P-HYDROXYBENZOATE ETHYLENE TEREPHTHALATE COPOLYESTER - STRUCTURE OF HIGH-MELTING CRYSTALS FORMED DURING PARTIALLY MOLTEN STATE ANNEALING

A liquid crystalline poly(ethylene terephthalate)/4-hydroxybenzoic acid (PET/HBA) copolyester forms high-melting crystals (recrystallizes) when annealed in the partially molten state above its nominal melting temperature (d.s.c. peak). The dynamic mechanical moduli increase by one order of magnitude within 3 h. The new melting temperature appears approximately 20 K above the recrystallization temperature. An additional equatorial WAXS reflection appears at d approximately 3.77 angstrom and is typical for a HBA-rich ordered phase. During recrystallization, the long HBA sequences in the copolymer chains are believed to match the unmolten highly ordered HBA crystallites which act as nuclei, and thus, crystal growting is induced. Partial melting provides for the presence of residual highly ordered crystallites and molecular mobility and is a necessary condition for the formation of high-melting crystals. If residual crystallites are eliminated by preheating to a higher temperature, isothermal annealing at the same temperature does not lead to recrystallization. The formation of the high-melting crystals can be thermally reversed by temporarily heating the samples to 300-degrees-C for approximately 1 min. This suggests that the observed phenomena are of a physical nature, and if chemical changes did occur, they had no noticeable effect on rheology, WAXS or d.s.c.