THERMAL HISTORY AND ENTHALPY RELAXATION OF AN INTERPENETRATING NETWORK POLYMER WITH EXCEPTIONALLY BROAD RELAXATION-TIME DISTRIBUTION

Differential scanning calorimetry (DSC) of an interpenetrating network polymer of composition 25% polyurethane-75% poly (methyl methacrylate ) shows a slowly increasing heat capacity, instead of the usual glass transition endotherm, whose onset temperature is not clearly discernib le. On aging of the polymer at several temperatures between 193 and 33 3 K, an endothermic peak is observed whose onset is in the vicinity of the respective temperature of aging. The area under these peaks incre ases with increasing aging time at a fixed temperature. The effects ar e attributed to a very broad distribution of relaxation times, which m ay be represented by either a sum of discrete structural relaxation ti mes of local network arrangement or by a nonexponential relaxation fun ction which is equivalent to a distribution of relaxation times. In ei ther view the vitrified state of the polymer can be envisaged as conta ining local structures whose own T(g)s extend over a wide range of tem perature. Aging decreases the enthalpy and produces an endothermic reg ion which resembles an increase in C(p) on heating because of relaxati on of that local structure. The interpretation is supported by simulat ion of DSC scans in which the distribution of relaxation times is assu med to be exceptionally broad and in which aging introduced at several temperatures over a wide range produces endothermic effects (or regio ns of DSC scans) qualitatively similar to those observed for the inter penetrating network polymer. (C) 1994 John Wiley & Sons, Inc.