Change of structure and phase behaviour during homo- and copolymerisation of (2-methacryloyloxyethyl)dodecyldimethylammonium bromide in a hexagonal lyotropic mesophase

Abstract

The γ-ray initiated polymerisation of (2-methacryloyloxyethyl) dodecyldimethylammonium bromide in a hexagonal lyotropic mesophase was investigated. The binary mixture with water and the ternary mixture with water and acrylamide were studied. Especially the changes in the structure and the phase behaviour during polymerisation were investigated. Small-angle X-ray scattering indicates that the polymerisation of the binary mixture is accompanied by a discontinuous increase of the d spacing from 3.08 to 3.36 nm at about 5% conversion although the hexagonal order of the system is retained. The increase originates from a conformational transition in the headgroup region of (2-methacryloyloxyethyl)dodecyldimethylammonium bromide and leads to a strong acceleration of the reaction rate. Evidence for the thermodynamic stability of the system during the entire reaction process is presented. Admixture of acrylamide to the binary system results in a decrease in the transition temperature from the hexagonal phase to the isotropic solution until for a system with (2-methacryloyloxyethyl)dodecyldimethylammonium bromide and acrylamide in equimolar ratio the mesophase has completely disappeared at room temperature. However, upon γ-ray irradiation the transition temperature is raised again and for systems which are not liquidcrystalline in the monomeric state a mesophase is eventually induced. The polymerisation rate of the ternary system is lower because the acrylamide induces a different surfactant conformation in the mesophase. IR spectroscopic analysis of the copolymer formed in the ternary system indicates that acrylamide is enriched at low conversion.