Onset and growth of conduction in polyimide Kapton induced by swift heavy-ion irradiation

In order to follow the insulator to conductor transition in a polymer when it is subject to a progressive degradation process, we have irradiated about 50 samples of poly(imide) Kapton with heavy ions and measured their conductivities as functions of temperature, electric field, and frequency. Electron spin resonance measurements gave the spin susceptibility, Linewidth, g factor, and relaxation times along each step of transforming the conductivity of the material. We defined a single damage parameter that characterizes a unique damage scale for the four different ions and the different irradiation energies that we used. From the point of view of the electronic properties, the degradation process consists mainly of the nucleation and growth of carbon disordered clusters which are rich in pi-conjugated bonds. During the transformation, the conductivity reflects the competition between two contributions, one of which is related to an initial disordered irradiation-doped polymeric substrate and the second to a hopping network progressively built from the merging of the clusters. The initial substrate contribution, visible in the first stage of irradiation, corresponds to a simple temperature-activated behavior and to a clear Poole-Frenkel dependence with the electric field, while the latter shows all the features of nonadiabatic hopping in a granular system. Adiabatic hopping within the clusters invades progressively all the sample in the final stage.