Resonant Raman study of polyparaphenylene-based carbons

A resonant Raman study of polyparaphenylene (PPP) prepared by the Kovacic and the Yamamoto methods and heat-treated at temperatures T_HT between 650 and 750°C has been performed using different laser excitation energies E_laser between 1.92 and 3.05 eV. For samples treated at low T_HT, the Raman spectra change with E_laser, and this behavior is ascribed to the coexistence of two forms of the PPP polymer (benzenoid and quinoid) as well as a disordered carbon material. For higher T_HT samples, only a dispersion of the position of the Raman band as a function of E_laser is observed, and this is explained as due to the carbonization of the original polymer. The transition temperature between these two regions of resonance behavior is lower for the Yamamoto-PPP samples than for the Kovacic-PPP samples.