Spectroscopic Investigations on the Effect of Proton Bombardment of Polyimide
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Spectroscopic Investigations on the Effect of Proton Bombardment of Polyimide David L Edwards1, Kim K de Groh2, Mary Nehls1, Sharon K Miller2, Bruce Banks2, Chris Stephens3, Ramon Artiaga4, Roberto Benson3, S. Balascuta, Jeffrey M. Zaleski5, and Mircea Chipara5 1
NASA MSFC Marshall Space Flight Center, Huntsville, Alabama; NASA Glenn, Cleveland, Ohio 3 University of Tennessee, Knoxville, Tennessee; 4 University of la Coruna, Ferol, Spain 5 Indiana University, Bloomington, IN. 2
ABSTRACT The effect of the radiation component of the space environment on polyimide films is reviewed. Experimental data obtained by electron spin resonance and dynamical mechanical analysis proved that the ionizing radiation generates free radicals with a long lifetime through a dominant chain scission mechanism. The radiation-induced shift of the glass transition of polyimide towards lower values confirms the decrease of the average molecular mass of the polymer during irradiation. The importance of polyimide for space exploration is critically analyzed. INTRODUCTION The main features of polyimide (see Table 1) of relevance for space applications are the reduced weight, the high glass transition temperature, the high dielectric strength, the thermal stability, and the excellent mechanical properties (high strength) [1-7]. The outstanding thermal stability, even in the presence of oxygen suggests the possibility to use this polymer in Low Earth Orbits (LEO), where atomic oxygen is present. Polyimide is a polymer liquid crystal, but this feature has been rarely exploited. TABLE 1. The physical properties of commercially available polyimide Kapton PHYSICAL PROPERTY Ultimate tensile strength Ultimate elongation Tensile modulus Glass transition temperature Thermal expansion coefficient Dielectric constant (1KHz) Dissipation factor (1 KHz) Volume resistivity
Kapton 33,500 72% 370,000 335 6x10-6 3.4 0.0018 1.5x1015
Units psi psi Psi K K-1 Ωm
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Polyimide is an excellent insulator and structural material. The high dielectric strength of polyimide (2,500 V/mil for Kapton) is preserved up to 4000C. Polyimide has been already used in space for inflatable structures, multilayered insulation, flexible circuit boards, thermal blanket, and acoustical fuselage blanket [1-3]. The transparent polyimide (LaRC) has a high transparency (90%), and a decomposition temperature of about 5500C. The polymer with a stiff molecular chain exhibits liquid crystal features. Recent studies revealed that the use of polyimide as insulator has to be considered with caution. High-fluence heavy ions as well as short circuit arcs are capable of producing conducting channels in polymide [8]. The pyrolisis like nature of these modifications that result in the production of graphite like conducting nanostructures, is responsible for the failure of polyimidebased insulators. A spectroscopic research on the effect of proton irradiation on polyimide aiming to a better understanding of the molecular basis of polyimide failures is reported. The final goal of this r
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