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

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Non-thermoelastic effects such as cure shrinkage of a polymer can play a role in residual stresses in composite parts. Studies have shown that cure shrinkage can place significant stresses on fibers. Therefore, the cure cycle of 3501-6 epoxy resins was modified to change its cure shrinkage characteristics to minimize the stresses. New cure strategies were developed using volumetric dilatometry, differential scanning calorimetry, dielectric cure monitoring, and a unique single fiber stress test method. Cure cycles were modified to balance the resin's thermal expansion with its cure shrinkage. In some cases, a region of constant volume was achieved for a short time. However, the cure shrinkage eventually dominated over thermal expansion in all cycles as the polymer gelled. Changing the cure cycle affected the degree of cure at the point where the fiber/matrix interface developed as well as the amount of cure shrinkage occurring afterwards. A higher degree of cure at this point leads to longer stress relaxation time. Furthermore, less cure shrinkage at this point leads to less stress on the fibers. Also, slow heating rates allow more time for the polymer to relax and relieve stresses caused by cure shrinkage. Finally, a cure cycle that minimizes stresses due to cure shrinkage has been demonstrated.

The inhomogeneous structure of polymeric composites causes internal stresses to develop due to matrix volume changes during processing. The volume changes occur during cure and during cooldown after the cure is completed. Most of the previous studies on residual stresses concentrated on stress development during cooldown. In this study, a new test method was used to monitor fiber stresses that develop during cure in single fiber model composites. The method was used to study the effect of changing the cure cycle on curing induced fiber stresses. It was seen that changing the cure cycle changes the resulting stress significantly. Also, it was shown that the cure-induced stresses and their contribution to final residual stresses vary for different resins. A cure cycle with almost zero cure-induced stresses is demonstrated. The new cycle was found to satisfy the cure requirements such as glass transition temperature and cure cycle duration.