Softening and Melting Temperatures of Aromatic Polyimides

Softening temperatures T_s and melting temperatures T_m of a great number of aromatic polyimides and their chemical derivatives have been studied. The results obtained are generalized. The analysis is based on the chemical classification of polyimides taking into account the presence and location of “hinge” atoms and atom groups making polymer chain flexible. If there are such “hinges” in both components of a monomer unit then T_s and T are distinctly revealed. For these polyimides the dependence of T_s and T_m on the number of “hinges” in a monomer unit cân be súccessfully explained by the well-known physical theories of transition temperatures of polymers. Other groups of polyimides have no distinct T_s and T_m and are usually considered as “infusible”. Calculations and experiments have shown that it is because in these groups the values of T_s and T_m are close to or higher than the value of the polyimide thermal degradation temperati e. The maximum values of T_s and T_m can be as high as ≃ 1000 and ≃ 1300°C, respectively. The influence of chemical structure on T_s and T_m for the whole class of aromatic polyimides can be understood if strong intermolecular interaction of benzimide rings and internal degrees of freedom in monomer units are taken into consideration. Simple models are used in this work for this purpose. The conclusions made here are thought to be valid both for aromatic polyimides and other related classes of polyheteroarylenes.