STRUCTURAL CHARACTERIZATION OF CELGARD(R) MICROPOROUS MEMBRANE PRECURSORS - MELT-EXTRUDED POLYETHYLENE FILMS

''Row nucleated lamellar'' structures are formed when highly crystalli ne polymers are melt-extruded and recrystallized under high stress. Po lyethylene (PE) and polypropylene (PP) films with row lamellar structu res have been utilized to produce microporous membranes. Birefringence measurements of melt-extruded PE films show that improved film orient ation can be achieved by annealing, extruding at higher speed, and usi ng higher molecular weight polymers. Images from scanning tunneling, a tomic force, and field emission scanning electron microscopy (STM, AFM , and FESEM) clearly show the lamellar structures in the melt-extruded PE and PP films. Microscopy results also show that surface lamellar t extures are more pronounced with thicker lamellae and are better align ed along the extrusion direction after annealing. X-ray diffraction re sults show that the increase in film orientation can be attributed to increased lamellar perfection and orientation during annealing and als o to better crystallite alignment along the machine direction with hig her extrusion speed or with higher molecular weight. High-resolution c apabilities of STM, AFM, and FESEM prove to be very effective tools in elucidating lamellar structures in polymeric membrane precursors and can be used as an aid in establishing structure-process-property relat ionships in making microporous membranes. (C) 1994 John Wiley & Sons, Inc.