Polypropylene blends with potential as materials for microporous membranesformed by melt processing

Novel microporous membranes with pore size ranging from 2 to 25 nm were pro duced from immiscible polypropylene blends via melt processing and post-ext rusion treatments. Systems containing polystyrene and polyethylene terephth alate as the minor phase components were employed as starting membrane mate rials at concentrations not exceeding 15 wt%. The blends were first compoun ded 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 micro porous structure and then post-treated at elevated temperatures to stabiliz e the porous structure, which consisted of uniform microcracks 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 morphologie s are presented. Finite element modeling of the stretching operation in the solid state yielded a successful interpretation of the blend response to u niaxial tension that resulted in microcrack formation. The processes develo ped in this work may be considered as solventless alternatives to phase inv ersion manufacturing practices for membranes containing mesopores. (C) 2001 Elsevier Science Ltd. All rights reserved.