THERMAL AND UV DEGRADATION OF POLYIMIDES AND SILICONES STUDIED IN SITU WITH ESR SPECTROSCOPY Kenneth Rasmussen (1),(2) , Günter Grampp (1) , Marc van Eesbeek (2) , Thomas Rohr (2) (1) Graz University of Technology, Institute of Physical and Theoretical Chemistry, Technikerstrasse 4/I, 8010 Graz, Austria, Tel.: +43 316 873 8721, Kenneth.Rasmussen@tugraz.at (2) ESA-ESTEC, Materials Space Evaluation & Radiation Effects Section, P.O. Box 299, 2200 AG Noordwijk ZH, The Netherlands, Tel.: +31 71 565 6777, Thomas.Rohr@esa.int ABSTRACT Thermal and UV degradation of several common space- grade polyimides and silicones was studied in situ using electron spin resonance (ESR) spectroscopy. With this method paramagnetic species, present in a sample, can be detected. Such species include radicals, transition metal ions and colour centres. From their various ESR signals, one may obtain concentrations and, depending on the material, also molecular structures. In the present case, the radicals formed during thermal or UV degradation of films of the polyimides Kapton HN and Upilex S as well as of bulk samples of the DC 93-500 and Elastosil S 690 silicones were studied. By recording subsequent spectra at a sufficient rate, the time dependence of the radical concentration could be followed, allowing more detailed study of the kinetics of the degradation process. The thermal degradation of Kapton HN was found to consist of two main processes, one being a radical termination and the other the actual degradation process. UV degradation experiments showed that the polyimides degrade at a much faster rate than the silicones. 1. INTRODUCTION With the current development pace of new polymers having a potential for space applications, a fast screening and cost effective method for pre-selection and basic performance characterization would be very valuable. One such method shall be presented, and its general feasibility demonstrated. Important goals were to set up an ESR facility at ESA/ESTEC and to develop a standard UV exposure test for screening purposes. With these in place, a complementary method for characterisation and space environmental performance evaluation of materials would be available. The main method of analysis is ESR spectroscopy, which deals with the resonant microwave absorption of samples in an external magnetic field. It is similar to the better known nuclear magnetic resonance (NMR) spectroscopy, but is restricted to paramagnetic compounds, such as organic radicals, complexes of transition metal ions and defects in crystals. This limitation, however, is also one of the strengths of the method as it can be applied very specifically. ESR spectroscopy, where the first derivative of the microwave absorption intensity is measured as a function of the magnetic field, may be used to determine the amount of spins present in a sample. Most often, this is done by comparing the double integral owing to an unknown sample to that of a known standard. The reason for this is that the magnitude of the ESR absorption depends heavily on spectrometer settings, as well as on temperature. Furthermore, ESR spectroscopy may provide information about the general type of radicals present, via the so-called g-factor, and sometimes also their structure, if sufficient hyperfine interactions can be detected. The idea of using ESR to study polymer degradation is not new and several investigations are reported in literature. For example, Ahn and co-workers studied the behaviour of thermally aged polyimide PMR-15 and characterised the generated radicals using conventional ESR [1, 2] as well as ESR imaging [3]. Also, in study by Blair et al. [4], the thermal ageing of several polysiloxane foams was investigated The UV degradation of Kapton H was reported by George et al. [5], who studied samples that been irradiated in either vacuum or air. These authors, among other things, investigated the dependence of the ESR signal on the duration of irradiation as well as on elevated temperatures and also, the stability of the generated radicals was studied. Additionally, they determined the relaxation parameters of the radicals. Other methods which have been used to generate radicals for use in ESR studies of polyimide degradation, include γ-radiolysis [6, 7], and irradiation with accelerated protons [8] electrons [9, 10] and heavy ions [11]. Most of these, however, were conducted ex situ, that is the sample was exposed to a source of degradation and subsequently transferred to the ESR spectrometer for measurement. An exception is the study of the UV degradation of Kapton, along with a series of other polyimides, conducted by Hill et al. [12]. Here samples