Porous structures controlled by segregation of ordered mesophases in poly(N,N-dimethylacrylamide) hydrogels polymerized from an isotropic AOT/water medium

Abstract

N,N-dimethylacrylamide with cross-linker (N,N′-methylenebis(acrylamide)) is polymerized in the presence of various amounts (4.5–51% v/v) of the anionic surfactant AOT. All initial mixtures are isotropic homogeneous solutions, but with polymerization they become microheterogeneous due to development of an anisotropic mesophase whose structure varies from lamellar to cubic to hexagonal (on increasing the AOT concentration). The polymerization is started in two ways: with initiator (AIBN), and induced by X-rays (without initiator). The hydrogels obtained at the end of polymerization with AIBN are analysed regarding swelling capacity and porous structure. Swelling increases with AOT concentration, and the rate of this increase is larger when the mesophase formed is hexagonal than when it is lamellar. These ordered domains induce the following porogen effects: (i) more macropores with a better defined geometry are formed when the mesophase is hexagonal. (ii) The mean diameter of the pores is controlled by the type of mesophase: 25 μm (lamellar); 55 μm (hexagonal). The evolution of polymerization induced by X-rays is followed during a period of time to study the development of the mesophase that occurs by a segregation process, and a model in four steps is proposed to explain the results.