STRUCTURAL-CHANGES THROUGH PHASE-TRANSITIONS IN LIQUID-CRYSTAL POLYMERS .1. A SPECTROSCOPIC STUDY OF POLYESTERS

Previous work on lattice dynamics and vibrational spectra of disordered chain molecules has allowed the identification of spectroscopic signals characteristic of the molecular microstructure. These data are used for the identification from the infra-red spectrum of the structural changes with temperature of a 'symmetric' liquid-crystal polyester and of a few model compounds. The K-S phase transition is prepared by the introduction of GTG' defects in the flexible spacers in a temperature range well inside the K phase. The S phase is characterized by the generation of a larger concentration of GTG' conformational kinks while no GG defects are observed. Thus the flexible chain does not coil in a 'liquid-like structure'. The same structural situation is observed for the I phase. These results have interesting implications for the description of the structural changes of this polymer molecule through phase transitions. A mechanism for the transition is proposed.