抄録
Birefringence was measured in the creep recovery of preoriented PVC cast films both of the unplasticized sort and of the plasticized sort. The PVC films were stretched under dead load at various temperatures ranging below and above glass transition temperature.
The measurements of temperature dependence of orientaion birefringence were conducted between room temperature and 160°C at a constant rate of temperature increase.
The experimental findings are as follows:
(1) Birefringence increases almost linearly between room temperature and Tg
(2) Tg is independent of the temperature at which the films are stretched.
(3) Another transition above Tg exists at 105°C, which does not depend on the stretching temperature, and is affected by plasticizer.
(4) Transition nearby the stretching temperature can be visualized easily at Tori using birefringence measurement.
The initial increase in birefringence is explained as having been caused by thermal expansion. The 105°C-transition is proposed to be a lower limit of melting point, Tm(min), of a microcrystalline structure formed by stretching. Tori corresponds to one of the state of the polymer association being observable under stretching between Tm(min) and Tm(max) which is the conventional primary transition.
