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

Glass-forming materials, including thermosetting and thermoplastic resins c ommonly used in polymer-matrix composites for high-performance applications , undergo structural relaxation when they are cooled through the glass tran sition region owing to the glassy non-equilibrium state. The process of mov ing towards equilibrium consists essentially of a densification of the matt er and follows complex paths deriving from its inherent non-linear and non- exponential character. The structural relaxation is observable in a laborat ory time-scale at temperatures below but close to T-g and is related to the durability of polymeric materials because it is characterized by changes o f structure-sensitive properties until equilibrium is approached. Polymer-b ased composites suffer the same shortcoming even if the properties are fibe r-dominated. In this paper sub-T-g, annealing studies have been carried out with both plain PEI and its carbon-fiber composites in order to observe th e kinetics of structural relaxation. The experimental technique used is dif ferential scanning calorimetry (DSC), which measures the enthalpy recovery during the structural relaxation. The investigation regarding the plain res in consisted of aging treatments at T-g-10 degrees C, T-g-20 degrees C and T-g-30 degrees C, for different annealing times ranging between 0 and 168 h . The experimental data were used to calculate the parameters of a long-ter m predictive model for the enthalpy relaxation based on the Narayanaswamy a pproach. The structural relaxation of the in situ resin (i.e, the resin con strained into the fibers lattice) was also investigated at T-g-20 degrees C taking into account the marked decrease of the glass transition temperatur e 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 o f undercooling (i.e, the same distance from T-g). It was found that the com posite aged faster than the plain matrix. The macroscopic effects of struct ural relaxation on PEI based composites were analyzed by fatigue tests in f our-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 cha racteristic strength, which resulted in a correspondingly higher fatigue li fe compared to the as manufactured materials. (C) 1999 Elsevier Science Ltd . All rights reserved.