Microporous PVDF membranes were prepared by immersion-precipitation in 1-octanol of casting dopes dissolved at different temperatures, with dissolution temperature affecting strongly the membrane microstructure. The effect of postcoagulation thermal annealing, which is an additional thermal parameter, on membrane microstructure and properties is probed herein. Membranes obtained were annealed at temperatures up to 160 °C, which is close to the melting point of PVDF polymer. Annealing leads to a substantial modification of the nano-scale fine structure of the membranes, while the overall-microporous structure is preserved. At elevated annealing temperatures, nano-grains, fibrils, and stick-like crystalline entities gradually eclipse, while globules develop more robust connections based on wide bands of crystal elements. Probing by X-ray diffraction and dynamic scanning calorimetry shows that crystallinity increases when annealing temperature and time are increased. As regards mechanical properties, the tensile strength of the membranes can be enhanced substantially, up to about 10 times, upon appropriate high temperature prolonged annealing.
Journal of Polymer Science Part B: Polymer Physics 47(19), pp.1880–1893