Process for preparing microporous hollow fibers

A process for preparing hollow open-celled microporous fibers which comprises:

• (a) providing from at least one polyolefin having both an amorphous component and a crystalline component, solidified polymeric non-porous hollow precursor fibers with a degree of molecular orientation characterized by a birefringence of from about 0.0008 to about 0.005;
• (b) contacting said hollow precursor fibers with a swelling agent, said swelling agent comprising a non-aqueous solvent having a Hildebrand solubility parameter at or near that of the polymeric hollow precursor fibers, for a time sufficient to permit absorption of the swelling agent into the hollow fibers;
• (c) stretching said hollow precursor fibers while in contact with the swelling agent, which is maintained at a temperature greater than the glass transition temperature of the precursor fibers and the freezing point of the swelling agent and less than the temperature at which the precursor fibers dissolve, at a degree of stretch of from about 150 to about 400% based on the initial precursor fiber length and a strain rate of from about 5 to about 150%/min.;
• and
• (d) removing the swelling agent from said hollow fibers while maintaining said hollow fibers in the stretched state.