HIGH-TEMPERATURE RECRYSTALLIZATION AND RHEOLOGY OF A THERMOTROPIC LIQUID-CRYSTALLINE POLYMER

The liquid crystalline melt of a random copolyester of 4-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) recrystallizes above the nominal melting temperature. The recrystallized HBA/HNA (r-HBA/HNA) seems to contain periodic HBA crystals as evidenced by an additional equatorial reflection at d = 3.8 angstrom and by its increased melting temperature (about 325-degrees-C). On the other hand, the d spacings in the WAXS pattern of the original low-melting crystals of this random copolyester are not suppressed by the recrystallization process. This suggests that the r-HBA/HNA consists of the original aperiodic crystal structure, however, reinforced by high-melting crystals with long HBA sequences. The r-HBA/HNA can be reversed by temporarily heating above 330-degrees-C. Any effect of a possible transesterification reaction could not be detected here. Dynamic mechanical experiments show that the rate of recrystallization depends on thermal history prior to melting, preheating temperature, recrystallization temperature, and degree of molecular orientation. Molecular mobility and nuclei (residual high-melting crystallites) are two necessary conditions for forming high-melting crystals. The recrystallization behavior opens up new possibilities for processing of liquid crystalline polymers near their melting temperature and in the supercooled state (increasing mechanical properties and thermal resistance).