Glass transition in homogeneous and heterogeneous interpenetrating polymer networks and its relation to concentration fluctuations

Abstract The glass transition of sequential poly(methyl acrylate)–poly(methyl methacrylate) interpenetrated polymer networks (IPNs) has been studied by means of temperature modulated differential scanning calorimetry. The poly(methyl acrylate) network was polymerised first, swollen up to equilibrium in methyl methacrylate monomer which was then polymerised. Ethyleneglycol dimethacrylate was used as cross-linking agent in a proportion of 0.1% or 10% by weight. Both networks in the IPNs had the same cross-linking concentration. The loosely cross-linked IPN presents clearly two glass transitions as corresponds to a phase-separated system. On the contrary a single broad glass transition can be observed in the highly cross-linked network. A simple model is presented to determine at each temperature the fraction of material which has a conformational mobility characteristic of a liquid. The temperature dependence of this liquid fraction calculated for the highly cross-linked network is very similar to the experimental temperature modulated differential scanning calorimetry thermogram. This allows one to conclude that at least a part of the broadening of the glass transition temperature interval of the IPN with respect to the pure component networks is expected even in a completely homogeneous blend due to the probability that the local composition is different than the average. A great part of the material vitrifies at temperatures lower than the glass transition that would correspond to the average composition, what can be explained by the temperature dependence of the length of cooperativity.