Study of the overtones and combination bands in the Raman spectra of polyparaphenylene-based carbons

A detailed study of the second-order Raman spectrum of the polymer polyparaphenylene (PPP) prepared according to the Kovacic method and heat treated at temperatures T_HT between 650 and 750 °C is presented. The Raman experiments have been performed with five different laser excitation energies in the visible range between 1.92 and 3.05 eV. Several Raman bands in the region between 2400 and 3400 cm⁻¹ have been detected and assigned to the overtones and combination bands of the two conformations of the PPP polymer (benzenoid and quinoid) that co-exist in our samples. Due to the carbonization process, these bands broaden and decrease in intensity with increasing heat treatment temperature, as is also observed for the corresponding first-order Raman features. The complete absence of these high-frequency Raman bands for PPP with heat treatment temperatures in excess of 750 °C indicates complete transformation of the polymer into a disordered carbon material.