Observation of two glass transitions in a thermotropic liquid-crystalline polymer

DSC and real-time variable-temperature synchrotron experiments have been performed on a thermotropic poly(etherester), PH31B32, with biphenyl units as mesogens and spacers with methyl substituents. It has been found that PH31B32 develops a low-ordered SmCalt mesophase with a rather slow rate of formation in such a way that the isotropic melt of this polymer can be easily quenched into the glassy amorphous state at the usual cooling rates of the calorimeter. Considerably high annealing times at temperatures above T-g are necessary to develop such a mesophase. The extent of transformation and symmetry of the mesophase have been determined by means of calorimetric and X-ray diffraction experiments. The DSC results show that the glass-transition temperatures of the amorphous and liquid-crystalline states are clearly different: 95 and 84 degreesC, respectively. Moreover, the results show that the isotropization temperature increases linearly with the annealing temperature (the temperature at which the mesophase has been formed). The synchrotron experiments show that the mesophase layer spacing is slightly lower for the higher annealing temperatures, indicating a slightly more compact structure. Although the WAXS profiles of the two phases (mesophase and amorphous) are rather similar, the mesophase has noticeably smaller most probable intermolecular distances and a smaller width of the WAXS broad peak. The temperature coefficients of these intermolecular distances are also different for the various phases, and clear discontinuities are observed at the glass transitions. The corresponding values of Tg obtained from these results are 94 and 84 degreesC for the amorphous and liquid-crystalline phases, respectively, in perfect agreement with the DSC results. The correlation lengths estimated from the widths of the mesophase layer peaks are of the order of the extended chain length of the polymer, so that no extensive chain folding seems to be present in the mesophase of PH31B32. This fact will favor the attainment of high degrees of liquid crystallinity. Moreover, the higher correlation lengths deduced for the higher annealing temperatures point to a segregation of defects from the smectic structure in a greater extent, which may explain the corresponding higher isotropization temperatures.