Degradation of radiation exposed polymethacrylamide (Rohacell (R) 31) foam used in composite hardware

摘要

Rohacell (R) foam is a critical structural component used to fabricate composite hardware for space applications. The effect of radiation exposure on Rohacell (R) 31 HFHT has been evaluated to determine the degree of chemical and mechanical degradation that may occur in a space environment. A Co-60 source was used for exposure levels ranging from 0 to 8 Mrad. Tensile tests showed significant decreases of 50% in tensile strength and 75% decreases in failure strain, after only a 4-Mrad exposure. However, the rate of degradation sharply decreased with additional exposure since crosslink density was more affected during the early stages of radiation exposure. Size exclusion chromatography and multiangle light scattering of the extracted material also provided evidence showing an increase in the concentration of lower molecular weight (M-w) fragments after exposure. The average M-w of the extracted material dropped by two orders of magnitude from the control to the 5-Mrad specimens, however changes in Mw between the 5- and 8-Mrad specimens only varied by 30% similar in trend to mechanical property data. LS analysis showed a structural conformation change from a heavily branched network to a more linear conformation. Dynamic mechanical analysis exhibited increases in the peak height and breadth of the tan delta curve with radiation dose. These increases in damping are a result of chain scission which results in increased segmental motion caused by decreases in the degree of crosslinking within the polymer structure. Gas chromatographymass spectrometry was used to probe the potential sites of radiation susceptibility within the PMI polymer structure and are discussed. The radiation susceptibility of this polymeric material should be properly evaluated when utilizing this material for space applications since end of life conditions may vary significantly from beginning of life mechanical and thermal behavior. (C) 2016 Wiley Periodicals, Inc.