A tubular film extrusion of poly(vinylidene fluoride): structure/process/property behavior as a function of molecular weight

Five different poly(vinylidene fluoride) (PVDF) resins spanning the molecular weights from 85,000–250,000 g/mol with nearly comparable polydispersities of ca. 2.0 were investigated with respect to their strain induced crystalline morphologies as produced by a tubular uniaxial film extrusion process. By holding the process time window constant through the use of fixed melt temperature, line speed, quench height and film thickness, it was noted that as molecular weight increased, the uniaxial films produced systematically change from nearly a spherulitic structure to that of an extremely high concentration of fibril nuclei with minimal growth of folded chain lamella developing perpendicular to the fibrils. The systematic variation in the morphology was directly coupled through consideration of the melt process time window in conjunction with the characteristic relaxation time of a given resin at a given temperature. The latter was determined through use of a Carreau–Yasuda analysis of the melt rheological behavior of each resin and this relaxation time, when correlated to the process time, produced a relative Deborah number. It was distinctly shown that when the Deborah number was considerably less than unity, little crystal orientation was observed in the morphological texture whereas in the range of unity, a distinct rise in crystalline orientation occurred leading to fibril nucleation with lamella side growth in the form of the well known row structure morphology. Exceeding a Deborah number of unity led to nearly full crystalline orientation saturation and to a very high concentration of fibril nucleai with relatively few orthogonal lamella observable. The morphological textures were investigated using AFM, WAXS, SALS, SAXS and birefringence. Use of thermal analysis to determine some of the polymorphic character of the PVDF crystal form was also undertaken.