Indian Journal of Pure & Applied Physics Vol. 46, February 2008, pp. 118-122 Thermally stimulated discharge current and fractional polarization studies in polyimide (Kapton-H) samples M S Gaur*, Ramlal, Prashant Shukla, Pooja Saxena & R K Tiwari Department of Physics, Hindustan College of Science and Technology, Farah (Mathura) U P *E- mail: mulayamgaur@rediffmail.com Received 31 January 2007; revised 24 October 2007; accepted 12 December 2007 In order to understand the relaxation processes responsible for thermally charged polyimide (PI) samples, the thermally stimulated discharge currents (TSDC) and fractional polarization thermally stimulated discharge current (FPTSDC) have been recorded. FPTSDC study is found to be having dipolar as well as space charge relaxation. The appearance of the second peak (i.e. β peak) at different poling field indicated that this peak is attributed to dipolar relaxation in polyimide. The peak position and activation energy do not change with the poling field (V p ), but it is very sensitive to the poling temperature (T p ). The magnitude of peak current has been found to vary with poling field and temperature. The poling field seems to change the reorientation of dipoles associated with β peak. The γ (first peak) and β (second peak) peaks were attributed to the motion of the side chain and the dipole-orientation associated with the residual reactive groups, respectively. The α peak (third peak) may be due to the space charge polarization or due to charge injected from the electrode. The activation energy and charge released have been found to vary with the poling field, poling temperature and poling time for this peak. FPTSDC spectra are showing two peaks which correspond to dipolar and space charge relaxation , respectively and are in good agreement with TSDC. Keywords: TSDC, FPTSDC, Dipolar relaxation, Space charge relaxation, Activation energy 1 Introduction Polyimides are a group of aromatic polymers that exhibit excellent mechanical, thermal and electrical properties. The structural composition of aromatic polyimides consists primarily of heterocyclic imides and aryl groups, which are linked sequentially by simple atoms or groups 1 as shown in Fig. 1. The dielectric constant is the measure of insulating ability of polymers. The materials having dielectric constant less than 4.0 (i.e. for example SiO 2 insulator) have been recognized by the electronic industries as being superior in electrical performance to ceramic materials. The growth of modern technology has posed a constantly increasing need for materials that can perform well under harsh conditions, such as elevated temperatures. Indeed, the initial excitement generated upon introduction of polyimides by Dupont in 1960s resulted from the outstanding thermo- oxidative stability of these materials. The dielectric constants of aromatic polyimides, such as Kapton-H, approach 3.0 (when dry), which has led to widespread use of these materials as thin film insulators 2,3 . Most of the previous studies 3-8 are concerned with transient current and conduction current over large range of temperatures, however, less work has been reported on dielectric relaxation study of PI. The molecular relaxations have been shown to affect numerous properties of polymers including chemical, thermal, mechanical and electrical. TSDC is one of the simple and reliable method for studying the secondary relaxation processes and phase transition over a wide range of temperature. The TSDC technique reveals the nature of various relaxation processes in dielectric insulating material. When a polymer is poled with high electric field and temperature, its dielectric polarization gets affected which can be investigated by TSDC. The appearance of various maxima in TSDC spectra will provide a deep insight into the molecular origin of dielectric and space charge relaxation processes in polymers. Inspite of several advantages of TSDC for characterizing the relaxation processes of polymers, Fig 1—Molecular structure of PI