Roles of Composition Heterogeneity on Ordering of a Liquid Crystalline Copolyester: Nematic/Nematic Biphase and Nonequilibrium Pattern Formed by Seeded Growth of LC Domains

Abstract

Roles of composition heterogeneity on pattern formation via phase separation and ordering of a main-chain type liquid crystalline (LC) copolyester have been investigated in a wide range of temperature. For this purpose, as-cast films of the neat LC copolyester having optically homogeneous and isotropic property, as a trapped nonequilibrium initial state, were first prepared by solvent casting. Ordering process of the copolyester films induced by isothermal annealing of the as-cast films at various temperatures was observed in situ and at real time under polarized optical microscopy. Upon increasing temperature T, we found new phases of N (nematic) single phase and N/N biphase in which nematic domains having high and low order parameters coexist, within the phase previously assigned as the semicrystalline phase. The N single and N/N biphase exist between the semicrystalline phase and N/I (isotropic) biphase in which nematic domains coexist with optically isotropic domains. In addition to this equilibrium or at least pseudo-equilibrium patterns, various interesting nonequilibrium textures were observed by first applying isothermal annealing to the specimen at various temperatures in the N/I biphase or in isotropic single phase (I) for a range of time, followed by quenching of it to room temperature. The textures observed are as follows; (i) “dendritic anisotropic LC texture” grown epitaxially toward isotropic matrix from the interface of preexisting anisotropic domains which were developed in the first isothermal annealing process and (ii) impurities-induced nonequilibrium Schlieren texture with disclination lines having strength s = +1 with φ = π/2 rather than thermodynamically more stable s = ±1/2, which is grown in the matrix phase from interfaces of preexisting small and spatially isolated round anisotropic domains. The preexisting domains act as seeds for the growth of the nematic domains whose directors tangentially orient around the seeds. The former nonequilibrium texture (i) is a consequence of a concentration gradient of molecular species having high mesogenic groups being created in the periphery of the preexisting anisotropic domains. The average concentration of the mesogenic group is expected to decrease with a distance from the interface of the anisotropic domains, and this concentration gradient is responsible for the epitaxial growth of LC phase. The later nonequilibrium texture (ii) reveals also the epitaxial growth in a sense that the interface of anisotropic domains, which are isolated and round in the isotropic matrix, acts as wedge disclination centers and controls energetically unfavorable orientation of directors of nematic liquids, when isotropic-to-nematic phase transition occurs in the isotropic matrix during the cooling process after the first annealing process.