Induced phase transitions by cross-linking polymerization of N,N-dimethylacrylamide within isotropic AOT/water phases.

Hydrogels of polydimethylacrylamide (PDMAA) cross-linked with bisacrylamide (BA) are synthesized in mixtures of 1,4-bis(2-ethylhexyl) sulfosuccinate sodium salt (AOT) and water with different concentrations. Before polymerization, all samples are isotropic. They become anisotropic due to a phase separation process induced by polymerization. The polymer, which stays in an isotropic microphase, induces the formation of a segregated AOT/water microphase containing ordered domains. These ordered domains present lamellar, cubic, and hexagonal lyotropic organizations, depending on the AOT concentration in the initial isotropic mixture. The lattice parameter of these ordered phases has been analyzed as a function of the composition of the microphases, which has been determined using a simple model, assuming that there is an interaction between the surfactant and the cross-linker that produces a distribution of the AOT between both the polymer rich phase and the lyotropic domains.