DSC, TMA, hot‐stage microscopy and WAXS have been used to examine the effect of concentration of angular units and molecular weight on the thermal transitions of poly(p‐oxybenzoate‐co‐m‐phenylene terephthalate)s. Copolymers containing molar fractions of m‐phenylenedioxy units between 0·20 and 0·30 are melt processable nematogenic materials. In contrast, poly(p‐oxybenzoate‐co‐p‐phenylene isophthalate)s remain nematogenic over a wider concentration range (0·20–0·40) of angular isophthaloyl units. Comparison of fibres produced from copolymers with the same fraction of angular units (0·25) indicates that those with isophthaloyl units have a higher modulus than those with m‐phenylenedioxy units. These differences are ascribed to differences in planarity of chains containing isophthaloyl and m‐phenylenedioxy units, respectively.
Thermotropic behaviour is observed in a number of aromatic polyesteramides containing p-phenylene or substituted p-phenylene groups. In such polymers, a substantial proportion of co-polymerised units lacking rigid rod structure can be tolerated without loss of thermotropic behaviour. Anisotropic melts can be formed at very short average chain length. The upper temperature limit of stability of the anisotropic phase is lower the shorter the average chain length a n d the greater the proportion of relatively flexible units. Some polyesteramides formally derived from p-N-methylaminophenol also exhibit thermotropic behaviour.
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