Structure development in oriented polyethylene films and microporous membranes as monitored by sound propagation

Microporous membranes of high-density polyethylene were prepared by melt-ex trusion followed by annealing and uniaxial extension. Crystallization at a high melt flow rate and subsequent annealing of the spun films with fixed e nds led to the formation of oriented hard-elastic materials with a high mod ulus of elasticity and a considerable work recovery. Uniaxial stretching of such systems along the orientation direction induced the formation of micr oscopic pores due to the specific structure of the hard-elastic material. A t some critical values of the processing parameters, throughflow channels w ere formed, converting the film into a microporous membrane permeable to li quids and vapors. Sound propagation, tensile measurements, and X-ray diffra ction techniques were used to characterize the structure and properties of the samples at individual stages of the process as a function of the proces sing parameters. In particular, it was shown that polar diagrams of the sou nd propagation velocity reflect sensitively the structural changes in the p rocess of porous structure formation. (C) 2001 John Wiley & Sons, Inc.