Raman scattering study of mesophase pitch

Abstract

First- and second-order Raman scattering from mesophase pitch was studied. In spite of the great complexity of this material, both spectra were found still to retain a common similarity to those of highly disordered graphites but also to exhibit several differences, which have been assumed to be caused by a higher degree of disorder in pitch. It has been established that the first-order 1600 cm −1 mode is narrowed almost by half and split into two components. Distortion of the perfect polycyclic conjugation of the pitch molecules resulting in significant variations in stress for their CC bonds and consequently in large variations in the in-plane stretching force constants was assumed to be the reason for this splitting. This conclusion is supported by similar spectral peculiarities observed in first- and second-order Raman scattering from donor and acceptor graphite intercalation compounds. During carbonization, mesophase pitch is converted into a carbonaceous material similar to any highly disordered graphite and their spectra become identical. Pitch carbonization was assumed to involve a kind of phase transition under which intermolecular interactions change from Van der Waals to covalent-like character. Correspondingly, the intensity ratio I 1600/ I 1350 versus heat-treatment temperature relationship was assumed to represent a curve of change of state during this transition.