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 polypara phenylene (PPP) prepared according to the Kovacic method and heat treated a t temperatures T-HT between 650 and 750 degrees C is presented. The Raman e xperiments have been performed with five different laser excitation energie s in the visible range between 1.92 and 3.05 eV, Several Raman bands in the region between 2400 and 3400 cm-l have been detected and assigned to the o vertones and combination bands of the two conformations of the PPP polymer (benzenoid and quinoid) that co-exist in our samples. Due to the carbonizat ion 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 b ands for PPP with heat treatment temperatures in excess of 750 degrees C in dicates complete transformation of the polymer into a disordered carbon mat erial.