Phase transition behaviors and physical properties of thermotropic poly(ester-imide)s with various molecular structures

Abstract

Various thermotropic liquid crystalline poly(ester-imide)s were synthesized from asymmetric N-(ω-carboxyalkylene) trimellitic imide and hydroquinone diacetate or methylhydroquinone diacetate. Their phase transition behaviors were investigated with the particular emphasis on the monotropic liquid crystalline behaviors. Poly(ester-imide)s with or without methylene side pendant group were synthesized from N-(ω-carboxyalkylene) trimellitic imide containing 4 and 6 methylene units. The polymers without methylene side group showed typical enantiotropic mesophase transition behaviors, which show melting transition and mesophase to isotropic phase transition during heating. But those without side group exhibit no crystallization during conventional cooling process - so called ````amorphous type mesophase transition,```` Their crystallization was extremely slow that approximately 32 hrs of annealing was necessary to complete in the fastest cases. The mesophases of all polymers are verified as nematic phase from the distinct peak shapes of elastic moduli and viscosities during temperature scan. Time-temperature superposition was tested for a amorphous thermotropic liquid crystalline polymer. The superposition holds for both isotropic and nematic phases but failed in biphase. A poly(ester-imide) containing 10 methylene units and methyl side pendant group was synthesized. On DSC cooling scans a exothermic process, which showed little dependence on the scan rate than other exotherms, was found. The morphology evolution showed the clear difference at the boundary of the exotherm. When cooled below the exothermic transition, tread-like morphology appeared on polarized light microscopy. Whereas, spherulic texture was observed above the transition. The monotropic mesophase transition was confirmed by the rheological temperature sweep measurements. The elastic moduli and complex viscosity exhibited unique hysteresis shape in cooling and subsequent heating scan. The mesophase w...