Long-term behaviour of PEI and PEI-based composites subjected to physical aging

Glass-forming materials, including thermosetting 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 owing to the glassy non-equilibrium state. The process of moving towards equilibrium consists essentially of a densification of the matter and follows complex paths deriving from its inherent non-linear and non-exponential character. The structural relaxation is observable in a laboratory time-scale at temperatures below but close to Tg, and is related to the durability of polymeric materials because it is characterized by changes of structure-sensitive properties until equilibrium is approached. Polymer-based composites suffer the same shortcoming even if the properties are fiber-dominated. In this paper sub-Tg annealing studies have been carried out with both plain PEI and its carbon-fiber composites in order to observe the kinetics of structural relaxation. The experimental technique used is differential scanning calorimetry (DSC), which measures the enthalpy recovery during the structural relaxation. The investigation regarding the plain resin consisted of aging treatments at Tg-10°C, Tg-20°C and Tg-30°C, for different annealing times ranging between 0 and 168 h. The experimental data were used to calculate the parameters of a long-term predictive model for the enthalpy relaxation based on the Narayanaswamy approach. The structural relaxation of the in situ resin (i.e. the resin constrained into the fibers lattice) was also investigated at Tg-20°C taking into account the marked decrease of the glass transition temperature resulting from the overall manufacturing process. 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 four-point bending geometry at different level of stress ratio R (the ratio of the minimum to the maximum stress). The aged samples showed a higher characteristic strength, which resulted in a correspondingly higher fatigue life compared to the as manufactured materials.