We have investigated oligomers, polymers, and a model of poly(4,4'-dioxy-2,2'-dimethylazoxybenzenedodecanedioyl). The molecular weights ranged from approximately 700 to 19000. In the oligomeric range, the kinetics of melt and cold crystallization, as well as the kinetics of macroscopic sample alignment in a magnetic field of ~1.8 T, is controlled by the aromatic end-group distribution. Phase transition temperatures increase with increasing molecular weight. A biphasic region (nematic + isotropic) is observed for all oligomers and polymers. Its range of thermal stability shrinks from ~40to ~5 °C as the molecular weight increases. The value of the glass transition temperature is nearly independent of molecular weight or development of crystallinity. The values of phase transition entropy at the N -* I transition and of the nematic order parameter increase with increasing molecular weight to reach a plateau at average chain lengths of 8-10 repeating units. Development of a polymeric mesophase, as characterized by maximum local chain extension, appears to require cooperativity between 8-10 repeating units per chain. The semicrystalline state of higher molecular weight samples is represented in terms of a fringed micelle model, with extended-chain segments forming cybotactic nematic domains.

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The order and mobility of a labeled flexible alkyl chain spacer in the linear thermotropic polymeric nematic liquid crystal poly (2,2'-dimethyl-4,4'-dioxyazoxybenzenedodecanedioyl-d2o) (poly[oxy(3-methyl-l,4phenylene)azoxy(2-methyl-l,4-phenylene)oxy(l,12-dioxo-l,12-dodecanediyl-c¡2o)]) is explored with deuterium NMR. The quadrupolar splittings of the spacer methylene segments in the nematic melt of the polymer are reported as a function of the temperature and are contrasted with observations on model compounds solubilized in a nematic solvent. The orientational order of the spacer CD2 segments is considerably larger than that of labeled chains appended to low molecular mass liquid crystals. Nonequivalent quadrupolar splittings along the labeled spacer indicate that the spacer does not simply adopt the all-trans conformation. The spacer flexibility does decrease substantially (relative to low molecular mass liquid crystals) as the temperature is lowered in the nematic phase. Speculations are advanced about the source of the large even-odd oscillations reported for thermodynamic properties of linear polymeric liquid crystals as the spacer length is varied.

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