EFFECT OF CROSS-LINKING DENSITY ON DELTA-C-P(T-G) IN AN EPOXY NETWORK

The difference in heat capacity of the liquid state and that of the gl assy state at the glass transition temperature, Delta C-p(T-g)=C-p(l) - C-p(g), has been determined in samples of epoxy resin with different extents of cure. The epoxy was a diglycidyl ether of bisphenol A cure d isothermally by phthalic anhydride as hardener in a temperature rang e between 30 and 130 degrees C for various periods of time. As crossli nking density increases, the Delta C-p(T-g) decreases. This result has been interpreted within the framework of 'strong' and 'fragile' behav iour of glass-forming systems. The decrease of Delta C-p(T-g) with inc reased crosslinking suggests a resistance of the glassy structure to t he breakdown due to a 'strengthening' effect of the crosslinked struct ure.