Microporous membranes of polyoxymethylene from a melt-extrusion process: (I) effects of resin variables and extrusion conditions

A two-part study utilizing polyoxymethylene (POM) was undertaken to investigate a three stage process (melt extrusion/annealing/uniaxial stretching) (MEAUS) employed to produce microporous films. In this first part, three POM resins (D, E, and Fl were melt extruded into tubular films (blowup ratio; BUR = 1), where resin D has a higher weight average molecular weight (M-w) than resin E, but both possess similar and relatively narrow molecular-weight distributions (MWD). In contrast, resin F is characterized by a distinctly broader MWD while its M-w is slightly higher than resin D. Specific attention was focused upon the morphological and crystal orientation results as a function MWD and M-w. A stacked lamellar morphology was obtained in each case from the melt extrusion; however, the type of stacked lamellar morphology, planar or twisted, and the orientation state was found to depend upon both the resin characteristics and the melt-extrusion conditions. Atomic force microscopy and wide-angle X-ray scattering were the main techniques utilized to study the melt-extruded films while dynamic melt rheometry in conjunction with the Carreau-Yasuda model aided in differentiating the melt-flow behavior of the three resins. Small-angle light scattering (SALS) was also employed to characterize the morphological state. (C) 2001 John Wiley & Sons, Inc.