Phase and Thermal Behavior of Liquid-Crystalline Block Copolymers

The phase behavior of liquid-crystalline (LC) block copolymers containing mesomorphic blocks with main-chain (MC) units and side-group (SG) substituents was investigated. The MC block consisted of an LC semiflexible polyester and the SG block was an LC polymethacrylate. Four different samples were prepared with the content of the SG blocks varying from 35 to 79 wt%. The thermal behavior of the copolymers was investigated using differential scanning calorimetry, and X-ray diffraction was used to characterize the phase structure. Both the MC and SG blocks formed smectic A and nematic mesophases. Thus, the occurrence of distinct and well-defined phase transitions clearly indicated that MC and SG blocks were phase-separated and showed their own phase behavior. Two different endothermic processes were identified in annealed samples that followed different kinetics. The low-temperature process is thought to involve the MC blocks in their own domains and was observed up to the smectic A to nematic transition temperature of these blocks. The high-temperature process involves the interface between MC and SG domains and is probably due to melting of crystallites grown during annealing. It is shown that the crystallization develops with kinetics affected by the degree of order of the LC state of the SG domains.