Polymer Engineering and Science, Vol.38, No.11, 1845-1853, 1998
Crystallization morphology of a thermotropic liquid crystalline polyimide
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 degrees C in the DSC cooling curve. The results of polarized light microscopy revealed that the polyimide quenched from 350 degrees C in air exhibited fine structure, frozen Liquid crystalline texture. The Liquid crystalline texture disappeared when the sample was heated to 298 degrees C. However, the polymer melt still exhibited, to some degrees, birefringence until the temperature reached 340 degrees 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 degrees C for a long time, and then reheated to 305 degrees 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 degrees C for a period of time, further crystallization could be observed using the spherulites formed at 286 degrees C as nuclei. Composite spherulites were developed if the low-high temperature crystallization process was repeated.