Polypropylene blends with potential as materials for microporous membranes formed by melt processing

Novel microporous membranes with pore size ranging from 2 to 25 nm were produced from immiscible polypropylene blends via melt processing and post-extrusion treatments. Systems containing polystyrene and polyethylene terephthalate as the minor phase components were employed as starting membrane materials at concentrations not exceeding 15 wt%. The blends were first compounded in a co-rotating twin-screw extruder and subsequently extruded through a sheet die to obtain the non-porous precursor films. These were uniaxially drawn (100-500%) with respect to the original dimensions at a temperature below the glass transition temperature of the minor phase to induce a microporous structure and then post-treated at elevated temperatures to stabilize the porous structure, which consisted of uniform micro-cracks in the order of a few nanometers in width. The effects of dispersed phase concentration and component melt rheology on the solid and microporous blend morphologies are presented. Finite element modeling of the stretching operation in the solid state yielded a successful interpretation of the blend response to uniaxial tension that resulted in microcrack formation. The processes developed in this work may be considered as solventless alternatives to phase inversion manufacturing practices for membranes containing mesopores.