Effect of curing history on ultimate glass transition temperature and network structure of crosslinking polymers

Abstract

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°C to the highest value near 180°C. Depending on the curing thermal history, some samples may only attain a T g,u of ∼100°C, even after post-curing at 200°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.