Effect of the degree of cure on the viscoelastic properties of vinyl ester resins

Abstract

Dynamic mechanical thermal analysis (DMTA) was performed on model vinyl ester resin (VER) samples prepared from bisGMA and styrene, partially cured using either benzoin ethyl ether (BEE) as a conventional photoinitiator or p-xylylene bis-( N, N-diethyldithiocarbamate) (XDT) as a photoiniferter, and on fully cured samples using benzoyl peroxide as a thermal initiator. Thermogravimetric studies showed that minimal styrene monomer was lost during the DMTA experiments. The DMTA experiments for the samples partially cured using BEE revealed two plateaus in the storage modulus and two peaks in the tan δ curves, attributed to trapped radicals gaining mobility and continuing polymerization after the samples were heated beyond their initial T g. Only a single transition was observed for samples cured using XDT because this system did not contain trapped radicals. Performing photopolymerizations with XDT thus allowed us to probe the effect of cure conditions on the glass transition temperature, rubber modulus and relaxational behaviour of partially cured resins. The breadth of the glass transition was characterized in the temperature and frequency domains (obtained by time–temperature superposition). Partially cured bisGMA/styrene blends showed a narrowing of the transition in the frequency domain with increased cure but the bisGMA system showed no variation. When partially and fully cured bisGMA/styrene blends were studied, the breadths of the frequency domains showed opposite trends as the styrene concentration was varied. Thus, the breadth of the transition could not be interpreted as a simple measure of the network heterogeneity but is believed to be related to the distribution of mobilities in the network structure which determine the distribution of relaxation times. Time–temperature superposition yielded shift factors which, when fitted to the WLF equation, indicated that the free volume at the glass transition temperature ( T g) and the free volume expansion coefficient depended on both crosslink density and also the ability of the monomer units to pack into the network.