Localized spins in partially carbonized polyparaphenylene

The results of electron spin resonance (ESR), static magnetic susceptibility, and electronic transport measurements of heat-treated polyparaphenylene (PPP) at various temperatures T-HT are presented as a function of measurement temperature T and magnetic field strength H. A crossover from a disordered polymer (H/C approximate to 0.6) to a disordered carbon (H/C0.2) is caused by heat-treatment above 700 degrees C, which gives rise to vast differences in spin environment. ESR measurements of PPP heat-treated to 700 degrees C show the presence of two spin centers, resulting in two distinct peak-to-peak linewidths of roughly 0.5 and 5 G, corresponding to spins localized within carbonized and noncarbonized regions, respectively. In polymeric regions, slowly diffusing spins (omega(j)approximate to 10(8) sec(-1)) experience a motionally narrowed hyperfine interaction, which is unresolved due to disorder. Carbonized regions, which control bulk conduction mechanisms, show evidence for a Korringa 1/T relaxation mechanism, typical of disordered metals; This is consistent with the transport behavior of PPP heat treated at or above 700 degrees C which indicates a variable range hopping mechanism for carriers localized within weakly conducting carbonized regions. On the other hand, spins in hydrogen-rich polymeric regions are highly localized and are associated with defects along PPP chains in the form of trapped polarons. {[}S0163-1829(99)14331-5].