Space charges induced in polymers by electron beam irradiation and their decay profiles

Electrons were irradiated to low-density polyethylene (LDPE), polyimide (PI), polyethylene terephthalate (PET), polyethylene naphthalate (PEN) and syndiotactic polystyrene (SPS). Many electrons were back scattered and large numbers of electrons and holes were generated in the film by ionization. Since the amount of negative charge observed under short-circuit after the irradiation is significantly smaller than the electron fluence, most electrons and holes generated by the irradiation were either recombined or transported to the electrodes. The amount of negative charge gradually decreases monotonically, showing the order of the decay speed, PI > LDPE > PET > SPS > PEN. The conductivity also shows the same order. When a dc voltage is applied to the sample, in which electrons and holes were generated abundantly by ionization, the electric field is enhanced significantly at the two electrodes due to the presence of positive charge near the cathode and negative charge near the anode.