the invention relates to a terpolymer of tetrafluoroethylene monomer , polyvinylidene fluoride monomer and hexafluoropropylene monomer for forming an ultrafiltration or microfiltration membrane , method of forming said membranes , and to the ultrafiltration or microfiltration membranes themselves . the invention also relates to a method of forming a polymeric ultrafiltration or microfiltration membrane including preparing a leachant resistant membrane dope which incorporates a leachable pore forming agent , casting a membrane from the dope and leaching the pore forming agent from the membrane . the invention also relates to a method of preparing a polymeric ultrafiltration or microfiltration membrane of improved structure including the step of adding a nucleating agent to the membrane dope before casting .