STRUCTURE GENERATION IN PTFE POROUS MEMBRANES INDUCED BY THE UNIAXIALAND BIAXIAL STRETCHING OPERATIONS

The morphology and its formation has been studied for the steady-rate stretching operation of polytetrafluoroethylene (PTFE) porous membrane s, which were prepared from PTFE fine powders with a mean diameter of approximately 2 x 10(2) nm through extruding and rolling operations pr ior to the stretching operation. The uniaxially stretched membranes we re produced by the unique stretching operation parallel to the rolling operation, and the biaxially stretched membranes by the dual operatio ns consisting of the parallel and the subsequent perpendicular stretch es. The inversion of the stretching direction, i.e., the first stretch ing operation perpendicular to the rolling operation and the second on e parallel has been observed to be impossible due to the occurrence of macroscopic fractures on the membrane during the first stretching ope ration. The uniaxially stretched porous membranes are comprised of fib rils completely oriented in the stretching direction and remaining isl and-like fractures of the rolled PTFE sheet. The second stretching ope ration provides a lattice-like porous structure by giving the island-l ike fractures further division along the second stretching direction a nd the fibrils slant from the original orientation. The stretching ope ration is possible within the range where the relative elongation of t he whole membrane along the second stretching direction is less than 5 0% of that along the first one, indicating that the fibrils yielded by the first stretching operation sustain the lattice-like porous struct ure induced by the second stretching operation. The distribution of th e slant angle of the fibrils is independent of the elongation in the s econd stretching operation, thus, the division of the island-like frac tures linked with the fibrils steadily proceeds during the second stre tching operation. (C) 1998 Elsevier Science B.V. All rights reserved.