CRYSTALLIZATION OF A THERMOTROPIC LIQUID CRYSTALLINE POLYIMIDE

Abstract

The crystallization behavior of a liquid crystalline polyimide synthesized from 1,2,4,5- benzenetetracarboxylic dianhydride (PMDA) and 1,3-bis[4-(4'-aminophenoxy) cumyl)] benzene (BACB) has been studied by means of polarized light microscopy (PLM) and differential scanning calorimetry (DSC). The critical work performed in this research consists of two parts: the first one is the observation of crystallization morphology by PLM; the second part is the investigation of phase transition kinetics by DSC. The polyimide studied is an enantiotropic liquid crystal, which exhibits two endothermic peaks at onset temperatures of 265 and 292°C during heating at 20°C/min. When it is cooled at 20°C/min from 400°C, three exothermic peaks are observed at onset temperatures of 292, 279 and 250°C. The results of polarized light microscopy reveal that this polyimide quenched from 350°C in air exhibited fine structures, corresponding to a 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 degree, birefringence until the temperature reached to 340°C where the polyimide was in the truly isotropic state. Isothermal crystallization experiments were carried out at both isotropic and non-isotropic temperature ranges. Two types of negative spherulites with lamellar structure and one type of positive needle-like crystals have been observed when crystallized at isotropic temperatures. Interestingly, if the crystallization temperature was below the liquid crystalline phase transition temperature, the entire field was covered by the liquid crystalline texture. However, when the sample was kept at 286°C for a longer period of time, and then reheated to 305°C to melt the liquid crystalline phase, the negative spherulites with loose structure could be observed. Surprisingly, if the sample was kept at 305°C for a period of time, further crystallization process could be observed using the spherulites formed at 286°C as the nuclei. Composite spherulites were developed if the low-high temperature crystallization process was repeated. The non-isothermal and isothermal crystallisation kinetics of this liquid crystalline polyimide was studied by means of DSC. The fan-conic texture as observed by PLM and the strong diffraction at small angle (~ 4°) by Wide Angle X-ray Diffraction (WAXD) confirmed that· this polyimide was a smectic liquid crystal. The phase transition process for the formation of a liquid crystal phase from isotropic melt is extremely fast and is completed in several seconds. The mesophase transition has a small Avrami parameter, n, whose magnitude is approximately 1. The isothermal crystallisation from 253 to 258°C has been examined. The average value n is -2.6 and the temperature dependent rate constant k changes about two orders of magnitude in the crystallisation temperature range of 6°C. The slope of ln k versus 1/(Tc?T) is calculated to be -2.4x105, which suggests nucleation control, via primary and/or secondary nucleation for the crystallisation process. During annealing at 230 to 250°C, a new phase (slow transition) is induced, which gradually increases with annealing time. At lower annealing temperatures (220-230°C), the slow transition process does not seem to be influenced by the crystals formed during cooling process and its Avrami parameter, n, is 0.3-0.4. However, the slow transition was hindered by the crystals formed during the cooling process when annealed at higher temperature (240-250°C).