The relationship between unpaired spin concentration, structure, and electrical conduction of a pyrolytic aromatic polyimide

When films of poly[N,N′‐(p,p′‐oxydiphenylene)pyromellitimide] are pyrolyzed in vacuum between 575 and 800°C., they are converted into products having electrical conductivities as high as 20 (ohm‐cm.)−1 measured at 25°C. This transformation of the polymer from an insulator into an electroconductor is accompanied by distinct changes in the electron paramagnetic resonance absorption (EPR) vs. time curves, x‐ray diffraction, elemental composition, and density. From the rates of EPR absorption, a tentative activation energy of 25 ± 3 kcal./mole was calculated for this internal transformation. The absence of polarization effects and the fact that repeated thermal cycling does not significantly diminish the observed electrical conduction suggest that it is electronic rather than ionic in character. The development of electrical conduction in the pyrolyzed polymer occurs well below the graphitization temperature and with the retention of considerable quantities of nitrogen and oxygen. Apparently a fused conjugated network with a large pool of π‐electrons and π‐orbital overlap develops and the reduction of interboundary resistance at sufficiently high temperatures causes the appearance of electrical conduction.