Effect of annealing on the molecular structure of chemically carbonized poly(vinylidene fluoride)

Changes in the IR spectra of a sample of partially dehydrofluorinated poly(vinylidene fluoride) during step-by-step annealing in the range of 95–180$^\circ$ have been studied. Dehydrofluorination was carried out chemically for 5 hours. Prior to the heat treatment, the sample was stored for 1945 hours at a pressure not exceeding 0.05 Torr and was removed from the pumped volume only for systematic measurements of the IR spectra. An increase in the final annealing temperature leads to a consistent decrease in the IR absorption in almost all frequency intervals. The only exception is the growth of a peak of unknown origin centered at 768 cm$^{-1}$. The analysis of the spectra in the frequency intervals of OH bonds oscillations showed that the thermal exposure increases the hygroscopicity of the material due to the destruction of closed pores of a nanoscopic size. The results of measurements of the spectral areas in the intervals of 960–1280 and 2800–3040 cm$^{-1}$ (corresponding to the stretching vibrations of C-F and C-H bonds) demonstrate their proportional decrease during annealing. Such proportionality may indicate that the elimination of both fluorine and ethoxy groups occurs as a result of either the same process or two related reactions. An alternative explanation is based on the hypothesis that the removal of fluorine leads to a more symmetrical arrangement of ethoxy groups relative to the carbon chain. In this case, a decrease in the absorption in the frequency range of 2800–3040 cm$^{-1}$ will occur without their cleavage.