Formation mechanism of porous structure in polytetrafluoroethylene (PTFE) porous membrane through mechanical operations

Polymeric porous membranes were prepared from polytetrafluoroethylene (PTFE ) fine powder by a series of mechanical operations, such as extrusion, roll ing, and stretching. The structure of the prepared porous membrane was well characterized by a spatial periodicity of nodes (domain of agglomerated PT FE particles) and fibril domains. The fibrils were highly oriented in the d irection of the stretching operation, providing pores in the polymeric memb rane as slit-like voids between adjoining fibrils. The unit size of the per iodic structure varied depending on the number averaged molecular weight of PTFE and the stretching conditions, the temperature of stretching, and the stretching rate and stretching ratio. A fibril consisted of several thread -like structures that were easily formed between PTFE particles due to the rolling operation in parallel with their direction. The dependence of the s teady tensile stress in the stretching operation on the PTFE molecular weig ht was much weaker than that presumed for noncrystalline polymeric systems. The activation energy of 11.3 kJ/mol for the growth of fibrils was only se veral times as large as the thermal energy at the ambient temperature. Thes e results imply that the thread-like structures can easily be pulled out of PTFE particles. This view is in accordance with the previously proposed mi crostructure in PTFE particles.