Gas transport in poly(vinylidene fluoride): Effects of uniaxial drawing and processing temperature

This paper reports on the gas sorption and transport properties of poly(vinylidene fluoride) (PVF 2 ) and on the effects uniaxial drawing and processing temperature have on these properties. Sorption and transport were first examined for “as‐received” commercial Kynar PVF 2 film at 35°C. This film was 54% crystalline by weight. Solubility, diffusion, and permeability coefficients were measured for He, H 2 , Ar, O 2 , N 2 , CH 4 , and CO 2 . The solubility coefficient and the diffusion coefficient D were correlated with the Lennard–Jones potential and mean molecular diameter of the gas, respectively. Uniaxial drawing of PVF 2 films was performed up to a draw ratio of 3.7 and over the temperature range 75–140°C. Transport properties were correlated with the extent of draw and drawing temperature. The permeability P and D were found to significantly decline with uniaxial drawing; the magnitude of this effect was dependent on both the drawing temperature and the molecular size of the penetrant considered. Roduetions in P and D became progressively more pronounced with increasing molecular diameter and with decreasing drawing temperature (down to a limit of about 75°C), which reflects an increase in effectiveness of drawing at low temperatures. PVF 2 films annealed above 75°C showed an increase in P and D as opposed to the effect of drawing. The solubility of various gases in PVF 2 was not found to be sensitive to processing treatments such as drawing and annealing, in agreement with the relatively small changes observed in free volume. PVF 2 films subjected to various treatments were characterized by DSC, density, birefringence, and dynamic mechanical measurements. Gas transport measurements appear to provide a more sensitive and hence more viable measure of the effectiveness of drawing than these other techniques. Drawing PVF 2 in the melt state was found to increase P and D , in contrast with the effects observed for solid‐state drawing. The results have been interpreted in terms of existing theories on morphology and microstructure in semicrystalline polymers.