Impact of Cross-Linking Density and Glassy Chain Dynamics on Pore Stability in Mesoporous Poly(styrene)

Mesoporous poly(styrene) (PS) containing various amounts of the cross-linker divinylbenzene was synthesized by a hard-templating routine using pressed pellets of fumed silica as templates. Porous polymers with a surface area of 215 m² g⁻¹ and porosities up to ∼35 vol % could be obtained. The impact of the cross-linker content and the processing on the porosity of the materials was investigated by nitrogen sorption and small-angle X-ray scattering. The surface area of the nascent porous PS increases and the pore size decreases with increasing amount of cross-linker. Furthermore, it was found that the porous polymers lose surface area at temperatures that are substantially below the glass transition temperature, T_g, of bulk polystyrene. The loss in surface area at temperatures below 65 °C suggests that partial pore collapse also may be induced in the glassy state. Finally, the stability of the mesopores against solvent swelling was investigated. A critical pore collapse was found at a cross-linker content of ∼20 wt %. In this context, also the existence of traced porosity, i.e., a nonaccessible mesostructure or porosity, could be confirmed.