Phase Behavior and Molecular Segregation Phenomena in Main Chain Thermotropic Liquid Crystalline Polymers.

Thermotropic main chain polymeric liquid crystals having regularly alternating mesogen moieties and flexible spacers are investigated in this study. The PLCs are prepared from the condensation of 4,4^' -dioxy-2,2^'-dimethyl azoxybenzene ("mesogen 9") and (alpha, omega)-alkane dicarboxylic acid halide (DDA9 for n = 10 and AZA9 for n = 7; n = number of the methylene spacer units). Molecular mass and spacer parity dependence of phase transitions such as glass transition, crystallization, melting, and nematic to isotropic (N/I) transition are studied using differential scanning calorimetry. In the N + I biphase, the conformational and orientational ordering of chains and selective partioning of chain lengths between the I and N components occurs rapidly. The coalescence of the I and N domains occurs until equilibrium (macroscopic) domain demixing is achieved upon isothermal annealing within the N + I biphase. Morphology of the biphase at various stages of coalescence is preserved by cooling to the solid phase. Chain length distribution tends to homogenize upon isothermal annealing within the pure nematic phase as well as the pure isotropic phase. The glass transition temperatures of DDA9 and AZA9 PLCs increase following Flory-Fox equation. DDA9 polymers are considered to have much less chain end free volume contribution than AZA9 polymers from the slope of that equation due to the highly ordered state of DDA9. The molecular mass dependence of Delta C_{p}(T_{g}) can be explained as the increase in the orientational and conformational orders of the nematic state with longer molecular chains. The chain rigidity of LC glasses is the cause of slower enthalpy relaxation than in the conventional amorphous glasses. Crystallization of the higher molecular mass DDA9 fractions from the ordered nematic state shows the double melting peaks indicating the two distinct levels of the crystalline morphology. From the thermal analysis of AZA9 and DDA9 PLC blends, the miscibility of blends are considered in terms of the depression of crystallization and melting behavior with increasing AZA9 fraction. The Delta H_{NI} of blends also indicates the interaction between the two components. The physical aging study illustrates the heterogeneity of blends from the multiple enthalpy relaxation peaks after annealing.