화학공학소재연구정보센터
Journal of Polymer Science Part B: Polymer Physics, Vol.36, No.12, 2111-2128, 1998
Thermodynamic characterization of the oriented state of bisphenol A polycarbonate as it pertains to enhanced physical aging
It has recently been demonstrated that hot-drawn samples of bisphenol A polycarbonate (PC) have a 50% higher volume relaxation rate than their isotropic counterpart even though the oriented samples have a lower initial free volume (i.e., higher density).(1) In an attempt to better understand this paradox, samples of unaged, hot-drawn PC were characterized thermodynamically and kinetically as a function of orientation. Heat capacity, hole energy, and T-g data indicate that the chain mobility is actually decreasing slightly with orientation, possibly due to the hindered motion brought about by tighter packing, stronger intermolecular bonding, and reduced free volume. Nonetheless, this decrease in localized mobility is in contradiction to the enhanced volume relaxation rates observed for the oriented samples. In contrast, dynamic mechanical data indicate an increase in the relaxation strength of the p-transition (-100 degrees C at 1 Hz) upon stretching for both the stretch and transverse directions. This implies that more segments are actively participating in the relaxation process for the oriented samples even though their individual localized mobility might be slightly lower. The net result is an increase in "effective" mobility for the oriented samples. It is conjectured that the enhanced relaxation strength of the oriented samples is a result of the stretching process somehow activating more of the chains into a higher energy state, and may be related to the physical aging concept of stress-induced rejuvenation/acceleration.