Determination of the melting enthalpy of β phase of poly(vinylidene fluoride)

Wide Angle X-ray Scattering (WAXS), Differential Scanning Calorimetry (DSC) and Fourier Transform Infrared spectroscopy (FTIR) analyses of phase composition and of thermal properties of PVDF samples, crystallized at temperatures 27 - 155 °C by casting from N,N-dimethyl formamide (DMF) solution, are reported. Samples obtained at 27 °C contain only β crystal phase and with increase of casting temperature content of β phase decreases in favor of α phase. Evaluation of combined: phase content (WAXS) and melting heat (DSC), leads to two fold higher than for 100 % α phase value of 100% β melting enthalpy, ΔH β 0 = 219.7 J.g -1 , which may be justified by strong polar interactions in β phase TTT conformation. The relation ΔH β 0 > ΔH α 0 leads either to the thermodynamic stability of β phase in whole temperature range (if T mβ 0 ≥ T mα 0 ) or to the limited temperature range of thermodynamic stability of α phase (if T mβ 0 < T mα 0 ).