Studies of the Phase Transitions, Structure, and Dynamics for Main-Chain Thermotropic Liquid Crystalline Polyethers and Polyurethanes with the Same Mesogen and Spacer Units

Abstract

Phase transition behavior, structure, and dynamics have been investigated for main-chain thermotropic liquid crystalline polyethers (EDMB-10) and polyurethanes (UDMB-10) with the same mesogen and spacer units by mainly using DSC and solid-state 13C NMR spectroscopy. The liquid crystallization temperature (Tlc), which corresponds to the isotropic melt → nematic phase transition temperature determined by DSC, is found to stay almost constant irrespective of the cooling rate from the isotropic melt for EDMB-10 while the crystallization temperature (Tc) greatly decreases with increasing cooling rate. This fact leads to the preparation of the liquid crystalline glass by quenching from the isotropic melt to ice−water. In contrast, Tlc and Tc for UDMB-10 are observed as an overlapped single exothermic peak in DSC, and both temperatures are found to significantly depend on the cooling rate. This fact suggests that somewhat larger-scale reorientation of the mesogen and spacer units, which may result in the formation of segmental assemblies through intermolecular hydrogen bonding, are inevitable for the liquid crystallization from the isotropic melt in the case of UDMB-10. The CSA spectra of the respective carbons for the mesogen and spacer units, which were successfully measured by the MAT method, have been elucidated in detail to characterize molecular motion of the constituent units in the EDMB-10 and UDMB-10 samples that were crystallized from the isotropic melt through the nematic phase: The mesogen units are found to be in the rigid state or undergo restricted fluctuation with amplitudes less than ±20° in the crystalline and noncrystalline regions for both samples. In contrast, the CH2 sequences are subjected to much enhanced motion such as cooperative t−g exchanges or the thermal fluctuation around chain axis with the planar zigzag conformation in the crystalline region for EDMB-10 or UDMB-10, respectively. Interestingly, similar cooperative t−g exchange motion is also allowed to occur for the noncrystalline CH2 sequences in UDMB-10 whereas the corresponding CH2 units adopt random conformation in EDMB-10. On the basis of these experimental results, the structural models have been proposed for the nematic phase in EDMB-10 and UDMB-10.