Association Phenomena Involving Hydrophobically Modified Polymers. Electrostatic and Hydrophobic Contributions

Abstract: The electrostatic and hydrophobic interactions in aqueous solutions of hydrophobically-modified water-soluble non-ionic or ionic polymers, their mixtures with a second polymer, nonionic or of opposite charge, and their mixtures with surfactants of opposite charge have been investigated. The relative strength of the electrostatic and hydrophobic contributions has been modulated by a variation of the polymer charge density and the solution pH, and by a variation in the degree and strength of the hydrophobic modifications, respectively. The repercussions of such interactions on the thermodynamics and dynamics of the systems under study have been quantified experimentally with the aid of phase behavior, rheology, and light scattering techniques, and have been rationalized theoretically in the context of dynamic scaling theory. The association between two oppositely charged hydrophobically modified polyelectrolytes can lead to the formation of soluble or insoluble mixtures depending on the mixture composition, the total polymer concentration, and the charge density of the polyions. The simultaneous presence of ionic and hydrophobic groups in both polymers results in a broad miscibility region (instead of the precipitate expected if only electrostatics were in effect), where the viscosity is three to four orders of magnitude higher than that of the solutions of the individual polymers. The physical networks obtained through the formation of mixed aggregates are especially reinforced in the proximity of phase separation as revealed by the existence of two relaxation modes, a fast diffusive mode and a slow mode (attributed to enhanced hydrophobic associations) in the time correlation data obtained from light scattering measurements. The network dynamics are strongly affected by the presence of the hydrophobic moieties and by the mixture composition, thus making the hydrophobically modified polyelectrolyte mixtures exhibit rheological properties of complex scaling behavior. However, the binary mixtures of unmodified polyelectrolytes as well as the single polyelectrolyte solutions exhibit a viscosity scaling behavior in good agreement with recent predictions of the dynamic scaling theory of polyelectrolytes in the semidilute regime. The interactions between polymers and surfactants of opposite charge in aqueous solutions can lead to the formation of transient networks, the rheological properties of which have been modulated via the addition of cyclodextrins. Cyclodextrins can form inclusion complexes with the hydrophobic moieties of surfactants, and thus they disrupt the polymer-surfactant network as evidenced by their ability to reverse the high viscosities exhibited by such networks.