Preparation and glass transition temperatures of elastomeric PolyHIPE materials

Highly porous elastomeric PolyHIPE materials have been prepared by polymerisation of the continuous phase of high internal phase emulsions (HIPEs) containing styrene, divinylbenzene and varying amounts of either 2-ethylhexyl acrylate or the corresponding methacrylate monomer. The glass transition temperatures of the resulting copolymers were investigated by differential scanning calorimetry (DSC). For each series,Tg was found to vary non-linearly with copolymer composition. By consideration of the comonomer unit sequencing along the polymer backbone, as deduced from reactivity ratios, the relationship between Tg and composition in each case has been qualitatively explained. Comparison between experimental results and values calculated using an entropic approach have been made. It is thought that the (meth)acrylate comonomers reduce the overall Tg by three mechanisms: by introducing large amounts of free volume, due to their bulky side groups; by providing a much higher degree of flexibility to the polymer chains; and by reducing the proportion of adjacent styrene units, which display particularly unfavourable steric interactions.