On the equation of state of crystalline polyvinylidene fluoride

Abstract

The crystalline state theory based on the cell model and discussed previously, is employed to describe the equation state of polyvinylidene fluoride (PVF₂). The recent experimental results on the compression ratios for PVF₂ (phases I and II) up to 14×10³ kg cm⁻² are well represented by a Tait relation. The comparison of theory and experiment for phase II shows satisfactory agreement for the isobar at atmospheric pressure and the isotherm at room temperature up to about 5×10³ kg cm⁻². For phase I such an agreement is observed only up to about 3×10³ kg cm⁻². Above these values, the theory predicts too large a compression. An empirical adjustment of the scaling pressure would be required to achieve agreement over the entire pressure range. This is in contrast to the earlier success for crystalline polyethylene over a much wider range. The difference between the two polymers in respect to the theory may arise from the asymmetry in the repeat unit of PVF₂. Finally we illustrate theoretical isotherms for low compressibility conditions and their excellent approximation by a Tait relation. The theory corresponds to highB-values for low compressibilities, whereas PVF₂ in both phases is characterized not only by a low compressibility, but also by its comparatively high isothermal volume derivative, particularly in phase I.