Relaxation processes in aromatic polyimide

Abstract

Electrical conduction in non-conjugated insulating polymers such as Kapton^(R) films was studied as a function of temperature, T (15-500 K) and electric field F (≤16 kV cm^-1). The time dependent isothermal transient currents (ITC) were also studied at different temperatures (15 K < T < 500 K). Thermally stimulated discharge current (TSDC) studies were carried out as a function of initial poling conditions. A distinct change in conduction mechanism as a function of T was observed at T≥300 K from DC conductivity, ITC and TSDC measurements. The origin of this behaviour is discussed in terms of transition of dipolar at low T to space charge mechanism at high T which can be attributed to carrier trapping and release of charge carriers from the trap states. The effective barrier energy, Δ for transport and hopping parameter λ(T) are estimated for Kapton on the basis of these studies. The results obtained for Kapton suggest that, for T < T_g at low or moderate fields, the transient currents are essentially governed by dipolar relaxation mechanisms.