Conformational Transitions in Polyelectrolyte Networks in Binary Solvents: Microheterogeneities in the Collapsed State

Abstract

Microheterogeneities in positively charged gels of diallyldimethylammonium bromide copolymerized with acrylamide and negatively charged gels of sodium and cesium methacrylate copolymerized with acrylamide appear as a result of the collapse of these gels in poor solvents (water- ethanol mixtures). Three possible reasons for microheterogeneities are analyzed: the polyelectrolyte effect (Le., competition between the attraction of the uncharged parts of the chains in poor solvent and electrostatic repulsion coupled with osmotic pressure of counterions), the ionomer effect &e., the formation of ionomer multiplet structure), and vitrification (i.e., partial formation of glassy kinetically frozen polyacrylamide-rich regions). Both macroscopic observations and SAXS experiments were made. The dry and water-swollen gels immersed in the water-ethanol mixtures show different final states at high ethanol contents, which proves the existence of kinetically frozen structures for these cases. For highly charged cationic gels we observed an increase of the scattering exponent which correlates with the volume phase transition while for smaller charge density this increase occurs at higher ethanol contents with the appearance of kinetically frozen structures. These results suggest that in the former case the microdomain structure appears mainly as a result of the ionomer effect while in the latter case the main factor is partial vitrification. In contrast to recent results in purely aqueous systems, no SAXS scattering maxima were observed, apparently due to the high irregularity of the microstructures.