EFFECT OF CURING HISTORY ON ULTIMATE GLASS-TRANSITION TEMPERATURE AND NETWORK STRUCTURE OF CROSS-LINKING POLYMERS

For a particular crosslinking polymer, it is often considered that the final state of crosslinking may be reached by post-curing at temperatures above its ultimate glass transition temperature, T(g,u), regardless of previous curing histories. Although this appears to be true over a certain range of variation in curing history, the T(g,u) and the network structure depend strongly on the curing history over a wide range of conditions for the homopolymerization of diglycidyl ether of bisphenol-A (DGEBA) catalysed by 2-ethyl-4-methyl imidazole. As degrees of cure and crosslink increase, the glass transition temperature, T(g), of a sample may increase from the monomeric DGEBA value of -22-degrees-C to the highest value near 180-degrees-C. Depending on the curing thermal history, some samples may only attain a T(g,u) of approximately 100-degrees-C, even after post-curing at 200-degrees-C for 16 h. Although the influence of thermal history on T(g) may rank second after the influence of the degree of cure, it is the most important factor on T(g,u) for a fixed resin and catalyst composition. The reversible physical ageing process appears to be the least influential on T(g). As all crosslinking reactions involve competing reactions with different kinetic parameters, we believe that these phenomena are universally observable to a greater or lesser degree in all crosslinking reactions.