Solid-state 13C NMR analysis of the crystalline-noncrystalline structure and chain conformation of thermotropic liquid crystalline polyester

The crystalline-noncrystalline structure and chain conformation of thermotropic liquid crystalline polyester (BB-8), composed of mesogenic biphenyl and spacer CH2 sequence units, have been characterized by high-resolution solid-state C-13 NMR spectroscopy. The sample was crystallized by cooling from the melt through the smectic A phase. C-13 spin-lattice relaxation measurements reveal that all resonance lines contain three components with different TIC values of 200-430, 9-38, and 0.4-5.5 s, which correspond to the crystalline, medium, and noncrystalline components, respectively. The high-resolution C-13 NMR spectrum for each component is selectively recorded by utilizing the difference in Tic and conformation of the spacer CH2 sequences of each component is evaluated by considering the gamma-gauche effect on C-13 chemical shift values. The noncrystalline component is found to adopt a characteristic conformation xttttttx in which the trans-gauche exchange conformations (x) are introduced at both ends of the CH2 sequence, whereas the all-trans conformation tttttttt is allowable for the crystalline and medium components. C-13 chemical shift anisotropy spectra of the respective carbons are obtained by the two-dimensional magic angle turning method to examine molecular motion of the mesogen and spacer units. The mesogenic phenylene units undergo rapid fluctuation around the bond axis of the units in the medium and noncrystalline components while the crystalline component is highly restricted in such molecular motion. On the basis of these results, a molecular motion model is proposed for the noncrystalline component, which corresponds to the supercooled smectic A phase, for the BB-8 sample.