Solid-State 13C NMR of Liquid Crystalline Polyesters: Variations in Morphology, Alignment, and Dynamics within a Homologous Series

The local ordering, morphology, and dynamics of aromatic cores and flexible alkyl spacers were analyzed for a homologous series of main-chain polymeric liquid crystals. 13C NMR experiments showed that the nematic ordering achieved by these synthetic polymers was retained into the solid state if their quenchings occur while remaining within the strong NMR magnetic field. The degree of orientation in the resulting glasses was investigated by variable-angle NMR experiments and found to differ between polymers with an even number of methylene units in the flexible spacer vs those with an odd number. To further discern at a molecular level the nature of these differences, the structures of these polyesters were examined by high-resolution solid-state 13C NMR. It was found that while the odd-chained series displayed a conformational annealing upon aligning, even-chained polymers were generally in all-trans conformations both for as-synthesized and for aligned samples. Variable-temperature 1D and 2D NMR experiments also illustrated substantial differences in the degree of motional dynamics between the odd and even polymer series: whereas considerable rigidity was exhibited by the even-numbered series all the way up to 150 °C, a relatively high flexibility displayed by the odd-methylene polymers. In unison, these measurements provide insight into the significant changes that can be imparted into the structure and dynamics of main-chain thermotropic polymers by subtle manipulations of their monomeric structures.