Crystallization morphology of a thermotropic liquid crystalline polymide

Crystallization morphology of an aromatic thermotropic liquid crystalline polyimide was studied by means of polarized light microscopy. The polyimide was synthesized from 1, 2, 4, 5-benzenetetracarboxylic dianhydride (PMDA) and 1, 3-bis[4'(4′ aminophenoxy) cumyl] benzene (BACB), which exhibited three exothermic peaks at onset temperatures of 292, 279, and 250°C in the DSC cooling curve. The results of polarized light microscopy revealed that the polyimide quenched from 350°C in air exhibited fine structure, frozen liquid crystalline texture. The liquid crystalline texture disappeared when the sample was heated to 298°C. However, the polymer melt still exhibited, to some degrees, birefringence until the temperature reached 340°C, where the polyimide was in the truly isotropic state. Isothermal crystallization experiments were carried out at both the isotropic temperature range and the non-isotropic temperature range. Two types of negative spherulites with lamellar structure and positive needle-like crystals formed from the isotropic melt have been observed. Interestingly, with a further decrease of the crystallization temperature to the liquid crystalline state temperature, the whole field was covered by the liquid crystalline texture. However, if the sample was kept at 286°C for a long time, and then reheated to 305°C to melt the liquid crystalline phase, negative spherulites with loose structures formed from the liquid crystalline state could be observed. Surprisingly, if the sample was kept at 305°C for a period of time, further crystallization could be observed using the spherulites formed at 286°C as nuclei. Composite spherulites were developed if the low-high temperature crystallization process was repeated.