Electron paramagnetic resonance and microwave conductivity in pyrolyzed polyacrylonitrile chains included in zeolites

It is possible to polymerize good quality polyacrylonitrile (PAN) chains, typically 360 monomers long, inside the channels of a zeolite. After suitable pyrolysis, these chains become conducting, as proved by microwave measurements and by electron spin resonance. This behavior contrasts with our preceding work with polypyrrole, where the presence of the counterions required for doping very probably blocks the mobility of the charge carriers. In the case of pyrolyzed PAN (PPAN), no such doping is needed, as the carriers appear through the creation of aromatic regions by pyrolysis. The conductivity values obtained prove that the imbedded polymer is a better conductor than the bulk one. Our EPR measurements prove that zeolite-imbedded PPAN undergoes a semiconductor-to-metal transition in the 70--150 K temperature range. Our microwave conductivity measurements imply that this metallic conductivity is restricted to clusters between which the carriers have to jump.