Microporous Membranes of Isotactic Poly(4-methyl-1-pentene) from a Melt-Extrusion Process: (II) Effects of Thermal Annealing and Stretching on Porosity

A two part study utilizing isotactic poly(4 -methyl-1-pentene) (PMP) was undertaken to investigate a three stage process (melt-extrusion/annealing/uniaxial-stretching) (MEAUS) to produce microporous films. In this report, the thermal annealing (second stage) and subsequent uniaxial-stretching (third stage) results of selected PMP films from three resins, labeled A, B & C, are discussed. From sequential analysis of the effect each stage had on the resulting microporosity, it was discovered that the melt -extruded precursor morphology and orientation, as a consequence of the first stage extrusion parameters and resin characteristics, were crucial to controlling the membrane permeability. The annealing parameters were also critical, where a temperature of 205°C applied for 20 min. under no tension was the optimum annealing condition for producing highly microporous PMP films upon stretching. For the conditions studied, the stretching parameters that were found to be the optimum for producing the desired characteristics in the final film were a cold and hot stretch temperature of 70 and 180°C, respectively. The cold and hot stretch extension levels concluded to be the best were a cold stretch extension of 80 percent followed by hot stretching to 90 percent and thus a total overall extension level of 170 percent for the processing window studied. However, these results were only with respect to resin A films while resin B and C samples could not be produced into microporous films via the MEAUS process. In the case of the resin B films, their inability to be formed into microporous films was due to the presence of a higher level of comonomer in this resin, which adversely impacted the αc relaxation. The resin C films were not formed into microporous films due to the relatively low crystalline orientation of the melt-extruded precursors from this resin. Matthew B. Johnson Chapter IV PMP Annealing & Stretching ______________________________________________________________________________ 116