Molecular mobility of thermoplastic aromatic polyimides studied by dielectric spectroscopy

Abstract

The dielectric behavior of five aromatic thermoplastic polyimides with a sequentially changing chemical structure of the diamine and dianhydride parts of the macromolecule is studied. The dielectric spectra have shown three ranges of dipole polarization relaxation: γ, β (in the glassy state) and α (in the highly elastic state). The γ process takes place due to local mobility of phenylene rings in the diamine moiety of the macromolecule. The β process is related with local mobility of phenylene rings in the diamine and dianhydride parts of the macromolecule with adjacent polar groups. For γ and β processes, the chemical structure of polyimides has virtually no effect on molecular mobility. The source of the α process is large-scale segmental mobility. Its molecular mobility is more caused by the structure of the dianhydride part rather than that of the diamine one. It is shown that there is a linear correlation between free volume and glass transition temperature, which is determined by the dielectric method.