Bb. Sauer et al., BROADENED GLASS-TRANSITION IN A LIQUID-CRYSTALLINE POLYMER STUDIED BYTHERMALLY STIMULATED CURRENTS, AC DIELECTRIC, DYNAMIC-MECHANICAL, ANDDSC METHODS, Journal of polymer science. Part B, Polymer physics, 31(12), 1993, pp. 1851-1859
Thermally stimulated currents (TSC), a.c. dielectric, dynamic mechanic
al (DMTA), and differential scanning calorimetry (DSC) methods were us
ed to study the glass transition in a thermotropic liquid-crystalline
copolyester. All the techniques were consistent in the determination o
f the main glass transition temperature. Using the high sensitivity of
the TSC thermal sampling method, it was shown that cooperative glass
transition-like relaxations occur down to 100-degrees-C below the main
glass transition. DSC was sensitive only to a broadening of the glass
transition to about ca. 30-degrees-C, so it was concluded that the th
ermal sampling method is sensing a very small fraction of cooperativel
y relaxing species which cannot be detected by DSC. Ac dielectric meas
urements and DMTA also indicated that the glass transition was broad,
but difficulties with overlapping transitions prevented quantitative d
etermination of the breadth of the glass transition. The results sugge
st that the broad glass transition, in this mostly amorphous LCP, is d
ue to chemical heterogeneity of the copolyester chain. Other evidence
indicates that the broadening is not due to the oriented nature of the
glassy state. Some discussion is presented concerning how the heterog
eneous nature of the LCP glass leads to compensation of the Arrhenius
curves obtained by the thermal sampling method. (C) 1993 John Wiley &
Sons, Inc.