Phase-transition behavior of a crystalline polymer near the melting point: case studies of the ferroelectric phase transition of poly(vinylidene fluoride) and the β-to-α transition of trans-1,4-polyisoprene

Abstract

Many crystalline polymers exhibit phase transitions between the low- and high-temperature phases during the heating process. Sometimes, however, the temperature-dependent measurement of the X-ray diffraction peaks and vibrational spectra leads us to the incorrect conclusion that these two phases transform in a direct solid-to-solid transition mode instead of the melt and recrystallization process. As a reliable method to confirm the transition behavior just below the melting region, we have developed a temperature-jump cell system, with which a time-dependent measurement of the X-ray diffraction peaks (and Fourier-transform infrared spectra) is performed in the rapidly changing temperature process from the low- to the high-temperature phase. This method was applied to two types of polymers and revealed that (i) the high-temperature phase transition of poly(vinylidene fluoride) is not a direct ferroelectric transition between the ferroelectric and paraelectric phases, as proposed in our previous studies (Polymer (1983), Polym. Bull. (1983)), but a melt of the ferroelectric phase (form I) followed by the recrystallization into the nonpolar crystalline phase (form III or V) and (ii) the β-to-α phase transition of trans-1,4-polyisoprene had been also controversial for many years but it was revealed to occur via a melt-recrystallization mechanism. Time-dependent measurements of X-ray diffraction and infrared spectra have been performed to reveal the thermally induced phase-transition mechanisms about the ferroelectric transition of poly(vinylidene fluoride) form I crystal and the β-to-α transition of trans-1,4-polyisoprene. In both cases, the transitions were found to occur by the mechanism of the melt of the original phase followed by the recrystallization to the another new crystal phase. These findings request us to change our previous understandings of the phase-transition behaviors of these polymers. This time-dependent measurement technique will be helpful for the reinvestigation of the phase transitions of the other polymers also.