usually suggested for organic monolayers.24,25

As was observed, at very low compression (large surfacearea per molecule) the molecules a lready formed initialmicellar m orphology. The monolayer deposited a t 20 nm 2

per molecule (twice th e limiting cross-sectional area)showed large circular domains of various sizes packedloosely with limit ed conta ct between th e doma ins (Figure5a).Th e average diameter ofth e domains was close to 400

nm , with som e structures as large as several m icronsacross. The exclusion of the larger domains resulted inthe calculated m ean diam eter of 300 n m , a ccurat elyrepresenting t he overall size of the domains. This surfacemorphology suggested the spont aneous long-ran ge order-i n g o f t h e m o l e c u l e s a t t h e a i r-water interface. Thecircular domains app eared to be two-dimensional micelleswith total th ickness 2.5-3 nm as determ ined by AFMtha t was close to ellipsometry m easuremen ts which wasindicative of pancakelike organization within th e mono-layer with PAA chains spreading thin beneath the PSdomains.12 The individual domains appear ed very un iformwith a low surface m icroroughness (below 0.15 nm ),indicatin g molecularly flat d omains with no indication of ordered intramonolayer structure (Figure 5b). The de-

crease in the thickness of the m onolayer as they weretra nsferred from the air-water interface to th e air-solidinterface was appar ently caused by the dehydra tion andcollapsing ofth e PAAchains in contact with solid substra te(see b elow).

U p on com p r e ss in g t h e m on ol a ye r t o 5 m N /m , t h emolecules ma intained the two-dimensional circular mi-cellar s tructure (Figure 5c). The num ber of circular

(24) (a)Ulman,A. An Introduction toUltrath in Organic Films: From Langmuir - Blodgett to S elf-Assembly ; Academic Press: Boston, 1991.(b) Spra tte, K.; Riegler, H . Makromol. Chem., Macromol. S ymp. 1991,46 , 113.

(25) (a) Roberts, G., Ed.; La n g mu i r - Blodgett Films; Plenum P ress:New York, 1990. (b) Petty, M. Langmuir - Blodgett Film s: An In troduc-tion ; Cambridge University P ress: New York, 1996.

T a b l e 2 . C o m p a r i s o n o f B o x M o d e l P a r a m e t e r s o f P A A6- b -P S 6 S t a r P o l y m e r

surface pressure (mN/m)

box m odel pa r a m et er s 0 2 10 20 22

fir st box len gth (n m ) 1.25 1.36 2.77 3.24 6.25electronic density (×10 3 e/nm 3) 0.365 0.377 0.376 0.362 0.386

secon d box len gt h (n m ) 1.05 1.60 1.77electronic density (×10 3 e/nm 3) 0.368 0.387 0.575

t h ir d box len gt h (n m ) 1.75 1.76 1.48 2.72electronic density (×10 3 e/nm 3) 0.315 0.327 0.331 0.317

fou r th box len gt h (n m ) 1.11 0.934 1.65 1.37 1.55

electronic density (×10 3 e/nm 3) 0.048 0.068 0.108 0.053 0.05su r fa ce r ou gh n ess (n m ) 0.20 0.173 0.264 0.281 0.28t ot a l t h ickn ess (n m ) 5.16 5.65 7.67 7.33 7.8P S t h ickn ess (n m ) 2.86 2.69 3.13 3.18 3.09




domains increased as t he average diameter decreased to200 nm. The circular domains partially fused, forminglarger a ggregates while the su rface coverage increased tonearly 50% of th e total su rface ar ea t hat becomes closeto the “jamming”limit ofth e random sur face arrangementof circular domains.26 The th ickness of the PS dom ainsremained n early the sam e (2.7 nm). The furth er increasein t he surface pressure to 10 m N/m showed additionaldensificat ion oft he monolayer (Figure 6a). The monolayerappeared a lm ost uniform with sm all irregular defectsp r e se n t t h r o u gh ou t t h e fi lm i n di ca t i n g t h e ci r cu l a rdomains merged intolarger pancakelike monolayers withonly a slightly higher thickness (3.1 nm). This value wasstill several t im es sm aller than the m axim um possiblelength of PS and PAA arms in their fully extended con-

form at ions an d indicat es their collapsed an d spread sta te.Upon further increase in the surface pressure to 30

m N /m t h e ci r cu l a r d om a i n s m e r ge d i n t o a v ir t u a l lycontinuous monolayer (Figure 6b). The redu ction in theaverage diameter of the circular structures was offset byincreasing in th eir num ber. The two-dimensional micellesformed a weblike continu ous morphology similar to th elarger aggregates composed of partially fused circulardom ains seen in Figure 6c. The coverage area of thesubstrate was calculated to be higher than 80% in mostlocations th at exceeded the limits for dense surface packingof circular domains an d indicated t heir merging with t he

forma tion ofun iform monolayer. The height of the domainsdetermined from AFM cross sections increased t o 3.5-4.0 nm wh ile the overall effective thickness obta ined frome ll ip s om e t r y i n cr e a se d t o n e a r ly 5 n m , i n d ica t i n g ap r e se n ce of a t h i n u n d e r ly in g l a ye r b el ow t h e t w o-dimens ional micelles. An explana tion for th is app eara nceof an underlying layer was determ ined t o be flattenedmolecules spread bet ween and un der t he circular micelles.This structural feature may be responsible for the factthat the observed surface micelles possessed an exceed-ingly high two-dimensionality with diameter being nearly100 times greater th an t heir thickness. It was interestingto note that such circular micelles represent 2D micro-str uctur es with facial amph iphilicity (hydrophobict op andhydrophilic bottom).Th is was in contr ast t o much smaller

surface micelles usually formed by linear diblocks.7

Finally, the sma ller symm etric star polymer (PAA25)6-s-(PS25)6 formed incomplete, defective domains with noindication of the circular two-dimensional micelles seenfor th e larger molecule (Figure 7a). At moderat e pressur e(10 m N/m ) the m olecules form ed large dom ains up toseveral microns in diameter with small, rounded holes of varying size distributed throughout the surface (Figure7a). The measur ed effective th ickness of the monolayerwas 2.6 nm , and the dom ain height was 2.3 nm . As th esurface pressure was increased to 30 mN/m, th e sur facecoverage increased with th e effective th ickn ess increasingto 3.7 nm. The domain s form ed a den se morph ology witha high concentration of cracklike defects running acrossmicron-sized surface areas (Figure 7b).

(26) Karim, A.; Tsukr uk, V. V.; Douglas, J. F .; Satija, S. K.; Fetters,L. J.; Reneker, D. H.; Foster, M. D. J . Ph y s. I I 1995, 5, 1441.

Figure 5. AFM images oft wo-dimensiona l micelles of(PAA30)6-s-(PS40)6 formed (a, b) at 0 mN/m su rface pressure (20 nm 2 pe rmolecule)an d (c) at low surface pressure (5 mN/m).The z rangefor topography (left) was 15 nm and for phase (right) was 20°.

Figure 6. AFM images oft wo-dimensional micelles of(PAA30)6-s-(PS40)6 formed at (a) moderate surface pressure (10 mN/m)and (b, c) high surface pressure (30 mN/m). The z range fortopography (left) was 10 nm and phase (right) was 10°.




G e n e r a l D i s c u s s i o n

Here, we will discuss m ainly the results related toasymmetric, long chain (PAA30)6-s-(PS 40)6 star polymerbecause of th e insufficient quality of the data for starmolecule with shorter chains. It seems that the decreaseofth e length oft he PAAan d PS arms in (PAA25)6-s-(PS 25)6

greatly influenced the interfacial behavior of th e star block copolymer, making its monolayer structure much lessdefined. Significan t decrease in th e PS block cont ent a ndthe length ofth e both types of chains r educed segregation

l ev el , t h e r eb y d is r u p t in g t h e for m a t i on of t h e t w o-dimensional circular micelles.Comparison of the monolayers parameters measured

by ellipsometr y, cont act angle, AFM,a nd X-ray reflectivityallows for general conclusions to be ma de on stru ctu ralreorganization ofth e monolayer in different p hysical stat es(Figure 8). In fact, this com bination of experim entaltechniqu es brings t o discussion very differen t bu t closelyrelated parameters ofthe monolayer morphology: overalleffective thickness of the monolayer at the air-waterint erface (X-ray r eflectivity) an d s olid s ur face (ellipsom-etry), the separate thicknesses of top PS dom ains andbottom PAA phase at both water (X-ray reflectivity) andsolid (AFM in combination with ellipsometry) surfaces,and the surface coverage with PS domains on the solid

surface along with the presence of the hydrophobic PSchains (AFM and cont act an gle). The tr ends observed forall these param eters are very instructive and allow tomak e nontrivial conclusions about su rface microstr uctur e.The results discussed here are very consist ent considerin gthat theywere obtained with four independent techniqueson two independent sets of samples at different surfaces.

The analysis of this tr end shows a consistent a nd sha rpincrease in the effective monolayer thickness at both waterand solid sur faces at very low sur face pressure below 5m N/m (Figure 8). For surface pressures higher than 5mN/m virtu ally constan t t hickness is observed. Consider-ing significant in crease in sur face coverage in th is ran ge,we can attribute m ost of the increase observed to theform ation of denser population of t he dom ains with

virtually u nchanged t hickness (except very low sur faceareas for pressures below 2 mN/m). This conclusion isalso supported by isotherm data which shows that thesur face area per molecule at su rface pressu res of5 mN /man d higher is very close to A 0, which is m ainly defined bythe rigid cores as we discussed above (Figure 2). Thecontact an gle initially increases sh arply to 95° and staysconstant in a whole range of pressures (Figure 8). Thisvalue is identical to th at observed for grafted PS chainsin solid stat e27 and indicates uniform composition of themonolayer surface composed of PS chains.

However, the thickness of the condensed monolayer atthe wat er sur face and on th e solid substra te differs morethan twice: 7.9 ( 0.5 nm vs 3.1 ( 0.5 nm (Figure 8). The

m uch sm aller total thickness for solid-supported LBmonolayers as compar ed with th e same monolayer at th ea ir-water interface indicates significant difference ininternal microstructure of the monolayers. Direct com-parison of the total thickness of the monolayer and theth ickn ess of th e PS pha se immediat ely allows concludin gtha t th e PAA chains su bmerged in water are extended to4.8 ( 0.6 nm but a re spread in th e thin layer of around0.3 nm at the solid surface. Constant extension of thePAAcha ins over almost a whole region ofsu rface pressu retest ed (from 5 to 30 mN/m) can be at tr ibut ed to the factoroflimited grafting density imposed by the pr esence oft he

(27) Luzinov, I.; Julth ongpiput, D.; Tsukr uk, V. V. Macrom olecules2000, 33 , 7629.

Figure 7. AFM images oft wo-dimensiona l micelles of(PAA25)6-s-(PS25)6 formed at (a) moderate surface pressure (10 mN/m)

and (b) high sur face pressu re (30 mN/m). The z range for thetopography image (left) was 10 nm and the phase (right) was10°.

F i g u r e 8 . Monolayer parameters determined by X-ray re-flectivity, ellipsometr y, cont act an gle,a nd AFM. Top: triangles,the tota l monolayer thickness at a ir-water int erface; squar es,the total monolayer thickness at solid surface;circles,PS domaint h i ck n ess at s ol id s u rface; i n vers e t r i an gl es , PS d omai nthickness at the air-water interface.Bottom: squar es, conta ctangles; circles, surface coverage.




sizable rigid cores which prevent t he t ruly bru sh r egimeand full extension of the PAA chains. This is in a sharpcont ra st with convent ional block copolymers with pointlike

junctions where the molecules un dergo constant chan geunder compression.7,28

The height of the PS domain determined by AFM wasconsistent with the thickness ofthe PS blocks determinedby X-ray reflectivity as well as ellipsomet ry a t a ll surfacepressures (Figure 8). Unlike PAA chains which un dergotremen dous spreading in th e course of tra nsfer to a solidsurface, the thickness of PS dom ains rem ains alm ostunchanged, around 3 nm. This last value is fairly close toth e dimen sions of th e collapsed PS chain s (Table 1). Thu s,despite the significant surface area available for randomcoiled PS chains provided by the large rigid core, evenhigher compression does not affect the state of the PSchains.

Finally, Figure 9 pr ovides a schemat ic represent ationoft he segregat ion beha vior oft he sta r molecules (PAA30)6-s-(PS40)6 a s r e v e a l e d b y t h i s s t u d y . A t t h e a i r-waterinterface, the PAA arm s subm erge into t he water sub-phase, creating a dense, hydrated PAA layer below thecore plan e with significan tly extended P AA chains in t heconden sed monolayer sta te. Toform a more sta ble micellarstructure, the PS chains a ggregate above the arom aticcore, creatin g a collapsed PS layer. Upon tra nsfer to th esolid substrate, the PAA chains dehydrate and spread

very thin between h ydrophobic PS layer a nd h ydrophilicsilicon substr ate (Figure 9). The moderate length of th ePS an d PAAchains allows th e cha ins to form dens e layers,occupying th e space directly above an d below the aroma ticcore. This is also responsible for the formation of stablecircular micellar structures with diameter of 200-30 0nm which include several thousand m olecules and aredistinguished two-dim ensional structures with “twofaces”: highly h ydrophobic and highly h ydrophilic. Re-markable stable molecular packing of the star moleculeswith virtua lly un chan ged char acteristics in a wide rangeof compressions is a signature of the amphiphilic starpolymer with rigid core studied here. We suggest thatthis is caused by limited grafting density of the PS andPAA chains atta ched to t he sizable r igid core whichprevents t he m onolayer to un dergo reorganization from“gaseous” to condensed stat e. These chan ges triggered bydecreasing sur face area per molecule usu ally result in awide var iety ofdot, spaghetti-like,r ectan gular, or lamellarsurface m orphologies observed in l inear or m odestlybran ched amph iphilic diblock copolymers of differenttypes.12,28-31 Stability of these micellar st ructur es at t hewater and solid surfaces over a wide range of surfacepressures is a signature of these amphiphilic star poly-mers, making them unique polymeric surfactants.

A c k n o w l e d g m e n t . The authors thank J.Holzmuller,M. Ornatsk a, an d S. Peleshank o for techn ical assistan cedurin g experiments. F un ding from th e National Science

Foundation, DMR-0308982 and Imperial Chemical In-dustries, SRF 2112, is gratefully acknowledged. TheMidwest Universities Collaborative Access Team (MU-C AT ) s e ct or a t t h e AP S , w h er e X-r a y s t u d ie s w er econducted,is supported by the U.S. Depart ment ofEn ergy,BasicE nergy Sciences,Office of Science,t hr ough th e AmesLaboratory under Contract W-7405-Eng-82. Use of theAPS was supported by the U.S. Department of Energy,Basic En ergy Ser vices, Office of Science, un der Contr actW-31-109-Eng-38.

LA048548X

(28) Tsukru k, V. V.; Genson, K.; Peleshanko, S.;Markut sya, S.;Lee,M.; Yoo, Y.-S. La n g mu i r 2003, 19 , 495.

(29) Williams, D. R.; Fredrickson, G. H. Macrom olecules 1992, 25 ,3561.

(30) Li, W.; Gersappe, D. Macrom olecules 2001, 34 , 6783.(31) Peleshanko, S.; Jeong, J.; Gunawidjaja, R.; Tsukruk, V. V.

Macromolecules 2004, 37 , 6511.

F i g u r e 9 . Schematic of (PAA30)6-s-(PS40)6 monolayer micro-stru cture: (a) top view ofth e core tocore packing behavior (thepolymeric ar ms are removed for clarity), side view of t hemolecule packing at the air-water interface (b), and t he air-solid interface (c).


kirsten l. genson et al- interfacial micellar structures from novel amphiphilic star polymers

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