SynopsisThe effect of an aqueous environment on the tensile properties of Kapton polyimide film has been investigated. Immersion of specimens in distilled water at 25" to 100°C for time periods ranging from one hour to several hundred hours resulted in a decrease in the ultimate tensile strength of the polymer from 23 ksi to approximately 14 h i , and a corresponding decrease in elongation to failure from 38% to approximately 5%. The kinetics of this decrease in mechanical properties are second order and yield an activation energy of approximately 15.6 kcal/mole. The reaction is slightly dependent on pH in the range 2.0 to 12.0, but is highly dependent on the p H in the range 0.4 to 2.0. The decrease in mechanical properties a t pH 2.0 to 6.0 appears to be due to hydrolysis of either uncyclized amic acid linkages or diamide functional groups present in the polyimide, whereas that a t pH below 2.0 is probably the result of hydrolysis of both imide and amide bonds. Prolonged reflux of the polyimide in water resulted in the extraction of a water-soluble, amide-containing material.
INTRODUCTIONResins such as polyesters, epoxies, and polyimides are used as organic matrices in high-strength composite structures that are subjected to varying environmental conditions. Environmental effects such as stress cracking, plasticization, and crazing are well known in nylons, hydrocarbon polymers, and epoxies. Very little information is available concerning the environmental degradation of polyimides. Several worker~l-~ have superficially studied the effect of water on the tensile properties of polyimides, but no detailed work has been presented showing the effect of such parameters as time and temperature of treatment or the pH of the solution. This investigation deals with a systematic variation of these parameters and the derivation of the kinetics for the aqueous degradation of the mechanical properties of polyimide film. A specific effort has been made to differentiate between chemical effects, such as hydrolytic chain scission, and purely physical effects, such as plasticization. The environmental effects of orqanic solvents and other solutions were also investigated and are the subject of a separate paper.4
synopsisHeat treatment a t temperatures of 224" to 310°C of hydrolytically degraded polyimide film has been shown to regenerate the ultimate tensile strength from 15.1 ksi to 24.3 ksi after 80 hr a t 224°C and to 25.8 ksi after 2 hr a t 310"C, whereas the elongation to failureis simultaneously increased from 6.3% to 38.3% a t 224OC and to 50.7% a t 310OC. Prior to degradation, the film had an ultimate tensile strength of 26.6 ksi and an elongation to failure of 62.0y0. The kinetics of this regeneration indicate that the reaction corresponds to the two-step recombination of the hydrolytically attacked amide linkages that were initially present in the "as received" polyimide film. The regenerated polyimide exhibits a much improved hydrolytic stability over the untreated material and specimens heat treated directly without prior aqueous degradation.
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