Melting of some Aromatic and Heterocyclic Oligomers

Abstract

The softening point of the aromatic or heterocyclic polymers is very high and usually undetected up to the decomposition point. The melting behavior of oligomers with the same basic structure of molecules as polymers has been measured by differential scanning calorimetry. The dependences of melting point, T_m and the enthalpy, Δntropy, Δlting on the degree of polymerization and the structure of molecules are discussed. The entropy of melting per ring for p-oligophenyl homologues decreases with the number of phenyl rings, n. For the longer p-phenyl chains, a sort of molecular arrangement would remain after melting; then, ΔS becomes low for the higher homologues, and T_m increases with n. A quite similar behavior was found for oligoacenes. On the other hand, T_m of m-oligophenyl homologues does not increase markedly with n. The asymmetric structure of the higher m-phenyls prevents the strong interactions between molecules in the crystalline state; therefore, AH becomes low for these molecules.

As for the benzimidazole derivatives, AH of dimeric molecules shows a larger difference than AS as compared with those of monomers. The energetic effect has a more significant influence on the melting temperature. The dimer molecules keep a kind of arrangement in the molten state. The lower AS of the dimers makes their T_m higher than that of monomers. The introduction of phenyl groups into molecules lowers AS significantly. The ether linkage on the backbone makes AS larger. Meanwhile, AH is also large by the polar nature of ether linkages. Benzimidazole derivatives show higher T_m than,-phenyls with the same number of rings, because of their lower AS.The hydrogen bonding among molecules is effective in keeping AS low.

In conclusion, the higher T_m of aromatic or heterocyclic oligomers are attributed to their large entropy of melting. The rigid ring structure increases AS by itself. Moreover, the longer chains of higher 2-phenyls and polar groups of the benzimidazole derivatives make a cause of high AS.