Rapid pH-dependent phase transition and elasticity of stimuli-responsive cationic poly(N,N-dimethylaminoethyl methacrylate) hydrogels prepared with a dimethacrylate crosslinker

Stimuli-responsive hydrogels prepared from poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) and its copolymers have attracted much interest to serve in biomedical and pharmaceutical applications. To investigate pH-dependent swelling and elasticity, a series of cationic hydrogels based on N,N-dimethylaminoethyl methacrylate were prepared by free radical crosslinking copolymerization at 60°C in the presence of tetraethylene glycol dimethacrylate as the crosslinker. The equilibrium swelling and the mechanical properties of the PDMAEMA hydrogels were investigated as a function of the gel preparation concentration. To explain the effect of pH on the equilibrium swelling of the hydrogels, pH-dependent swelling studies were carried out in solutions of pH ranging from 2.1 to 10.7. It was found that the PDMAEMA hydrogels exhibit a rapid pH-dependent phase transition in aqueous solutions; that is, the gels first remain in the swollen state at acidic pH then collapse in a very narrow range of pH. The results showed that the volume of PDMAEMA hydrogels in acidic conditions is about 10- to 40-fold larger than that in the basic pH region. By using the Flory-Rehner theory, the characteristic network parameters of the PDMAEMA hydrogels were calculated and good agreement obtained between the swelling equilibria of hydrogels and their mechanical properties over the whole range of gel preparation concentration.