a method for making a semi - permeable microporous polymer sheet , useful as a separation membrane or the like , including extruding a melt - blended mixture of a high - temperature thermoplastic polymer and a leachable glass to form sheet and then leaching the glass from the sheet to leave a microporous relict polymer network , wherein the permeability of the sheet is improved by plasma - etching the surface of the sheet before or after leaching . hydrophilic or hydrophobic porous membranes may be provided .