A SMALL-ANGLE NEUTRON-SCATTERING STUDY OF A SEMIFLEXIBLE MAIN-CHAIN LIQUID-CRYSTALLINE COPOLYESTER

A main-chain semiflexible copolyester containing 4,4'-dihydroxy-alpha,alpha'-dimethylbenzalazine as the mesogenic unit has been studied by small-angle neutron scattering (SANS). The polyester shows a nematic phase in the temperature range between 160 and 250-degrees-C. The polymer chain conformation, in the nematic phase range, was investigated for mixtures of deuterated and hydrogenous polymers, aligned using a magnetic field (B0 = 0.6 T). Due to the occurrence of a transesterification reaction, polymer chains consisting of deuterated and nondeuterated blocks of different lengths are formed, as a function of annealing time. The average degree of polymerization and the radius of gyration of the blocks were measured from the neutron scattering profiles. Changes in chain anisotropy have been observed in the nematic phase range, as predicted by recent theories developed for semiflexible main-chain liquid crystalline polymers (LCPs). A persistence length of 120 angstrom was measured in the temperature range from 175 to 196-degrees-C for a mixture consisting of fully deuterated LCP and unlabeled polymer. The scattering from the pure polymers aligned in the mesophase was also investigated. The results suggest that the disclination structure associated with a domain structure, typical of liquid crystalline systems, may be observable by SANS.