The effect of physical aging on the durability of carbon fiber reinforced PEI

Glass forming materials, including thermoset and thermoplastic resins commonly used in polymer matrix composites for high performance applications, undergo structural relaxation when they are cooled through the glass transition region, due to the glassy non equilibrium state. The process towards the equilibrium follows complex paths deriving from its inherent nonlinear and nonexponential character. In this paper sub-Tg annealing studies have been carried out with both neat PEI and its carbon fiber composites. The in-situ resin (i.e. the resin constrained into the fibers lattice) showed a marked decrease of the glass transition temperature. The comparison between the plain and the in situ matrix was done on the basis of the same degree of undercooling (i.e. the same distance from Tg). It was found that the composite aged faster than the plain matrix. The macroscopic effects of structural relaxation on PEI based composites were analyzed by fatigue tests in a four-point bending geometry at different level of stress ratio R (the ratio of the minimum to the maximum stress). The aged samples showed an higher characteristic strength which resulted in a correspondingly higher fatigue life compared to the as manufactured materials.