Melting Behavior of Some Oligomers of Heterocyclic Polymers by Differential Scanning Calorimetry

Abstract

The softening point of polybenzimidazole involving aromatic ether linkage in the main chain is not measurable below degradation of the polymer. The various oligobenzimidazoles and benzothiazole and benzoxazole derivatives with analogous structures were synthesized and their melting point Tm and heat of fusion ΔH were measured by differential scanning calorimetry. The entropy of fusion ΔS was calculated by ΔS = ΔH/Tm. The accuracy of the method was examined and the effect of imidazole ring on the melting behavior was investigated.

Hydrogen bonds owing to N-H bond of imidazole rings enhance ΔH markedly compared to oxazole and thiazole rings. It causes the higher melting point of imidazole derivatives. The values of ΔH and ΔS for benzimidazole dimer are smaller than for corresponding oxazole dimer and it suggests that a kind of structure is remaining even after melting of imidazole dimers. Along with the higher ΔH the oligobenzimidazole having diphenyl ether linkage shows a large ΔS due to molecular mobility by the internal rotation around ether linkages.