A tubular melt extrusion of poly(vinylidene fluoride): structure/process/property behavior as a function of molecular weight (M{sub}w)

摘要

Five poly(vinylidene fluoride) (PVDF) resins, R1-R5, of narrow molecular weight distribution (ca. 2.0) but of different weight average molecular weights M{sub}w's (85 - 250 Kg/mol) were melt extruded in tubular film form with a blow up ratio (BUR) of unity. The objective was to produce a stacked lamella structure that could serve as a precursor for a later process step that converts this film into a microporous membrane. Four of the resins were in pure form R1-R4 whereas the fifth, R5, contained a small amount of plasticizer to facilitate processing due to its high molecular weight. Comparisons were made of how M{sub}w influences film morphology under a given set of process conditions. WAXS systematically showed an increase in crystal orientation as M{sub}w increased for fixed conditions. A Carreau-Yasuda fit of the melt rheological data provided a characteristic relaxation time and this variable was correlated to the respective morphologies produced. It was shown that nearly spherulitic-like textures could be induced with the lowest M{sub}w material whereas highly concentrated fibril nucleated morphologies were promoted with the highest M{sub}w under identical process conditions. It was demonstrated that by blending the resins, R2 & R4, the desired stacked lamellar structure could be fine tuned with regard to morphological features.