Disk Morphology and Disk-to-Cylinder Tunability ofPoly(Acrylic Acid)-b-Poly(Methyl Acrylate)-b-Polystyrene

Triblock Copolymer Solution-State Assemblies

Zhibin Li,† Zhiyun Chen,‡ Honggang Cui,† Kelly Hales,† Kai Qi,‡Karen L. Wooley,*,‡ and Darrin J. Pochan*,†

Material Science and Engineering and Delaware Biotechnology Institute, University ofDelaware, Newark, Delaware 19716, and Center for Materials Innovation and Department of

Chemistry, Washington University, Saint Louis, Missouri 63130

Received April 15, 2005. In Final Form: May 17, 2005

Disk and cylindrical micellar assemblies were formed through self-organization of poly(acrylic acid)-b-poly(methyl acrylate)-b-polystyrene (PAA-b-PMA-b-PS) amphiphilic triblock copolymers with organicdiamines as counterions in water/ tetrahydrofuran (THF) solvent mixtures. The system was investigatedby means of transmission electron microscopy and cryogenic transmission electron microscopy. It wasfound that the assembled-state morphologies could be modified by alteration of the type and concentrationof cationic diamine counterion undergoing interaction with the negatively charged, polyelectrolyte PAAcorona block, the relative amount of water in the water/THF mixture, and the hydrophobic block chainlength. Multivalency of the organic amine counterion was critical for disk formation. It was furtherdemonstrated that a single block copolymer underwent disc-to-cylindrical micellar transitions reversiblywith variation in the relative water/THF ratio. The ability to form disks beginning from either THF-richor water-rich solutions indicated that the disk morphology was thermodynamically stable and that THFwas important in keeping the micellar structure from becoming kinetically frozen. The nanoassemblieswere produced having low size dispersities and were stable for at least one month. Intermediate structuresbetween disks and cylinders were also observed, indicating two distinct kinetic pathways between the twomicelle structures.

Introduction

In recent years, different micellar structures such asspheres, cylinders, lamellae, and vesicles have beenstudied using amphiphilic diblock copolymers.1-5 Disklikemicellar structures are relatively rare compared to thesecommon micellar structures. It has also been recognizedthat triblock copolymers can provide an important role inmaking the phase diagrams more complicated than thoseobserved for diblock copolymers in solutions.6-8 Anexample of triblock architecture producing unique micellarstructure is the polymeric disk micelles that have beenonly recently observed experimentally, using a nonionictriblock copolymer in aqueous solutions.9 Another exampleof unique triblock micellar structure includes polymerictoroid formation, reported for an ionic triblock copolymerpoly(acrylic acid)-b-poly(methyl acrylate)-b-polystyrene(PAA-b-PMA-b-PS).10,11 In the present study, disk forma-

tion has been examined through the self-assembly of thesame PAA-b-PMA-b-PS ionic triblock copolymer. Theassembled structures were formed with the ionic poly-electrolyte PAA as the corona block and organic diaminesas counterions in water/tetrahydrofuran (THF) solventmixtures. It was observed that by using the same triblockcopolymer but varying the type and amount of diamines,disk or cylindrical micelles could be selectively formed. Inaddition, the disk or cylindrical morphology could also beaccessed via more traditional means, such as by changingthecompositionof thesolventmixtureorbyusingpolymerswith different hydrophobic block chain lengths. Usingorganic counterions to tune the micellar morphologyformed from amphiphilic polymers with a polyelectrolytecorona block provides an experimentally simple way tomodify micellar structure compared to the synthesis of anentirely new block copolymer.

To understand the morphological assembly of chargedtriblock copolymers, interfacial curvature and chainstretching within micelles can be considered.12,13 Theinterfacial curvature is dictated by volume and confor-mational differences between hydrophobic and chargedhydrophilic blocks and the interfacial energy betweenthem. As the interfacial energy increases, blocks stretchaway from the interface and resultant micellar structuresprefer to form flat interfaces to minimize interfacialcontact. There are several ways to adjust the interfacialcurvature and consequently tune the micellar structure,one way being manipulaton of the hydrophilic block

* To whom correspondence should be addressed. E-mail: pochan@udel.edu (D.J.P.); klwooley@artsci.wustl.edu (K.L.W.).

† University of Delaware.‡ Washington University.(1) Jain, S.; Bates, F. S. Science 2003, 300, 460-464.(2) Won, Y.-Y.; Davis, H. T.; Bates, F. S. Science 1999, 283, 960-963.(3) Won, Y.-Y.; Brannan, A. K.; Davis, H. T.; Bates, F. S. J, Phys.

Chem. B 2002, 106, 3354-3364.(4) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728-1731.(5) Discher, B. M.; Won, Y.-Y.; Ege, D. S.; Lee, J. C. M.; Bates, F. S.;

Discher, D. E.; Hammer, D. A. Science 1999, 284, 1143-1146.(6) Epps, T. H., III; Cochran, E. W.; Hardy, C. M.; Bailey, T. S.;

Waletzko, R. S.; Bates, F. S. Macromolecules 2004, 37, 7085-7088.(7) Li, Z.; Kesselman, E.; Talmon, Y.; Hillmyer, M. A.; Lodge, T. P.

Science 2004, 306, 98-101.(8) Matsen, M. W.; Bates, F. S. Macromolecules 1996, 29, 7641-

7644.(9) Lodge, T. P.; Hillmyer, M. A.; Zhou, Z.; Talmon, Y. Macromolecules

2004, 37, 6680-6682.(10) Pochan, D. J.; Chen, Z. Y.; Cui, H. G.; Hales, K.; Qi, K.; Wooley,

K. L. Science 2004, 306, 94-97.

(11) Chen, Z.; Cui, H.; Hales, K.; Li, Z.; Qi, K.; Pochan, D. J.; Wooley,K. L. J. Am. Chem. Soc. 2005, 127, 8592-8593.

(12) Matsen, M. W.; Bates, F. S. J. Chem. Phys. 1997, 106, 2436-2448.

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7533Langmuir 2005, 21, 7533-7539

10.1021/la051020n CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 06/28/2005

conformation/corona volume through the addition ofcounterions. The hydrophilic block can form a coil vs amore condensed globular structure by using mono- vsmultivalent counterions, respectively, to screen electro-static interactions. The counterion-induced change of thepolyelectrolyte conformation consequently changes thevolume ratio of corona vs core. Additionally, when organiccounterionsareused, thecounterion itself canalso increasethe corona volume by strongly interacting with polyelec-trolyte chains and, despite the decrease in electrostaticrepulsion, increase the corona volume due to the addedcounterion volume. It is well known that polyelectrolytesize can be changed by the concentration and valency ofcounterions.14-21 When monovalent, the effect of thecounterion is generally ascribed to electrostatic repulsionscreening. It has been established experimentally14-16 thatthe larger the amount of the added monovalent salt, thecloser the macromolecular behavior of polyelectrolytesolutions approaches that of conventional, unchargedmacromolecular solutions. It has also been establishedthat multivalent counterions can strongly condense flex-ible polyelectrolytes or bundle rigid DNA moleculesbecause of the electrostatic attraction between the mul-tivalent counterions and the charges along the polyelec-trolyte or between DNA molecules.17-21 In flexible, linearpolyelectrolytes, phase separation (condensation) occursbecause of electrostatic bridging between monomers viamultivalent counterions along the polyelectrolyte chain.For relatively rigid polyelectrolyte DNA, counterionbridging cannot cause complete molecular collapse butrather intermolecular bundling and toroid formation.22

Even though counterion effects have been well studied inpolyelectrolyte/DNA systems, they have not been fre-quently used to explain or modify the morphology ofsynthetic amphiphilic block copolymer micellar structuresin solution. Herein, different types and amounts of diaminecounterions are used to modify polyelectrolyte corona blockstructure and consequent micellar morphology.

In addition to corona charge/volume manipulation, onecan also change the solvent properties to adjust the volumeof each polymer chain block segment. Solvent propertiescan be adjusted by using different ratios of solvents,selective for one block or the other, in miscible solventmixtures. This method has been applied within severallaboratories to manipulate the micellar structure of blockcopolymers.23-31 In the present study, different waterratios in water/THF cosolvent mixtures were used to study

micellar structural changes. As the water content in-creased, the solubility and volume of the PAA (withcounterions) block increased together with greater sepa-ration of ion pairs, while the solubility and volume of thehydrophobic blocks decreased. Therefore, the relativeeffective volume of corona vs core could be manipulated.Finally, the volume ratio of blocks was also tuned bysynthesizing polymers with different relative block chainlengths.

Therefore, in this paper micellar structure tunabilitywas studied, with variation of several parameters involvedin directing the assembly process to understand thefundamentals of disk formation. The three experimentalmethods that were used to tune the micellar structuresinclude (1) type and amount of the counterions tomanipulate corona volume; (2) solvent mixtures, used notonly for micelle tunability but also for the determinationof the stability and reversibility of disk formation; and (3)polymer block segment chain lengthsthree differenttriblock copolymers with identical PAA and PMA chainlengths but different PS chain lengths were tested.

Experimental SectionMaterial. Triblock copolymer, PtBA-b-PMA-b-PS, was first

synthesized by sequentially incorporating tert-butyl acrylate(tBA), methyl acrylate (MA), and styrene (S) according to areported atom transfer radical polymerization (ATRP) proce-dure.32 The PtBA segment was later converted into PAA to obtainthe amphiphilic PAA-b-PMA-b-PS triblock copolymer. Threedifferent polymers were synthesized with PAA and PMA chainlengths fixed at repeat units 99 and 73, respectively, and the PSchain lengths were 66, 101, and 203, designated as S66, S101,and S203, respectively (Table 1). AR grade ethylenediamine(EDA), ethylenedioxy-bis-ethylenediamine (EDDA), ethanol-amine, and 2-aminoethyl methyl ether were obtained fromAldrich Chemical Co. and were used as received. HPLC gradeTHF was obtained from Aldrich Chemical Co. and was used asreceived. Water was obtained from a Barnstead NANOpureDiamond water system.

Micellar Solution Preparation. Polymeric micellar solu-tions were made by first dissolving the polymer in THF at roomtemperature followed by adding organic diamine for a targetedamine-to-acid functional group molar ratio. The polymer/amineTHF solution was then pipetted into different THF/water solvent

(14) Dubois, E.; Boue, F. Macromolecules 2001, 34, 3684-3697.(15) Ermi, B. D.; Amis, E. J. Macromolecules 1997, 30, 6937-6942.(16) Prabhu, V. M.; Muthukumar, M.; Wignall, G. D.; Melnichenko,

Y. B. Polymer 2001, 42, 8935-8946.(17) Ha, B. Y.; Liu, A. J. Phys. Rev. E: Statistical Physics, Plasmas,

Fluids, Relat. Interdiscip. Top. 1999, 60, 803-813.(18) Kuhn, P. S.; Levin, Y.; Barbosa, M. C. Phys. A (Amsterdam)

1999, 266, 413-419.(19) Li, A. Z.; Marx, K. A. Biophys. J. 1999, 77, 114-122.(20) Bloomfield, V. A.; Wilson, R. W.; Rau, D. C. Biophys. Chem.

1980, 11, 339-343.(21) Belloni, L.; Olvera de la Cruz, M.; Delsanti, M.; Dalbiez, J. P.;

Spallaa, O.; Drifford, M. Nuovo Cimento Soc. Ital. Fis., D 1994, 16,727-736.

(22) Bloomfield, V. A. Biopolymers 1998, 44, 269-282.(23) Alexandridis, P.; Ivanova, R.; Lindman, B. Langmuir 2000, 16,

3676-3689.(24) Fuse, C.; Okabe, S.; Sugihara, S.; Aoshima, S.; Shibayama, M.

Macromolecules 2004, 37, 7791-7798.(25) Lodge, T. P.; Pudil, B.; Hanley, K. J. Macromolecules 2002, 35,

4707-4717.(26) Lund, R.; Willner, L.; Stellbrink, J.; Radulescu, A.; Richter, D.

Phys. B (Amsterdam) 2004, 350, e909-e912.(27) Ma, Q.; Remsen, E. E.; Kowalewski, T.; Wooley, K. L. Polym.

Mater. Sci. Eng. 2001, 84, 96-97.(28) Rager, T.; Meyer, W. H.; Wegner, G. Macromol. Chem. Phys.

1999, 200, 1672-1680.

(29) Zhang, L. F.; Eisenberg, A. Macromolecules 1999, 32, 2239-2249.

(30) Zhang, W.; Shi, L.; An, Y.; Shen, X.; Guo, Y.; Gao, L.; Liu, Z.;He, B. Langmuir 2003, 19, 6026-6031.

(31) Zhang, W.; Shi, L.; An, Y.; Gao, L.; Wu, K.; Ma, R. Macromolecules2004, 37, 2551-2555.

(32) Ma, Q.; Wooley, K. L. J. Polym. Sci., Part A: Polym. Chem.2000, 38, 4805-4820.

Table 1. Chemical Structures of Polymer andCounterions

7534 Langmuir, Vol. 21, No. 16, 2005 Li et al.

mixtures to produce final water content in THF (w/w) rangingfrom 0% to 90% with polymer concentration fixed at 0.1 wt%. Forexample, 20 mg of S101 (8.3 × 10-7 mol of polymer or 8.3 × 10-5

mol of acid functional group) was first dissolved into 2.0 mL ofTHF followed by adding 6.1 µL of EDDA (4.1 × 10-5 mol of EDDAor 8.3 × 10-5 mol of amine functional group) counterions; 200 µLof polymer/amine solutions was then pipetted into solventmixtures with 1000 µL of THF and 800 µL of H2O to obtain a 0.1wt% polymeric micellar solution of amine-to-acid functional groupmolar ratio of 1:1 at 40% water. The solutions were stored in testtubes with caps and sealed with Parafilm M barrier film. Smallaliquots (ca. 10 µL) were obtained from the solution at differenttimes for evaluation of the micellar structure. To test the stabilityand kinetics of disk formation, a reversibility test was alsoperformed by adding different amounts of THF back into 90%water content samples to reach final water content between 45%and 70%.

Transmission Electron Microscopy (TEM). For conven-tional bright-field TEM imaging, grids were prepared by applyinga droplet of polymer solution directly onto carbon-coated copperTEM grids and allowing to dry under ambient conditions afterimmediate wicking away of most of the solution. Once dry,negative staining was performed using ca. 1 wt% uranyl acetatesolution. For cryoTEM, a thin film of polymer micellar suspensionon a carbon-coated copper TEM grid was plunged into liquidethane at ca. -170 °C using a Leica EM cryo preparation system.The vitrified sample was transferred to a Gatan 626 cryo transferholder under liquid nitrogen protection. The cryo sample wasmaintained at ca. -170 °C in the holder during the entire TEMobservation. TEM images were obtained in bright-field mode at200 kV accelerating voltage on a JEOL 2000FX transmissionelectron microscope on both Kodak 4489 electron imaging filmand Gatan CCD.

Results

Figure 1 contains TEM micrographs of S101 triblockcopolymer that exhibit the tunability of micellar structurebetween disklike micelles and cylindrical micelles bychoosing the correct type and amount of the counterionsand the correct solvent composition. Disklike micelles wereobtained (Figure 1A) using EDA as the counterion, anamine-to-acid functional group molar ratio of 1:1, andsolvent composition of 40% water/THF. By changing thetype of the counterion (using EDDA as the counterion,having a larger molecular structure relative to that ofEDA) and keeping all of the other conditions the same, amixture of cylindrical and toroidal micelles (with aminority of spherical micelles) was obtained (Figure 1B).(For the sake of simplicity, the local micellar structuresof toroids and cylinders are considered to be the same dueto similar interfacial curvature between blocks and similarcross-sectional micellar radii. Details regarding toroidalvs cylindrical assemblies are currently under investiga-tion.) Similar behavior was also observed by changing thecounterion concentrations. By decreasing the concentra-tion of the larger EDDA counterions to change the amine-to-acid molar ratio from 1:1 to 0.3:1, disklike micelles wereformed (Figure 1C). Furthermore, increase of the relativewater composition in the solvent mixture to 80% ratherthan 40% resulted in the formation of a cylindricalmorphology, together with a minority of spheres (Figure1D). From Figure 1A-D, it is demonstrated that disksand cylinders can be formed intentionally and selectivelyby changing the type and amount of counterions or thesolvent compositions.

To confirm the disk geometry, a tilted micrograph,collected from the sample illustrated in Figure 1C, ispresented in Figure 1E. Ellipsoidally shaped structureswere easily observed at the tilt angle of 45°, indicatingthat the micelles were, in fact, flattened, disklike objects.In-situ cryo TEM was also performed to confirm the diskformation in the system (Figure 1F). Disk prosections,

both parallel (arrows 1) and perpendicular (arrows 2) tothe electron beam axis can be seen clearly. Figure 2illustrates the expected disklike micellar morphology thatis generated in solution. The hydrophobic PS and PMAare packaged within the core domain and are partiallysolvated by THF, while the hydrophilic PAA and oppositelycharged EDDA counterions comprise the micelle corona.

To further study the ability to selectively produce disk-shaped or cylindrical assemblies, polymers with differenthydrophobic PS chain lengths were investigated underconditions that allowed for observation of the effects ofcounterion, solvent, and polymer block chain length.Unlike sample S101, for which a simple change in divalentcounterion volume induced a micelle geometry change,S203 (having a hydrophobic chain length twice that of PSin S101) exhibited the same disklike geometry by usingeither EDA or EDDA as counterion and an amine-to-acidmolar ratio of 1:1 at 40% water (Figure 3A and B). When

Figure 1. TEM micrographs from dilute solutions of S101triblock copolymers with disk micelles and cylindrical micelles.A. EDA as the counterion, amine-to-acid functional group molarratio of 1:1, and solvent with 40% water and 60% THF. B. EDDAas the counterion, amine-to-acid functional group ratio of 1:1,and solvent with 40% water and 60% THF. C. EDDA as thecounterion, amine-to-acid functional group ratio of 0.3:1, andsolvent with 40% water and 60% THF. D. EDDA as thecounterion, amine-to-acid functional group ratio of 0.3:1, andsolvent with 80% water and 20% THF. E. Tilted TEMmicrograph of image in C. F. Cryo micrograph for sample Csolution: arrows 1, the disks are parallel to the electron beamaxis; and arrows 2, disks are perpendicular to the electron beamaxis. All scale bars are equal to 200 nm.

PAA-b-PMA-b-PS Copolymer Solution-State Assemblies Langmuir, Vol. 21, No. 16, 2005 7535

the counterion concentration was increased from 1:1 to2:1 with EDDA as counterion, disks (Figure 3B) were nolonger stable and short cylinders, among a minority ofspheres, were observed (Figure 3C). When the watercontent was increased from 40% to 80%, while keepingthe other solution conditions the same as in Figure 3B,short cylinders and spheres were formed (Figure 3D). S66polymer, with the shortest hydrophobic chain length, gaveresults similar to those obtained for S101, producing disks(Figure 4A, Disks prefer to form stacks at these solutionconditions. The local micellar structures are consideredto be the same between disks and stacks of disks. Detailsregarding the interactions between disks and why diskslike to stack together are currently under investigation)or cylindrical micelles (Figure 4B) in the presence of EDAor EDDA as counterions, respectively. No micellar ge-ometry change from disk to cylinder was observed forsolutions of S66 with change of the concentration of thecounterion or the water composition. Table 2 summarizesthe detailed experimental results regarding the effect oftype and amount of counterion, water composition, andhydrophobic chain length for all three polymers studied.

DiscussionCounterion Effect. The results presented in Figures

1, 3, and 4 illustrate that the micellar structure can betuned by adjusting counterion type and concentration.Using counterions to adjust polyelectrolyte structure and,consequently, change micellar morphology formed bycharged amphiphilic block copolymers provides a facilemethod to modify micellar structure, relative to thechemical synthesis of new block copolymers with differentblock lengths. Since multivalent counterions bind to thecharges along the polyelectrolyte chain, they remain inthe corona and can both decrease effective corona volume,by both screening electrostatic repulsion and collapsingpolyelectrolyte chains, and increase the effective coronavolume due to the additional volume of the counterion.

The relative importance of each effect can be ascertainedby the resultant micellar structure formed. In the resultspresented here, the effects on micellar structure aredominated by different degrees of corona swelling due tomolecular volume differences between counterions. Insolutions of both S66 and S101 with 1:1 amine-to-acidratio, when the molecular volume of the counterion wasincreased (EDA vs EDDA) while the ionic strength washeld constant, different micellar structures were observed.With the smaller EDA counterion, the smaller effectivecorona volume produced the disk morphology with lowinterfacial curvature (Figures 1A and 4A). The larger

Figure 2. Cartoon schematic of disklike micelle with crosssection showing hydrophobic PS (red) and PMA (purple) coreand hydrophilic PAA (yellow) corona with closely associatedEDDA counterions (blue).

Figure 3. TEM micrographs from dilute solutions of S203triblock copolymers with disk micelles and cylindrical micelles.A. EDA as the counterion, amine-to-acid functional group ratioof 1:1, and solvent with 40% water and 60% THF. B. EDDA asthe counterion, amine-to-acid functional group ratio of 1:1, andsolvent with 40% water and 60% THF. C. EDDA as thecounterion, amine-to-acid functional group ratio of 2:1, andsolvent with 40% water and 60% THF. D. EDDA as thecounterion, amine-to-acid functional group ratio of 1:1, andsolvent with 80% water and 20% THF (arrows show perforateddisks, details vide infra). All scale bars are equal to 200 nm.

Figure 4. TEM micrographs from dilute solutions of S66triblock copolymers with disk micelles and cylindrical micelles.A. EDA as the counterion, amine-to-acid functional group ratioof 1:1, and solvent with 40% water and 60% THF. B. EDDA asthe counterion, amine-to-acid functional group ratio of 1:1, andsolvent with 40% water and 60% THF. All scale bars are equalto 200 nm.

7536 Langmuir, Vol. 21, No. 16, 2005 Li et al.

EDDA counterions produced cylinders due to the increasein effective corona volume and consequent higher inter-facial curvature between hydrophobic-hydrophilic blocks(Figures 1B and 4B). Similarly, an increase in thecounterion concentration was also observed to increaseeffective corona volume and consequently change themicellar structure from disks to cylinders. Disk-to-cylindertransitions were observed with an increase in counterionconcentration in both S101 and S203 solutions with EDDAas the counterion at 40% water content. In addition to thecounterion volume effect discussed here, other possiblemechanisms such as effects of inter- vs intra-PAA chaincondensation via diamine counterions are also currentlyunder investigation.

To observe the effect of counterion valency, monoamineswere also utilized during the assembly processes. Themonoamines tested were 2-aminoethyl methyl ether (CH3-OCH2CH2NH2) and ethanolamine (HOCH2CH2NH2) tokeep chemical functionality and hydrophobicity similarto divalent EDDA molecules, and monoamine-to-acrylicacid ratios ranging from 0.2:1 to 4:1 were employed.Importantly, only spheres were observed for the threepolymers with monoamine as the counterion. Therefore,screening of charges along the polyelectrolyte chain wasnot sufficient to condense corona volume and lower theinterfacial curvature and change micellar geometry. Thus,a divalent counterion is required to form disks in thissystem. It is well-known that multivalent counterions canbe used to condense polyelectrolyte chains.15,18,21 In thiswork, organic diamines, which can be considered asdivalent counterions with different molecular volumes,condensed polyelectrolyte chains through intramicellarinteractions, without causing extensive aggregation andprecipitation of the block copolymers from solution.Further details regarding specific polyelectrolyte chainconformations at different multivalent counterion con-centrations and their importance for disk formation arealso currently under investigation.

Solvent Effect. The micellar structure can also betuned by adjusting the solvent content. Through increaseof the water content from 30% to 90% in water/THFmixtures, the solubility of the hydrophobic PS and PMAblocks decreases while the solubility and electrostaticeffects within the coronal layer may have increased.During this process, an interfacial curvature increase wasobserved because of the volume decrease of the hydro-phobic core and, additionally, volume increase of thecorona. For instance, the micellar structure of S101 withEDDA counterions at an amine-to-acid ratio of 0.3:1 wentthroughdisks,disk-cylinder intermediates, cylinders,andfinally formed a mixture of cylinders and spheres as thewater content increased from 30% to 80%, Figure 5.

The reversibilities of the morphological assembliesresulting from the reversible core-shell volume changeswith solution addition, as well as the stability of the disk

Table 2. Summary of the Structures Observed from TEMa

S66 S101 S203

%water

EDA1:1

EDDA0.2:1

EDDA0.5:1

EDDA1:1

EDA1:1

EDDA0.3:1

EDDA0.5:1

EDDA1:1

EDA1:1

EDDA0.5:1

EDDA1:1

EDDA2:1

30 B B+S B B B D R+D R+B D+B D+B D R+D40 D S+R R R D D R+D R D+B D D R+S50 - S+R - - D D - R D+B+S D+S D R+S60 D S R R D D R+S R+S D+S D+S D+R R+S70 - S+R - - D D+DR - R+S D+S D+S D+R+S R+S80 D R R+S S+R D+S R R+S S+R S+D S+D+R S+DR S+R90 - S+R - - S+D S+R - S S+D S+D S+R Sa Disks can be formed by tuning the counterion type and concentration, water composition, and hydrophobic chain length. A dash (-)

in the table indicates a solution condition not explicitly studied. In addition, when several different structures coexisted, the structure listedfirst was the majority structure observed (e.g., S+R means sample was predominantly spheres with a minority population of cylinders).EDA: ethylenediamine, shorter diamine. EDDA: ethylenedioxy-bis-ethylele diamine, longer diamine. B: bulk phase (Lamellar, etc.). D:disks. R: cylinders, toroids. DR: disk-to-cylinder intermediate. S: spheres.

Figure 5. Water effect on S101 with EDDA counterions andamine-to-acid functional group ratio of 0.3:1 and with watercontent of A, 30%; B, 40%; C, 50%; D, 60% (arrows show disksperforating); E, 70% (arrows show cylinders growing peripher-ally out from disks); and F, 80%. All scale bars are equal to 200nm.

PAA-b-PMA-b-PS Copolymer Solution-State Assemblies Langmuir, Vol. 21, No. 16, 2005 7537

micellar morphology, were investigated. S101 sampleswith molar ratio of 0.3:1 amine to acid were used sincethey originally formed disks and cylinders at differentwater contents (disks at lower water content, cylinders athigher water content, Figure 5). By adding increasingamounts of THF directly into the 90% water contentsample (Figure 6A), samples with final water content equalto 70%, 60%, and 45% were obtained. Disk-to-cylinderintermediate structures were observed at 70% water(Figure 6B), while disks were observed at 45% water(Figure 6D). These results indicate that disk formation isnot dependent on a kinetic pathway, rather discs can formstarting from either THF-rich or water-rich solutions bythe addition of water or THF, respectively.

Micelle transformation in amphiphilic polymers at highwater content is kinetically possible due to the presenceof organic cosolvent in the micellar core, thus producingfluid micelles.33 The kinetics of micellization in diblockcopolymers has been studied in the literature1,4,33-35 inboth single-solvent and organic/water cosolvent systems.Water is the single solvent in most cases for amphiphilicblock copolymers. A glassy hydrophobic core leads to no(or very slow) unimer exchange in the micellar systems,and the micellar structure is kinetically frozen.4,36 By usinga water/organic cosolvent mixture, micelle transforma-tions can be made accessible. Early work from Nagarajanet al.34 showed that diblock copolymer micelles canselectively swell with one of the cosolvents. By using

sedimentation velocity, an insight of unimer/micelleequilibrium was observed through the hybridization ofnumerous block copolymer micellar pairs.33 By adding anorganic cosolvent, Roger et al.28 reported that the unimer-micelle exchange equilibrium was not kinetically hinderedbut controlled by a strong thermodynamic preference forthe aggregated state. Stam et al.35 also reported that theefficiency of unimer/micelle exchange can be tuned bycosolvent using a steady-state fluorescence spectroscopymeasurement. These literature results are consistent withthe reversibility experiments performed here in thepresence of THF in water. The experiments suggest thatthe micellar core is not kinetically frozen because of thepresence of the THF. Therefore, the disk is expected to beat a thermodynamically stable state rather than akinetically captured intermediate state.

Disk-to-cylinder intermediates have been observed inS101 from either adding water into THF rich solutions(Figure 5) or adding THF into water-rich solutions (Figure6). Two possible mechanisms can be proposed for theobserved transformations between disklike micelles andcylinders: disks perforate to form cylinders (arrows 1 inFigure 5D and E) or cylinders grow out from the peripheryof disks (arrows 2 in Figure 5E). Cylinders growing fromthe periphery of disks has been reported by Bates andco-workers by mixing two nonionic copolymers withdifferent hydrophilic lengths.36 Their octopuslike struc-tures have disklike cores and cylindrical “arms” alwayssymmetric in number. Intramicellar segregation was usedto explain the formation of the disk and cylindrical regionsbecause of local interfacial curvature preferences. Themechanism for disk-to-cylinder intermediate structuresreported here might be also due to intramicellar segrega-tion from polymer molecular weight polydispersity. Forthe reverse transformation from cylinders to disks, unevenTHF distribution inside the micellar core would produceregions with more THF that prefer disk features with amore swollen hydrophobic core relative to cylindricalregions.

Block Copolymer Chain Structure Effect. The moretraditional block copolymer parameter of relative chainlength can also be used to tune micellar structure. Insolutions with EDDA as counterion and amine-to-acidmolar ratio of 1:1 at 40% water content, an interfacialcurvature decrease was observed as the hydrophobic chainlength increased from S66 and S101 (cylinders) to S203(disks). Keeping all the other conditions the same, theincrease of the PS chain length can increase the hydro-phobic core volume relative to corona and decrease theinterfacial curvature from cylinders to disks. The changeof micellar structure with hydrophobic chain length wasnot observed in the case of EDA counterion. The coronacannot reach the required volume with the small EDAcounterion to consequently change micellar morphology.

The kinetics of micelles to reach equilibrium was alsostudied with the three different PS chain length polymers.For all S66 and S101 suspensions studied, final stablestructures were reached within 1 day. However, for S203,final stable structures could be reached only after severaldays. The longer the hydrophobic block of the copolymer,the slower it is for the micelles to reach the thermody-namically stable state.

Table 2 provides additional experimental informationto further understand the parameters discussed in theprevious sections: (i) Less counterion or water is requiredfor S101 to reach cylindrical micelles than that for S203,and this is consistent with the hydrophobic chain lengtheffect; (ii) a mixture of different micellar morphologiescan coexist under some conditions, perhaps due to the

(33) Tian, M.; Qin, A.; Ramireddy, C.; Webber, S. E.; Munk, P.; Tuzar,Z.; Prochazka, K. Langmuir 1993, 9, 1741-1748.

(34) Nagarajan, R.; Barry, M.; Ruckenstein, E. Langmuir 1986, 2,210-215.

(35) van Stam, J.; Creutz, S.; De Schryver, F. C.; Jerome, R.Macromolecules 2000, 33, 6388-6395.

(36) Jain, S.; Bates, F. S. Macromolecules 2004, 37, 1511-1523.

Figure 6. Reversibility tested for S101 by adding THF backinto 90% water and 10% THF content sample (A) with amine-to-acid functional group ratio of 0.3:1, and the final water contentof B, 70%; C, 60%; and D, 45%. All scale bars are equal to 200nm.

7538 Langmuir, Vol. 21, No. 16, 2005 Li et al.

uneven distribution of THF in different micelles or polymermolecular weight polydispersity. From Table 2, one cansee that by introducing multiple variables in the system,such as counterions, solvents, and block chain lengths, aspecific micellar structure can be obtained easily.

So far, only the morphology change based on theinterfacial curvature induced by the volume change inthe micellar core or corona has been discussed. However,the presence of triblock architecture and ionic characterof the corona may also be of fundamental importance. Inamphiphilic block copolymers, chains stretch away froma corona interface due to repulsive interactions and thusform a strong segregation between the blocks. For triblockcopolymers, three segregations between A and B, B andC, and C and A blocks can drive the polymers into thesuperstrong segregation regime (SSSR), originally pro-posed by Semenov and co-workers.37 A disk micellemorphology was predicted to be stable in the SSSR. Morerecently, disks formed from a nonionic triblock copolymer,poly(ethylene oxide)-b-polystyrene-b-fluoro(1,2-polybuta-diene) (OSF), have been observed by Lodge et al.9 withdisk formation attributed to the fact that the nonionictriblock copolymer may reside in the SSSR. Interestingly,Semonov originally indicated that the SSSR might onlybe accessible using ionic polymers to obtain significantinteraction strength. To the best of our knowledge, noionic triblock has been reported to obtain disk structure.Disks formed from anionic triblock copolymers reportedhere seem to be the complement to Semenov’s and Lodge’sstudies.

By analogy to amphiphilic block copolymers, surfactantmolecules can also form disklike micelles. This has alreadybeen reported both experimentally and theoretically. Amodel calculation has predicted that, for sodium dodecylsulfate (SDS) surfactant, the micellar structures canchange from cylinder, to disk, and then to vesicles withrespect to increase of salt concentration at certain sur-factant concentration.38 Zemb and co-workers observeddisk formation experimentally by mixing cationic andanionic surfactant, cetyltrimethylammonium hydroxide,and myristic acid, respectively, to adjust micelle surfacecharge density.39 Amphiphilic block copolymers havegenerally exhibited similarity to surfactants in finalaggregated micellar structure (spheres, cylinders, andbilayers), but no comparison between surfactant and

polymeric disks has been made in the literature. Thequestions arise as to whether amphiphilic block copoly-mers form disks similar to the surfactant mechanismrather than the SSSR mechanism and whether triblockcopolymers are required for disks to form. To understandthe theory for amphiphilic polymeric disk formation incosolvent systems, several different diblock copolymersare currently under investigation for direct comparisonswith triblock copolymer analogues.

Conclusions

The assembly of PAA-b-PMA-b-PS in water/THF co-solvent systems was studied in the presence of mono- anddiamines having different molecular sizes as the coun-terions. Disk formation was observed for three differenttriblock copolymers with varying lengths for the PS chainsegment. By tuning the type and concentration of diaminecounterion and/or water composition in the cosolvent, diskor cylindrical regions of the phase diagram were accessedsuccessfully and reversibly. The nature of the counterionwas found to be important for disk formation, withmultivalency being required and smaller diamine sizesfavoring disk formation. Disks observed were ratherhomogeneous in size and stable for at least one month.Since disks could be formed from either THF-rich or water-rich regions, it can be concluded that disks were ther-modynamically stable. THF is important in keeping themicellar structure fluid and not kinetically frozen. Disk-to-cylinder intermediates were observed, and two possiblemechanisms seem to be operational: the transition fromdisks into cylindrical micelles and vice versa. The resultsindicate that by introducing multiple variables in thesystem, such as counterions to modify the hydrophiliccorona block volume and organic solvent to increase themobility and volume of the hydrophobic core, one canaccess a variety of micellar structures from the same blockcopolymer molecule.

Acknowledgment. The authors acknowledge supportof this research by the National Science Foundation underNSF-NIRT Grant No. DMR-0210247 and NSF Grant No.0301833. D.J.P. also acknowledges the Dupont YoungFaculty award for support. W. M. Keck College ofEngineering electron microscopy lab at University ofDelaware is also acknowledged. We thank J. L. Turnerfor creation of the disk schematic in Figure 2. We alsothank Dr. Chaoying Ni and Frank Kriss for their helpduring the TEM measurements.

LA051020N

(37) Semenov, A. N.; Nyrkova, I. A.; Khokhlov, A. R. Macromolecules1995, 28, 7491-7500.

(38) Eriksson, J. C.; Ljunggren, S. Langmuir 1990, 6, 895-904.(39) Zemb, T.; Dubois, M.; Deme, B.; Gulik-Krzywicki, T. Science

1999, 283, 816-819.

PAA-b-PMA-b-PS Copolymer Solution-State Assemblies Langmuir, Vol. 21, No. 16, 2005 7539