When CO2 dissolves into a polypropylene (PP), its crystallization kinetics changes These changes were studied, in the expectation that the information would reflect on the behavior of other semicrystalline polyolefins. The isothermal crystallizatior rate of the PP‐CO2 solutions was measured using a high‐pressure differential scanning calorimeter (DSC), which performed calorlmetric measurements while keeping the polymer in contact with pressurized CO2. Although the measured crystallizatior rate followed the Avrami equation, the value of the crystallization kinetic constant was different from that measured for PP crystallized in air under atmospheric pressure. The dissolved CO2 decreased the overall crystallization rate of PP within the nucleation dominated temperature region. This suggests that the dissolved CO2 decreases the melting and the glass transition temperatures and prevents formation of critical size nuclei.
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 treatment at temperatures higher than 320°C. Shrinkage diminished and the mechanical strength increased due to the partial melting of PTFE. The increase 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 particle. The decrease in shrinkage was caused by the transformation of fibrils, formed as a collection of ribbon‐like structures, into a massive fibrous structure, which inhibited the reformation of particles. The most important change 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 with larger spatial periodicity. A heat treatment above melting temperature of PTFE was the most effective. However, it was necessary to control the temperature and time in order to restrict the coarseness of the porous structure of the membrane.
Polymeric porous membranes were prepared from polytetrafluoroethylene (PTFE) fine powder by a series of mechanical operations, such as extrusion, rolling, and stretching. The structure of the prepared porous membrane was well characterized by a spatial periodicity of nodes (domain of agglomerated PTFE particles) and fibril domains. The fibrils were highly oriented in the direction of the stretching operation, providing pores in the polymeric membrane as slit‐like voids between adjoining fibrils. The unit size of the periodic 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 steady tensile stress in the stretching operation on the PTFE molecular weight 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 several times as large as the thermal energy at the ambient temperature. These results imply that the thread‐like structures can easily be pulled out of PTFE particles. This view is in accordance with the previously proposed microstructure in PTFE particles.
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