Investigation Of The Liquid-Liquid Transition In Atactic Polystyrene And Syndiotactic Polystyrene

Atactic polystyrene (aPS), which is a poor polar polymer, is one of the most important engineering materials, and Syndiotactic polystyrene (sPS), which is a crystalline polymer, is a novel thermoplastic engineering plastic.Alought many articles have been published on the study of aPS and its blends using TSDC method, the results did not agree well enough with each other, furthermore the research of sPS by the method of TSDC is not enough. Therefore, it is both theoretically and practically significant to study the molecular movement and localized state of PS, especially the liquid-liquid transition above the glass temperature.This paper studied the liquid-liquid transition and space charge of aPS by using TSDC method, and a further research was accomplished through changing the polarization field strength, polarization temperature and heating rate; dielectric relaxation measurement was applied as a complementary method to asistant the results of TSDC. Besides them, the VTF equation is used in the liquid-liquid transition of TSDC spectra.This experiment mainly includes the following sections:1. There are four distinct peaks showed in TSDC spectra from 27 to 200℃. Compared with the result of DSC, TSDC spectra show that peakαcorresponds to glass transition, peakτis related to liquid–liquid transition, and peakρand peakρ'belong to the space chargecurrent peak,βpeak is found by separating peaks which is related to the motion of Sub-Tg molecular chain segments. Curve fitting for theτpeak shows that theτpeak becomes stronger and higher of peak temperature with the increase of polarization temperature because of being polarized more completely. With the increase of polarization field strength, the position of peakαand peakτdoes not change and the value of them increases linear with the increase of field strength; the position of peakρand peakρ'also does not change and the value of them increases with the increase of field strength, but the relationship of them is nonlinear. As the heating rate increases, all peaks move towards higher temperature, and peak value also increases. Forτpeak, activation energy is decrease with the increase of temperature. By comparing the TSDC of sPS, the difference of the molecular structure between aPS and sPS can be illuminated.2. There are two obvious relaxation processesβandαobserved in the dielectric relaxation spectra of aPS (10K, 100K and 1M Hz) in the range of 25～160℃. Theαrelaxation is corresponds the glass transition andβrelaxation is corresponds the secondary relaxation. We can correlate between DRS and TSDC atαrelaxation with WLF equation.3. Through interpretation the spectrum of TSDC around glass transition and liquid-liquid transition using VTF equation, some important parameters can be calculated by VTF simulation. It is proved that the molecular motion of the amorphous polymers is fitting VTF formula between Tg and TLL, the result is more rational than the conclusion that amorphous polymers is fitting WLF formula between Tg and Tg + 100℃.