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Rheology of Block Polyelectrolyte Solutions and Gels: A Review Abigail S. Kimerling,† Willie E. (Skip) Rochefort,‡ and Surita R. Bhatia*,† Department of Chemical Engineering, UniVersity of Massachusetts, 159 Goessmann Lab, 686 North Pleasant Street, Amherst, Massachusetts 01003-9303, and Department of Chemical Engineering, 102 Gleeson Hall, Oregon State UniVersity, CorVallis, Oregon 97331-2702
We review recent experimental results on the rheology of block polyelectrolyte solutions, focusing mainly on diblock and triblock architectures and the effects of block polyelectrolyte concentration, block composition, solution pH, and ionic strength. Micellar solutions of diblock polyelectrolytes are typically viscoelastic liquids at a low concentration; however, gel formation can occur at higher concentrations when micelle corona chains begin to interpenetrate or interact. The rheology can be tuned by varying pH or ionic strength or through the addition of hydrophobic groups along the corona chain. Triblock architectures form elastic gels, analogous to neutral telechelic associative polymers. However, the charged nature of the backbone promotes the formation of a highly networked structure, enabling gel formation at lower concentrations than those observed with neutral telechelics. Finally, we briefly describe recent work on creating block copolypeptide gels with unique rheological and self-assembly characteristics and new architectures for creating thermoreversible block polyelectrolyte gels. Introduction We are honored to participate in this special issue celebrating Bill Russel’s 60th birthday. Our contribution concerns the rheology of block polyelectrolyte solutions. Bill’s own work on curved polyelectrolyte brushes1,2 and the mobility of polyelectrolyte-coated colloids3,4 is quite relevant to many of the systems described below. The term “block polyelectrolytes” typically refers to block copolymers in which at least one block is a polyelectrolyte. As compared to neutral (i.e., nonionizable) block copolymers and neutral associative polymers, the charged nature of block polyelectrolytes gives us additional handles for tuning solution interactions and rheology, namely, the pH and ionic strength of the surrounding media. Although the selfassembly of block polyelectrolytes has been well-studied both experimentally (see recent reviews5,6) and theoretically,7-11 the literature on the rheology of these systems is much sparser. Often, block polyelectrolytes can be used to create strong elastic gels at lower concentrations than achievable with neutral polymers. In this review, we focus mainly on recent results for synthetic block polyelectrolytes with diblock and triblock architectures. Although hydrophobically modified polyelectrolytes and polyelectrolyte complexes are also known to form associative structures, the rheology of these polyelectrolytes is beyond the scope of this review. Diblock Polyelectrolytes and Related Systems The most widely studied diblock polyelectrolyte solutions are those based on poly(styrene-b-acrylic acid) (PS-PAA). As a result of the hydrophobic nature of the PS block, this diblock copolymer forms spherical micelles in aqueous solutions (Figure 1), although several other kinetically trapped morphologies such as rods, lamellae, and vesicles are possible.12,13 PAA is a weak polyelectrolyte; the chain conformation and, therefore, solution properties depend on solution pH as well as ionic strength. Rheological studies thus far probed the effects of copolymer * To whom correspondence should be addressed. E-mail: sbhatia@ ecs.umass.edu. † University of Massachusetts. ‡ Oregon State University.
Figure 1. Association of amphiphilic diblock polyelectrolytes into a spherical micelle under aqueous solution conditions.
concentration,14-16 polyelectrolyte block composition,16-18 added salt,14 and surfactant.16 Currently, our group is examining the effects of block length, pH, and ionic strength. Micellar Diblock Gels. Korobko et al.14 have studied the rheology of PS-PAA with 20 PS and 85 PAA units in aqueous solutions at concentrations from 4.5 to 44 g/L (approximately 0.45-4.4 wt %) and at 50% neutralization of the PAA block. All of the solutions exhibited Newtonian plateaus at low shear rates, and at higher copolymer concentrations, shear thinning was observed. The zero-shear viscosity, ηo, defined as the limit of the viscosity at low shear rates, was found to increase dramatically with concentration. The most dilute solution has a zero-shear viscosity one order of magnitude greater than water, while the zero-shear viscosity of the highest concentration was four orders of magnitude greater than water. The presence of excess salt (1 M KBr) yielded solutions with viscosities similar to that of water; this decrease was caused by salt ions shielding the charges on the PAA blocks, causing chain collapse and weaker intermicellar interactions. These solutions were also examined in oscillatory shear experiments. At all concentrations, the loss modulus, G′′, was greater than the storage modulus, G′, for the frequency range tested. At the lowest copolymer concentration, samples behaved as viscoelastic liquids, with G′ ∼ ω2 and G′′∼ ω1, where ω is the frequency of oscillation in radians per second. For the higher concentrations, G′ ∼ G′′∼ ω1, indicating a transition to a gel. Korobko et al.14 suggest that the solutions display elasticity when the micelle corona layers interpenetrate at a high enough copolymer concentration. Our studies on PS-PAA have focused on the effects of solution ionic strength and pH on rheological properties. The
10.1021/ie051034o CCC: $33.50 © 2006 American Chemical Society Published on Web 02/10/2006
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Figure 4. PS-PAA micelles associated via ethyl acrylate stickers in the PAA block.
Figure 2. Viscosities for 1.0 wt % PS-PAA (molecular weight 1.4 K-40.2 K) solutions at 0.020 M NaOH and varying NaCl concentrations: 0.099 M (2), 0.049 M (0), 0.0094 M (b), 0.000 90 M (b), and no salt (×).
Figure 3. Storage modulus G′ (filled symbols) and loss modulus G′′ (open symbols) for 1.0 wt % PS-PAA (molecular weight 1.4 K-40.2 K) solutions at 0.020 M NaOH and varying NaCl concentrations: 0.099 M (2), 0.049 M ([), 0.0094 M (9), 0.000 90 M (b), and no salt (1).
PAA blocks can be ionized by the addition of sodium hydroxide to the aqueous solution. The ensuing corona expansion leads to an increase in solution viscosity and in both storage and loss moduli. By adding sodium chloride, the charges are shielded, leading to a decrease in corona size and a corresponding decrease in viscosity and elasticity. This is illustrated in Figure 2, where a decrease in viscosity of a 1.0 wt % PS-PAA solution is observed with increasing solution ionic strength at 0.020 M NaOH (pH ∼4). These solutions were all shearthinning with no apparent Newtonian plateau in the shear rate range measured. The solutions also had comparable slopes, with the power-law fits (η ∼ γ˘ n-1) having an average n of 0.3. Similar decreases in viscosity with an increasing salt concentration have been seen for PS-PAA solutions at different pH’s and with different block makeups. Dynamic shear experiments for 1.0 wt % PS-PAA at 0.020 M NaOH displayed decreases in both moduli as the ionic strength increased (Figure 3). At low salt concentrations, the solutions were elastic gels with G′ > G′′ and G′ nearly independent of ω. As the salt concentration increased, the gap between G′ and G′′ grew smaller and G′ became frequency-dependent. Further investigations into the effects of block length, pH, and ionic strength are in progress. Diblock Micelles with Attractive Interactions. A related mechanism for gel formation was proposed for solutions of PSPAA with unhydrolyzed ethyl acrylate groups in the PAA block.15-18 In this case, the hydrophobic ethyl acrylate units in the micelle corona associated, causing intermicellar attraction
(Figure 4). All of the solutions were formulated at pH 10 and had no salt added to increase the ionic strength. As the copolymer concentration was increased, the solutions transitioned from viscoelastic fluids to elastic gels, with G′ independent of ω and G′ greater than G′′. However, similar to the fully hydrolyzed system studied by Korobko et al.,14 dynamic light scattering measurements suggested that gel formation occurred at a very high effective volume fraction of micelles, corresponding to either interpenetration or compression of the polyelectrolyte corona.15 There is evidence that these systems behave as attractive glasses, with the strength of the intermicellar attraction dependent on the fraction of ethyl acrylate groups in the PAA chain.15,18 The critical polymer concentration for gel formation was much lower than that in the fully hydrolyzed case, approximately 1.5-4.0 wt % polymer, and was found to depend on the strength of intermicellar attraction. An upturn in G′ at high strains was observed, indicating strain hardening, which is unusual for polymer solutions.16 Increasing the degree of hydrolysis or decreasing the concentration resulted in an increase in the critical strain γc. Polyelectrolyte-Covered Colloids. Colloidal particles with hydrophobic cores and polyelectrolyte grafts on the core surface are similar in structure to diblock polyelectrolyte micelles. The two differ in that the core-corona interface is much sharper for the colloidal particles due to polydispersity in micelleforming diblock polylectrolytes. Also, the number of polyelectrolyte chains is fixed for a colloidal particle, unlike micelles, which can have different aggregation numbers depending on solution conditions.19 Particle cores are typically much larger than micelle cores, with a diameter of about 100 nm, and are made of tightly packed hydrophobic polymer chains. The polyelectrolyte grafts form a closely packed brush since the distance between grafts is shorter than the graft length. Thus, the particles have more corona chains than similarly sized micelles and will have stronger interparticle repulsions. Experimental particles have been made with PS cores and PAA grafts (denoted here as PS/PAA)20 and a mixture of PS and poly(butyl acrylate) (PBA) in the core with poly(methacrylic acid) (PMAA) grafts (denoted here as PS-PBA/PMAA).21 Solutions of colloidal particles exhibited an increase in viscosity with an increased volume fraction.20,21 At low volume fractions, the PS/PAA particles at pH 10 had a viscosity independent of the shear rate; the solution was Newtonian due to a lack of strong particle interactions or particle deformation. However, once the volume fraction, φ, was large enough for interparticle repulsion to be significant (φ ∼ 0.01), shear thinning was observed.20 Particle deformation caused by interactions led to the decrease in viscosity. The high-frequency modulus, G′∞, increased with the mass fraction of PS-PBA/PMAA particles.21 The polyelectrolyte grafts give the colloidal particles a chargedependent structure and rheology. Uncharged grafts form coils that are densely packed around the core, while charged grafts form fully stretched chains under no-salt conditions. Changes in pH cause the particle radius to vary, thus altering the solution viscosity. In PS/PAA and PS-PBA/PMAA particles, the brush
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Figure 5. Flowerlike micelle network formed by telechelic polyelectrolytes in an aqueous solution. The polyelectrolyte midblock is not charged in part a and is highly charged in part b, creating a preference for bridges.
layer was constant at low pH up to values near the acid pKa. The brush grew longer as the pH rose until the chains were fully ionized and extended, and the brush layer remained constant for any further pH increase.19,21 When exposed to strong shear flows, the weakly charged PAA brushes expanded, a typical neutral polymer response.20 Strongly charged PAA brushes in the same situation compressed due to the overlap of the particle electric double layers, which were distorted by flow.20 The addition of salt to charged colloidal particle solutions had the effect of contracting the polyelectrolyte grafts. As the brushes collapsed and the particle radius decreased, a drop in viscosity was observed.20,21 Solutions with increased ionic strength also had smaller G′∞ values due to a decrease in the particle volume fraction. The addition of salt to systems at pH’s 7 and 9 did not impact the stability of the solutions. However, PS-PBA/PMAA particles with uncharged grafts (pH of 4) exhibited particle aggregation when salt was added.21 Triblock Polyelectrolytes A great deal of rheological research has been conducted by Tsitsilianis and co-workers on triblock polyelectrolytes with either similar end blocks (ABA) or different end blocks (ABC).22-26 Much of the work has focused on telechelic polyelectrolytes which have PAA center blocks and hydrophobic end blocks.22-25 In aqueous solutions, these polymers form flowerlike micelles, with the hydrophobic end blocks comprising the micelle cores and the PAA chains adopting either a loop or bridge conformation (Figure 5). Similar to neutral telechelic polymers, the bridging chains result in a networked solution structure and enhanced viscosity and elasticity. However, the use of a polyelectrolyte midblock enables control over the formation of bridges versus loops. When the polyelectrolyte midblock is highly charged, the formation of loops is unfavorable, and more bridges are formed between micelles, creating large aggregates.23,24 These larger structures bring about interesting rheological behavior, including gel formation at very low polymer concentrations. ABA Telechelic Polyelectrolytes. An as example, poly(sodium acrylate), PANa, the sodium salt of PAA, was used in a PS-PANa-PS block polyelectrolyte studied by Tsitsilianis et al.22,23 The pH of the solutions was not reported; however,
the PAA was neutralized with NaOH, making the pH dependent on polymer concentration. The PS blocks were quite short, 23 monomer units, and the structures formed in solution appeared to be at equilibrium despite the experimental temperature below the glass transition temperature, Tg, of PS.23 G′ and G′′ were found to be approximately independent of ω for c g 0.4 wt %, indicating that the PS-PANa-PS was an elastic gel. Decreasing the degree of ionization of the PAA block (i.e., using a more acidic solution) decreased the storage modulus, but the solution remained a gel with G′ independent of ω. Dynamic strain sweeps yielded low values of the critical strain for all samples, indicating a weak physical gel. The critical strain was independent of the concentration above 0.2 wt %. Interestingly, the plateau modulus was higher and the critical strain was lower than those of similar weak gels formed by neutral telechelics.22 In steady shear, these systems displayed Newtonian behavior for c e 0.2 wt % and shear-thinning profiles for c g 0.4 wt %, with no Newtonian plateau visible. Both the steady shear and complex viscosities increased with the concentration. At c g 0.4 wt %, the solutions displayed a yield stress, again suggesting the formation of a weak physical gel. Finally, the phase separation predicted for telechelics in dilute solutions27 was not seen for the PS-PANaPS solutions, presumably due to electrostatic repulsions.23 However, with the addition of NaCl, the charges in the PAA block were screened, and phase separation occurred. ABC Triblock Polyelectrolytes. Another strategy to promote bridging chains is to use ABC architectures. Studies on poly(styrene-b-sodium acrylate-b-n-butyl methacrylate), PS-PANaPnBMA, showed similar rheological behavior to that of PSPANa-PS.24,25 The pH of the solutions was not reported; however, the PAA was neutralized with NaOH, making the pH dependent on polymer concentration, and no salt was added. Depending on solution conditions, two different hydrophobic cores could be formed because of PS and PnBMA incompatibility. Thus, there was a greater propensity for bridging, leading to network formation at lower concentrations than the ABA copolymer. At very low shear rates (below 10-4 s-1), solutions of various concentrations were found to have a Newtonian plateau.25 At greater shear rates, the solutions were shear thinning, with those at higher concentrations displaying three distinct regimes. Within the concentration range 0.1 < c < 0.2 wt %, a five-order increase in the zero-shear viscosity was observed.25 The authors interpret this as being indicative of network formation; however, it is worth noting that other mechanisms may account for the increase in viscosity, and efforts are underway to visualize network formation in these systems via cryo-TEM. At 0.3 wt %, the ABC triblock polyelectrolyte was shear thinning with the same slope throughout, while a comparable AB diblock (same-sized blocks) was Newtonian, with a five-order difference between the two. Three thinning regions were observed at 0.6 wt % and above, and these solutions exhibited yield stresses. Sharp decreases in viscosity were attributed to the breakage of the infinite threedimensional network and further breakage of the finite clusters into still smaller pieces. At 0.6 wt %, the network breakage was reversible as no hysteresis loop was observed, with the only difference being that the yield stress was lower when a decreasing shear stress was employed. When the concentration was increased to 0.8 wt %, the three shear-thinning regimes were still present, but the yield stress and viscosity increased. The 0.8 wt % solution had a hysteresis loop, with lower viscosities measured when the stress was decreased. This change in behavior was attributed to a slower structure reformation for the more concentrated solution.
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PS-PANa-PnBMA concentrations at 0.3 wt % and above had dynamic shear results that showed the solutions were elastic, with G′ > G′′ throughout the frequency range tested.25 Both moduli increased with concentration. At c g 0.6 wt %, G′ was about 1 order of magnitude greater than G′′ and G′′ was independent of ω. By 1.0 wt %, G′ was nearly independent of ω, indicating that the solution was an elastic soft solid. The steady shear results from PS-PANa-PnBMA were found to differ from those of PS-PANa-PS at the same concentration and roughly the same block lengths.24 Both triblocks displayed the three shear-thinning regions at 0.8 wt %. Yet the ABC triblock had a yield stress 1 order of magnitude greater, and the viscosity at a given stress was also greater, indicating more associations in the network. Polyampholytes. Rheological characteristics of the triblock polyampholyte poly(acrylic acid-b-2-vinlypyridine-b-acrylic acid) (PAA-P2VP-PAA) have also been examined by Tsitsilianis and co-workers.26 This water-soluble polymer contains both positive and negative charges; with more P2VP units, the chains were positively charged. Because of the opposite charges, at certain pH values, the triblock copolymer was not soluble in water, so experiments were conducted at pH 3.4 with no added salt, where it was found that PAA-P2VP-PAA self-assembled into a network structure despite the lack of hydrophobic associating units. Above the percolation concentration cg, a transient network formed due to interchain attractions, and above the transition concentration c′, a three-dimensional network formed. The solution viscosity increased with concentration, and the zero-shear viscosity displayed three different regimes of concentration dependence. At semidilute concentrations (c < cg), the viscosity was Newtonian for all shears and ηo ∼ c0.58, close to the predicted dependence for unentangled polyelectrolytes.26,28 At higher concentrations, the viscosity had a slight thickening and then thinning with increasing shear. The zeroshear viscosity in the intermediate regime (cg < c < c′) had a greater dependence on the concentration than in the concentrated regime. At concentrations above cg, a hysteresis loop was observed, with the zero-shear viscosity matching in the concentrated regime (c > c′), but not matching in the intermediate regime. These different response regimes were due to the associations formed in solution, with intramolecular attraction occurring at low concentrations and an intermolecular network forming above cg. The dynamic shear experiments found that the solution was a viscoelastic fluid in the intermediate regime, with G′ and G′′ both dependent on ω and a crossover from G′′ > G′ to G′ > G′′ seen at the lowest frequencies. Above c′, the moduli were almost independent of the frequency and G′ > G′′, so the triblock solution was an elastic gel. A strain sweep found that above cg strain hardening occurred, with γc independent of the concentration. The increase at the critical strain was greater at lower concentrations. The plateau modulus had greater dependence on the concentration in the intermediate regime than in the concentrated regime, as seen for the zeroshear viscosity. Other Architectures Block Copolypeptides. A special class of block polyelectrolytes is the amphiphilic block copolypeptides, such as those comprising poly-L-lysine and poly-L-leucine, which are similar to natural polypeptides and have different solution properties than other block copolymers.29 The leucine block is hydrophobic and forms an R helix in aqueous solutions, while the cationic hydrophilic lysine block has a stretched coil configuration. In
deionized water solutions, the R helixes packed together to form twisted fibers (fibrils), which triggered gelation at lower concentrations than typical block copolymers. As the diblock copolypeptide concentration increased, the solutions transitioned from viscoelastic fluids to gels, with the gel concentration identified as where G′ ) G′′. Both G′ and G′′ were observed to increase with the diblock concentration. Lysine blocks greater than 100-150-units-long were needed for gelation to occur. Increasing the lysine block length created gels at lower concentrations but did not necessarily make stronger gels. An increase in the leucine block length with a simultaneous decrease in the lysine block decreased the gelation concentration and significantly increased the gel strength. Strain sweeps found that G′ decreased at high strains, with the critical strain decreasing with the diblock copolypeptide concentration. Thus, at higher concentrations, the gels were more brittle. Mixtures of different leucine block lengths were found to destroy the gel structure, as did racemic mixtures. Triblock structures with lysine end blocks were also investigated, which were similar to the diblocks in rheology and block length requirements for gelation. Lengthening the leucine block while simultaneously decreasing the lysine end blocks made the gels noticeably stronger but did not greatly influence the gelation concentration. Recent work has also been done with copolypeptides that, while not constructed of peptide blocks, fold to form amphiphilic structures that assemble, creating gels in aqueous solutions.30-33 Polyelectrolytes Grafted to Temperature-Sensitive Block Copolymers. Another method used to give PAA additional functionality is to graft PAA to a Pluronic [poly(ethylene oxideb-propylene oxide-b-ethylene oxide), PEO-PPO-PEO] backbone, resulting in a multiple-response PEO-PPO-PEO-PAA molecule that is sensitive to solution temperature, pH, and ionic strength. Bromberg34 studied a Pluronic-PAA molecule that consisted of nearly equal parts Pluronic and PAA in neutral aqueous solutions with no added salt. This copolymer formed micellelike aggregates with PPO cores and the ability to form bridges due to the PAA grafts, creating an elastic network. The copolymer solutions were non-Newtonian at most shear rates, having a short Newtonian plateau only at the lowest shear rates measured. Above the gelation temperature, elastic gels were observed for all of the concentrations studied. The moduli fit the single-mode Maxwell model below the crossover frequency but deviated at higher frequencies, indicating multiple relaxation times. An increase in gel strength was obtained when the copolymer concentration was increased. The crossover frequency decreased as the concentration increased; thus, the maximum relaxation time rose. Conclusions Block polyelectrolytes display interesting rheology, often forming highly elastic gels at a lower concentration than their neutral counterparts. The coupling of polyelectrolyte blocks to thermally responsive copolymers yields systems that are even more rheologically complex, with evidence of multiple relaxation processes. Some rheological characteristics of block polyelectrolyte solutions can be understood in terms of existing frameworks. However, several issues need to be more fully explored, including the degree of interpenetration of polyelectrolyte coronas in micellar solutions and the effect of pH and ionic strength on the fraction of bridging chains in triblock polyelectrolyte solutions. It is clear that future work will be necessary to fully understand how to control block polyelectrolyte solutions to obtain desired rheological properties.
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Acknowledgment This material is based upon work supported under a National Science Foundation Graduate Research Fellowship for A.S.K. Partial support was also provided by the NSF-supported UMass MRSEC on Polymers (DMR-0213695). Literature Cited (1) Hariharan, R.; Biver, C.; Mays, J.; Russel, W. B. Ionic Strength and Curvature Effects in Flat and Highly Curved Polyelectrolyte Brushes. Macromolecules 1998, 31, 7506. (2) Hariharan, R.; Biver, C.; Russel, W. B. Ionic Strength Effects in Polyelectrolyte Brushes: The Counterion Correction. Macromolecules 1998, 31, 7514. (3) Hill, R. J.; Saville, D. A.; Russel, W. B. Electrophoresis of spherical polymer-coated colloidal particles. J. Colloid Interface Sci. 2003, 258, 56. (4) Hill, R. J.; Saville, D. A.; Russel, W. B. Polarizability and complex conductivity of dilute suspensions of spherical colloidal particles with charged (polyelectrolyte) coatings. J. Colloid Interface Sci. 2003, 263, 478. (5) Fo¨rster, S.; Abetz, V.; Mu¨ller, A. H. E. Polyelectrolyte Block Copolymer Micelles. AdV. Polym. Sci. 2004, 166, 173. (6) Cohen Stuart, M. A.; Hofs, B.; Voets, I. K.; de Keizer, A. Assembly of polyelctrolyte-containing block copolymers in aqueous media. Curr. Opin. Colloid Interface Sci. 2005, 10, 30. (7) Shusharina, N. P.; Linse, P.; Khokhlov, A. R. Micelles of Diblock Copolymers with Charged and Neutral Blocks: Scaling and Mean-Field Lattice Approaches. Macromolecules 2000, 33, 3892. (8) Borisov, O. V.; Zhulina, E. B. Effect of Salt on Self-Assembly in Charged Block Copolymer Micelles. Macromolecules 2002, 35, 4472. (9) Zhulina, E. B.; Borisov, O. V. Self-Assembly in Solution of Block Copolymers with Annealing Polyelectrolyte Blocks. Macromolecules 2002, 35, 9191. (10) Zhulina, E. B.; Borisov, O. V. Theory of Morphological Transitions in Weakly Dissociating Diblock Polyelectrolyte Micelles. Macromolecules 2005, 38, 6726. (11) Shusharina, N. P.; Zhulina, E. B.; Dobrynin, A. V.; Rubinstein, M. Scaling Theory of Diblock Polyampholyte Solutions. Macromolecules 2005, 38, 8870. (12) Cameron, N. S.; Corbierre, M. K.; Eisenberg, A. Asymmetric Amphiphilic Block Copolymers in Solution: A Morphological Wonderland. Can. J. Chem. 1999, 77, 1311. (13) Zhang, L. F.; Eisenberg, A. Multiple Morphologies of ‘Crew-Cut’ Aggregates of Polystyrene-b-poly(acrylic acid) Block-Copolymers. Science 1995, 268, 1728. (14) Korobko, A. V.; Jesse, W.; Lapp, A.; Egelhaaf, S. U.; van der Maarel, J. R. C. Structure of strongly interacting polyelectrolyte diblock copolymer micelles. J. Chem. Phys. 2005, 122, 024902. (15) Bhatia, S. R.; Mourchid, A. Gelation of Micellar Block Polyelectrolytes: Evidence of Glassy Behavior in an Attractive System. Langmuir 2002, 18, 6469. (16) Bhatia, S. R.; Mourchid, A.; Joanicot, M. Block copolymer assembly to control fluid rheology. Curr. Opin. Colloid Interface Sci. 2001, 6, 471. (17) Bhatia, S. R.; Crichton, M.; Mourchid, A.; Prud’homme, R. K.; Lal, J. Tuning interactions between novel polyelectrolyte micelles. Polym. Prepr. (Am. Chem. Soc., DiV. Polym. Chem.) 2001, 42, 326.
(18) Grandjean, J.; Mourchid, A. Re-entrant glass transition and logarithmic decay in a jammed micellar system. Rheology and dynamics investigation. Europhys. Lett. 2004, 65, 712. (19) Ballauff, M. Nanoscopic Polymer Particles with a Well-Defined Surface: Synthesis, Characterization, and Properties. Macromol. Chem. Phys. 2003, 204, 220. (20) Marra, A.; Peuvrel-Disdier, E.; Wittemann, A.; Guo, X.; Ballauff, M. Rheology of dilute and semidilute suspensions of spherical polyelectrolyte brushes. Colloid Polym. Sci. 2003, 281, 491. (21) Fritz, G.; Scha¨dler, V.; Willenbacher, N.; Wagner, N. J. Electrosteric Stabilization of Colloidal Dispersions. Langmuir 2002, 18, 6381. (22) Tsitsilianis, C.; Iliopoulos, I. Viscoelastic Properties of Physical Gels Formed by Associative Telechelic Polyelectrolytes in Aqueous Media. Macromolecules 2002, 35, 3662. (23) Tsitsilianis, C.; Iliopoulos, I.; Ducouret, G. An Associative Polyelectrolyte End-Capped with Short Polystyrene Chains. Synthesis and Rheological Behavior. Macromolecules 2000, 33, 2936. (24) Tsitsilianis, C.; Katsampas, I.; Sfika, V. ABC Heterotelechelic Associative Polyelectrolytes. Rheology Behavior in Aqueous Media. Macromolecules 2000, 33, 9054. (25) Katsampas, I.; Tsitsilianis, C. Hierarchical Self-Organization of ABC Terpolymer Constituted of a Long Polyelectrolyte End-Capped by Different Hydrophobic Blocks. Macromolecules 2005, 38, 1307. (26) Bossard, F.; Sfika, V.; Tsitsilianis, C. Rheological Properties of Physical Gel Formed by Triblock Polyampholyte in Salt-Free Aqueous Solutions. Macromolecules 2004, 37, 3899. (27) Semenov, A. N.; Joanny, J.-F.; Khokhlov, A. R. Associating Polymers: Equilibrium and Linear Viscoelasticity. Macromolecules 1995, 28, 1066. (28) Dobrynin, A. V.; Colby, R. H.; Rubinstein, M. Scaling Theory of Polyelectrolyte Solutions. Macromolecules 1995, 28, 1859. (29) Breedveld, V.; Nowak, A. P.; Sato, J.; Deming, T. J.; Pine, D. J. Rheology of Block Copolypeptide Solutions: Hydrogels and Tunable Properties. Macromolecules 2004, 37, 3943. (30) Ozbas, B.; Rajagopal, K.; Schneider, J. P.; Pochan, D. J. Semiflexible Chain Networks Formed via Self-Assembly of b-Hairpin Molecules. Phys. ReV. Lett. 2004, 93, 268106. (31) Ozbas, B.; Kretsinger, J.; Rajagopal, K.; Schneider, J. P.; Pochan, D. J. Salt-Triggered Peptide Folding and Consequent Self-Assembly into Hydrogels with Tunable Modulus. Macromolecules 2004, 37, 7331. (32) Pochan, D. J.; Schneider, J. P.; Kretsinger, J.; Ozbas, B.; Rajagopal, K.; Haines, L. Thermally Reversible Hydrogels via Intramolecular Folding and Consequent Self-Assembly of a de NoVo Designed Peptide. J. Am. Chem. Soc. 2003, 1254, 11802. (33) Schneider, J. P.; Pochan, D. J.; Ozbas, B.; Rajagopal, K.; Pakstis, L.; Kretsinger, J. Responsive Hydrogels from the Intramolecular Folding and Self-Assembly of a Designed Peptide. J. Am. Chem. Soc. 2002, 124, 15030. (34) Bromberg, L. Self-Assembly in Aqueous Solutions of PolyetherModified Poly(acrylic acid). Langmuir 1998, 14, 5806.
ReceiVed for reView September 14, 2005 ReVised manuscript receiVed January 11, 2006 Accepted January 12, 2006 IE051034O
