Numerical modelling of radiation-induced conductivity of polymers

Nowadays for describing and predicting of radiation-induced conductivity of polymers Rose-Fowler-Vaisberg (RFV) model is widely used. But now there is an insistent need for RFV-model modification for taking into account electron transport bipolarity and geminate character of radiation-induced conductivity process at it's early stage. Besides there is a need for taking into account the possibility of analysis the dose and annealing effects during arbitrary variation of dose rate and temperature that is very important for spacecraft outer polymeric coating electrization process calculation. For some applications the so called time-of-flight effects can be very important because they can significantly affect the transient current kinetics.The generalized Rose-Fowler-Vaisberg model for bipolar transport is presented. It is shown that taking into account the carrier transport bipolarity significantly affects transient current curves in comparison with the monopolar case. Besides the dose and annealing effects are rather sensitive to carrier transport bipolarity.Currently, the main approach to radiation-chemical reactions in the condensed phase is based on the ion-pair mechanism of radiolysis, according to which the action of ionizing radiation on a substance reduces to the generation of isolated ion pairs in substance volume, interaction between which can be ignored. The space-time evolution of such pairs called geminate has been studied using the Onsager model, which is in turn based on the diffusion-drift approximation for describing the motion of electrons as classical particles that obey the Smoluchowski equation. A program for numerically solving the Smoluchowski equation with a modified initial condition taking into account the drift shift of electrons that experience thermalization in an external electric field was developed. The probability of survival and the polarization current of isolated ion pairs were calculated. Shift effects were shown to be especially strong in the region of medium electric fields on the order of 107 V/m and noticeably weaker in both low and high fields. This was related to the proportional relation between the drift shift and electric field applied. The program was used to critically analyze the available experimental data on pulsed photoconductivity of polyacenes.Theoretical description of the time of flight effects in radiation-induced conductivity of polymers on the base of multiple trapping model is based on the solution of the initial-boundary problem for corresponding system of integral-differential equations. In this work numerical solution of this system is presented and precise description of transient current curves is achieved. (authors)