Surfactant Responsive Cross-Linked Polymer Hydrogels

Sammanfattning: The focus of the thesis is on chemically cross-linked hydrogels which drastically change their volume when immersed in solutions of different ionic surfactants (with or without added salt). The two main questions addressed are: (1) How useful are simple gel swelling experiments to the study of polymer-surfactant interactions?, and (2) How does the volume of a cross-linked polymer gel change when it is immersed in a surfactant solution, both with respect to the equilibrium swelling and with respect to transient volume changes? Most of the gels studied were prepared by cross-linking linear polymers, which were well studied derivatives of hydroxyethyl cellulose (HEC). Therefore, it was possible to compare the results from the gel swelling experiments with results from previous studies on the same polymer-surfactant systems using other experimental methods, such as solubility studies, NMR self-diffusion measurements, and fluorescence measurements. For comparison, chemically simpler vinyl type gels were synthesized and included in the study. It is concluded that the gel swelling experiment is a simple, but potent, method to study polymer-surfactant interactions. It is especially useful for distinguishing between associating and nonassociating polymer-surfactant pairs, for detecting critical association concentrations (cac:s), and for studying the changes in the polymer dimensions that occur within the one- and two-phase regions of the corresponding mixtures of linear polymers and surfactants. The gel-surfactant systems studied are classified into three groups: (1) slightly hydrophobic nonionic gels, (2) polyelectrolyte gels (with oppositely charged surfactants), and (3) hydrophobically modified gels. For the two first groups generic swelling isotherms are discussed. The hydrophobicity of both the gel polymer and the surfactant tail is shown to be of great importance. The thesis also includes two literature reviews, which place surfactant responsive gels in a broader scientific context. The first review deals with the history of cross-linked gels, and the second with general swelling trends of responsive hydrogels.