Transitions and Relaxations in Mesophase Polymers: Thermotropic Liquid Crystalline Polyesters with Mesogenic Groups in the Main Chain

Abstract

The thermal and dynamic viscoelastic behavior of some thermotropic liquid crystalline polyesters based on α, ω-bis(4-hydroxybenzoyloxy)alkanes and terephthalic acid is reported. Original samples obtained by solution polymerization followed by precipitation showed physical and dynamic-mechanical properties typical of semicrystalline polymers. Quenching of the polymers from the LC state gave rise to glass-like mesomorphic modifications that retained a glass transition at considerably higher temperature T _8m than that of the corresponding amorphous material T _8a. However, the formation of a new crystal form with respect to the original sample could not be prevented and the polymers so obtained were in fact partially mesomorphic and partially crystalline materials. All the quenched samples exhibited by dynamic-mechanical and calorimetric analyses a first order solid-solid transition independent of the length of the flexible spacer. Such a phenomenon was interpreted as an order-disorder transition of the material quenched from the LC state.