Polymeric hydrophilic hydrogels with flexible hydrophobic chains. Control of the hydration and interactions with water molecules

Acrylic hydrogels based on 2-hydroxyethyl methacrylate, H, have been synthesized by copolymerization reaction of this monomer with 2-ethylhexyl acrylate, E, using AIBN as initiator. Reactivity ratios mere estimated from copolymerization reactions carried out in solution and at low conversion, by using both linearization and nonlinearization methods. They were round to be r(E) = 0.29 and r(H) = 2.54. The swelling behavior of the hydrogels was studied by immersion of copolymer films prepared by bulk copolymerization, in water at different temperatures. Equilibrium water uptake was strongly dependent on composition and decreased with the E content in the copolymer. Fickian behavior was observed in all cases for reduced sorption coefficients lower than 0.4. The diffusion coefficient was found to decrease with increasing E content in the copolymer. A value of apparent activation energy for the diffusion process of 8.8 kJ/mol was obtained for poly(2-hydroxyethyl methacrylate); however, H/E copolymers did not obey the Arrhenius behavior over the short temperature interval studied. Differential scanning calorimetry has been used to study the organization of water in the copolymer hydrogels. The amounts of nonfreezable water for the copolymers with 5 and 10 wt % of E content were found to be 28 and 26 wt %, respectively. The surfaces of the copolymer films were characterized by contact angle measurements. The surface free energy decreased noticeably with the presence of 10 wt % of the flexible hydrophobic monomer, the reduction of the polar component being higher than that of the dispersive component. Segregation of the components in microdomains, the H segments being the continuous phase, is postulated tentatively in the light of the results obtained.