Morphology change in polytetrafluoroethylene (PTFE) porous membrane causedby heat treatment

The morphological changes and improvement of membrane properties caused by heat treatment were investigated for polytetrafluoroethylene (PTFE) porous membranes prepared from a fine powder by extrusion, rolling and stretching. The properties of the membrane were significantly changed by heat treatmen t at temperatures higher than 320 degrees C. Shrinkage diminished and the m echanical strength increased due to the partial melting of PTFE. The increa se in mechanical strength was caused by suppression of new fibril formation as a result of the loss of folded ribbon-like crystalline structures that facilitated fibril structures to be pulled out of the original PTFE particl e. The decrease in shrinkage was caused by the transformation of fibrils, f ormed as a collection of ribbon-like structures, into a massive fibrous str ucture, which inhibited the reformation of particles. The most important ch ange of the porous structure caused by the heat treatment was the union of nodes in the direction of stretching resulting in a PTFE porous membrane wi th larger spatial periodicity. A heat treatment above melting temperature o f PTFE was the most effective. However, it was necessary to control the tem perature and time in order to restrict the coarseness of the porous structu re of the membrane.