(173ar) Characterisation of the Crystal Polymorphs of Polyvinylidene Fluoride Using FTIR, Raman and DSC Measurements

Polyvinylidene fluoride (PVDF) is a polymer of considerable practical interest because of its amazing piezo and pyroelectric properties, in addition to it being chemically inert and mechanically strong. It is a semicrystalline polymer existing in five known crystalline polymorphs namely α, β, δ, γ and η, of which α, β, and γ are the more commonly encountered polymorphs. α form is non-polar whereas β and γ are the polar forms. β form having the highest polarisation density is the most desired form for ferroelectric applications. It is important to calculate crystallinity (X_C) and individual polymorph fraction with high accuracy. Some IR absorption peak positions are still ambiguous for the polymorphs and there have been multiple instances of confusion, especially between β and γ. Dynamic scanning calorimetry (DSC) can be used for finding crystallinity, but only for the PVDF sample containing α polymorph alone since for the other phases, the value of the specific enthalpy of melting, H_o has not been found yet. Peak positions can be more reliably assigned if spectra for pure crystalline polymorphs and the amorphous phase can be obtained. In this work, PVDF was processed into samples having predominantly α, β, and γ crystalline phases. These samples were analysed with FTIR, Raman spectroscopy, and DSC. For better qualitative and quantitative analysis of the phases, the spectra were fitted with the Gaussian peaks. Peaks exclusive to the amorphous phase were identified and used for the amorphous phase quantification. For the quantification of individual crystalline polymorphs in the sample, IR absorption coefficients for their respective peaks were calculated. Thickness of the samples was also calculated using FTIR spectra. DSC was used together with FTIR data to obtain the values of H_o for the α, β, and γ polymorphs and the values for H_o of β and γ were found not to be equal to that of α were found not to be equal as opposed to the assumption in the literature.