Polymers in Aqueous Media

16 Hydrophobic Polyelectrolytes

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U l r i c h P. Strauss Department of Chemistry, Rutgers, The State University of N e w Jersey, New Brunswick, N J 08903

The incorporation of hydrophobic side chains into a polyelectrolyte can produce profound changes in the physical chemical behavior of the parent macromolecule. The hydrophobic attractions of the side chains can cause contractions in the molecular dimensions and aggregate formation. The presence of intramolecular micelles allows the solubilization of compounds with normally low water solubility. The solubilized compounds can have sizable effects on both intra-and intermolecular interactions. Selected studies carried out to determine the effects of hydrophobic group content and size, electrical charge, ionic strength, solubilization, and temperature on intramolecular micelle formation and intermolecular aggregation are reviewed. This chapter also discusses the solubilization of fluorescent probes, which has recently been found especially useful for following conformational transitions and for determining the size of the micelles.

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t i o n that, because soap m o l e c u l e s form m i c e l l e s spontaneously i n aqueous solution, m i c e l l e formation c o u l d b e e x p e d i t e d b y c h e m i c a l l y attaching surfactant m o l e c u l e s to p o l y m e r s . O u r first s u c h " p o l y s o a p " was p r e p a r e d b y the p a r t i a l q u a t e r n i z a t i o n o f p o l y ( 2 - v i n y l p y r i d i n e ) w i t h n - d o d e c y l b r o m i d e (J). V i s c o s i t y measurements i n d i c a t e d that this polysoap e x h i b i t e d abnorm a l l y compact m o l e c u l a r d i m e n s i o n s i n aqueous solutions, a result suggesting i n t r a m o l e c u l a r m i c e l l e formation. T h i s i d e a was c o n f i r m e d b y the fact that the polysoap s o l u b i l i z e d n o r m a l l y w a t e r - i n s o l u b l e h y d r o c a r b o n s . T h e a m o u n t s o l u b i l i z e d was greater than that s o l u b i l i z e d b y an e q u a l a m o u n t of c h e m i c a l l y related s i m p l e soap. I n contrast to the monosoap, n o c r i t i c a l

0065-2393/89/0223-0317$06.00/0 © 1989 American Chemical Society

In Polymers in Aqueous Media; Glass, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

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POLYMERS IN A Q U E O U S M E D I A

micelle concentration was necessary before solubilization occurred with the polysoap. The solubilization was proportional to the polysoap concentration, a finding confirming that the micelles were intramaeromoleeular (I). The early studies (2-5) showed that the solubilized hydrocarbons had pronounced effects on the viscosity of the polysoap solutions, and that these effects were both qualitatively and quantitatively different depending on whether the hydrocarbon was aliphatic or aromatic.


Effect of Hydrophobicity on Polysoap Behavior To study the effects of varying the number of soap molecules attached to a polymer chain while holding the total linear charge density constant, derivatives of poly(4-vinylpyridine) were prepared by quaternizing to various extents with n-dodecyl bromide and completing the process with ethyl bromide. In this way, close to 100% substitution was attained (6, 7). The variety of viscosity behavior of the derivatives, dissolved in dilute aqueous electrolyte, is illustrated in Figure 1 (7). A logarithmic scale was used to fit the reduced viscosity curves on the same graph. The curves fell into two distinct groups. The top two curves characterize an all ethyl bromide derivative (PEB) and a derivative with 6.7 mol % of the pyridine groups substituted with n-dodecyl bromide and most of the remainder substituted with ethyl bromide (6.7% polysoap). These curves indicate highly extended molecular dimensions typical of normal polyelectrolyte behavior. O f the two derivatives, the 6.7% polysoap has a lower intrinsic viscosity but a larger increase of the reduced viscosity with concentration; this result shows the intra- and intermolecular effects, respectively, of the hydrophobic attractions between the dodecyl groups. The bottom three curves, corresponding to 13.6%, 28.5%, and 37.9% polysoaps, show intrinsic viscosities about 2 orders of magnitude smaller than those of the top two curves and indicate compact conformation brought about by intramolecular micelle formation. The sharp change in intrinsic viscosity from the top to the bottom group suggests the existence of a critical dodecyl group content necessary for micelle formation, which might be considered the intramolecular equivalent of the critical micelle concentration of nonpolymeric surfactant (7). The steeper slope of the 37.9% polysoap curve compared to those of the 13.6% and 28.5% polysoap curves indicates intermolecular aggregation brought about by hydrophobic interactions. The behavior of polysoaps is also strongly influenced by their concentration, the ionic strength, and temperature. A typical example is presented in Figure 2, which shows the effect of added potassium bromide on the reduced viscosity of the 37.9% polysoap at two polysoap concentrations and two temperatures. The primary effect of the increase in ionic strength is a suppression of the electrostatic repulsions between the ionic groups. The decreased repulsions result in an intramolecular contraction (a well-known



Figure 1. Effect of dodecyl group content on reduced viscosity of poly(4vinylpyridine) derivatives in 0.0226 M KBr at 25 °C. Key: I, FEB; 2, 6.7% polysoap; 3, 13.6% polysoap; 4, 28.5% polysoap; 5, 37.9% polysoap. (Reproduced from ref 7. Copyright 1956 American Chemical Society.) polyelectrolyte effect manifesting itself in the initial negative slopes of the curves), and in an intermolecular aggregation evident from the positive slopes at higher ionic strengths. The intermolecular aggregation is clearly much more pronounced in the 6% than in the 1% polysoap solution. The effects of a temperature increase from 25 to 45 °C are most significant where the intermolecular interactions are greatest, a finding indicating that the aggregates are destabilized by heat. The 6% solution at 25 °C turned into a clear thixotropic jelly when the potassium bromide molality exceeded 0.035. Similar homogeneous gels could also be obtained with many other polysoap systems. The aggregation and disaggregation processes of the 37.9% polysoap were slow enough at 25 °C to be observed by appropriate viscosity


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Figure 2. Effect of KBr on reduced viscosity of 37.9% polysoap. Key: · , O, 1.00% polysoap solution at 25 and 45 °C, respectively; ©, Θ , 6.00% polysoap solution at 25 and 45 °C, respectively. (Reproduced from ref 7. Copyright 1956 American Chemical Society.)

experiments, but too fast at 45 ° C ; therefore, the aggregates are stabilized by a substantial activation energy (7). Polysoaps with more than 40 mol % of the pyridine groups substituted with dodecyl bromide were not watersoluble at room temperature (8). These results and their interpretations were later confirmed by light-scattering experiments (9) and by studies on similar polyvinylpyridine derivatives (10, 11).

Solubilization Effects Organic molecules with limited water solubility were solubilized by those polysoaps for which viscosity studies indicated micellar behavior (6). The solubilization had varied effects on the viscosity of the polysoap solutions, depending on the nature of the solubilized compound and the concentration


16.

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Hydrophobic Polyelectrolytes

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of the polysoaps (2-6). Aliphatic solubilized compounds depressed the viscosity under all conditions, whereas aromatic compounds depressed the viscosity at low polysoap concentrations but produced increasingly prominent viscosity maxima as the polysoap concentration was raised. Still more complex and pronounced viscosity effects were observed with amphiphilic compounds, such as long-chain aliphatic alcohols and nitroaromatic compounds, for which viscosity minima were observed at all polysoap concentrations. Maxima similar to those induced by aromatic hydrocarbons were found at high polysoap concentrations. With the help of supplementary light-scattering and electrical conductDownloaded by IOWA STATE UNIV on October 18, 2014 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0223.ch016

ance studies, the following method of accounting for these viscosity effects emerged (12,13): Aliphatic hydrocarbons are solubilized in the hydrocarbon core of the micelle where they cause a small contraction in the molecular dimensions of the polysoap molecules. Aromatic and amphiphilic molecules are solubilized by bridging the hydrocarbon and polar domains of the micelles. As a result, the polar regions are significantly contracted.

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phobic groups are then exposed to the aqueous environment and, in turn, cause aggregate formation. The molecular weight of the aggregates increases until the solubilization limit is reached. The viscosity maxima occur because the aggregation proceeds from asymmetric to globular complexes.

Conformational Transitions In addition to the polyvinylpyridine derivatives, the alternating copolymers of maleic acid and alkyl vinyl ethers have provided useful information about the behavior of hydrophobic polyelectrolytes. For these polyacids, hydrophobicity can be controlled by the alkyl group size, and the charge density can be varied by p H (14, 15). With the alkyl group containing up to three carbon atoms, the macromolecules behave as typical polyacids (16), whereas when the alkyl group contains 12 or more carbon atoms, the macromolecules behave like typical polysoaps (17). The copolymers whose alkyl group size falls between these extremes have compact conformations (typical of polysoaps) at low p H where their charge density is small and random coil conformations at high p H where their charge density is large (14, 15). This effect is illustrated most strikingly with the fluorescence behavior of a chemically attached dansyl probe (18). The emission of this probe near 500 nm is large in nonpolar and small in polar media. In Figure 3, the fluorescence intensity at 520 nm of a dansylated butyl copolymer is given as a function of a, the degree of deprotonation (defined so as to equal 2 at complete deprotonation) (18). At low a, the fluorescence intensity is large, a result that indicates that the probe molecules are surrounded by nonpolar groups. This conformation would be expected from compact micellar regions consisting of the nonpolar butyl groups. As the ionic charge on the polyacid is increased, the fluorescence intensity drops sharply, a result that indicates


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a transition from the compact micellar conformation to a random coil state in which the butyl groups are separated from one another and the probe is surrounded by a polar aqueous environment. The figure also shows that added electrolyte stabilizes the compact structure. The initial fluorescence intensity is increased, and the conformational transition is shifted to higher values of a. The stabilization is presumably due to a reduction in repulsion between the ionized groups of the polyacid. Changes in the polymer concentration had no significant effect on the fluorescence behavior; therefore, the transition was an intramolecular rather than an intermolecular phenomenon. In contrast to the butyl copolymer, the dansylated methyl copolymer, whose fluorescence is also shown in Figure 3, undergoes no transition; its fluorescence is essentially constant over the whole ionization range. The low value is characteristic of the aqueous en vironment that the probe experiences in the random coil conformation.

Micelle Size The conformational transition of the butyl copolymer, as well as those of the pentyl, hexyl, and octyl copolymers, has also been observed by viscosity (15), by potentiometric titration (14,15,19), and by calorimetric studies (16,

OC Figure 3. Dependence of F 520, the fluorescence emitted at 520 nm by dansylated copolymers, on the degree of deprotonation, a. Key: Θ , 3, ©, butyl copolymer in water, 0.2 M NaCl, and 0.5 M NaCl, respectively; ·, methyl copolymer in water. (Adapted from ref. 18. Copyright 1975 American Chemical Society.)


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20, 21). Analysis of the potentiometric titration data suggested that the transition could be described in terms of a cooperative break-up of uniformly sized small micelles formed from adjacent chain elements. A method was developed for estimating the size of these micelles from potentiometric titration data. By this method, the micelle for the butyl copolymer was found to comprise close to 20 repeat units (22, 23); however, this method was not easy to apply to the copolymers with larger hydrocarbon groups because the conformational transition was not sharp enough. A method based on fluorescence quenching that did not depend on the nature of the transition was used to determine the micelle size of the hexyl Downloaded by IOWA STATE UNIV on October 18, 2014 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0223.ch016

copolymer (24). The basic idea underlying this method is that, in a solution containing luminescent probe and quencher molecules, both solubilized in an excess of micelles, the quenching will be inversely related to the number of micelles, because the more micelles there are, the smaller is the chance of both a probe and a quencher molecule inhabiting the same micelle (25-27). The hexyl copolymer used in our study had a degree of polymerization of 1700.

T h e fluorescent probe was tris(2,2'-bipyridine)ruthenium(II) ion

[Ru(bpy) ], the quencher was 9-methylanthraeene (9-MeA), and the sol3

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vent was an aqueous 0.1 M L i C l solution. The fluorescence experiments were supplemented with solubilization experiments; from these, the distribution of the 9-MeA between the polymer molecules and the solvent molecules, as well as the extent to which the polymer was in micellar form, could be simultaneously determined. The results indicated that the micelles inside the domain of a macromolecule encompassed approximately 24 repeat units, and that this micelle size was independent of the polymer concentration, of the probe concentration, and the extent to which the polymer was micellized.

Conclusion The research reviewed here demonstrates that polyelectrolytes experience a variety of profound alterations in their physicochemical behavior with the controlled introduction of hydrophobicity into their chemical structure. The wide range of untried structural variations that can be envisioned ensures continuing exploration in this fertile area of research and gives promise of further attractive applications.

References 1. 2. 3. 4. 5.

Strauss, Strauss, Layton, Strauss, Layton,

U. P.; Jackson, E . G . J. Polym. Sci. 1951, 6, 649. U. P.; Jackson, E . G . J. Polym. Sci. 1951, 7, 473. L. J.; Jackson, E . G . ; Strauss, U. P. J. Polym. Sci. 1952, 9, 295. U. P.; Layton, L. H.; J. Phys. Chem. 1953, 57, 352. L. H.; Strauss, U. P. J. Colloid Sci. 1954, 9, 149.



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6. Strauss, U . P.; Gershfeld, N. L. J. Phys. Chem. 1954, 58, 747. 7. Strauss, U. P.; Gershfeld, N. L.; Crook, Ε . H. J. Phys. Chem. 1956, 60, 577. 8. Strauss, U. P.; Assony, S. J.; Jackson, E . G.; Layton, J. H. J. Polym. Sci. 1952, 9, 509. 9. Strauss, U . P.; Williams, B. L . J. Phys. Chem. 1961, 65, 1390. 10. Woerman, D.; Wall, F. T. J. Phys. Chem. 1960, 64, 581. 11. Inoue, J. Kolloid Z. 1964, 195, 102. 12. Strauss, U . P.; Slowata, S. S. J. Phys. Chem. 1957, 61, 411. 13. Richlin, J. Ph.D. Thesis, Rutgers University, 1964. 14. Dubin, P.; Strauss, U . P. J. Phys. Chem. 1967, 71, 2757. 15. Dubin, P. L . ; Strauss, U . P. J. Phys. Chem. 1970, 74, 2842. 16. Martin, P. J.; Strauss, U . P. Biophys. Chem. 1980, 11, 397. 17. Itko, K.; Ono, J.; Yamashita, Y. J. Colloid Sci. 1964, 19, 28. 18. Strauss, U . P.; Vesnaver, G . J. Phys. Chem. 1975, 79, 2426. 19. Strauss, U . P.; Schlesinger, M . S. J. Phys. Chem. 1978, 82, 571. 20. Delben, F.; Crescenci, V. J. Solution Chem. 1978, 7, 597. 21. Martin, P. J., Morss, L. R.; Strauss, U . P. J. Phys. Chem. 1980, 84, 577. 22. Strauss, U . P.; Barbieri, B. Macromolecules 1982, 15, 1347. 23. Barbieri, B. W.; Strauss, U . P. J. Phys. Chem. 1985, 18, 411. 24. Hsu, J. L.; Strauss, U . P. J. Phys. Chem. 1987, 91, 6238. 25. Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978, 100, 5951. 26. Chu, D . Y.; Thomas, J. K. J. Am. Chem. Soc. 1986, 108, 6270. 27. Chu, D.Y.; Thomas, J. K. Macromolecules 1987, 20, 2133. R E C E I V E D for review March 31, 1988. A C C E P T E D revised manuscript November 1, 1988.


Polymers in Aqueous Media

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