Methods: Five monomer mixtures were prepared: 100%BisGMA [G1], 80%BisGMA/20% HEMA [G2], 70%BisGMA/30% HEMA [G3], 60%BisGMA/40% HEMA [G4] and 50%BisGMA/50% HEMA [G5]. The initiator system was 0.5 wt% camphorquinone and 0.5 wt% dimethylaminoethyl methacrylate. Three samples of each mixture were applied on a microscopic slide, covered with a Mylar strip and light-cured with a conventional halogen light-curing unit at 400 mW/cm2 for 20 s. Micro-Raman spectroscopy was done within 5 min after light-curing and 24 h and 72 h thereafter. Three point spectra were taken randomly from the surface of each sample. The RDB was determined according to the formula: RDB = (1 – Rcured / Runcured)x100. For 24 h and 72 h analysis, samples were kept in air at 37ºC. The data were analysed using ANOVA at 95% significance level.
Results: The highest initial RDB was found in groups [G1] and [G2] (51.04% and 56.02%, p>0.05). Groups [G3], [G4] and [G5] had RDBs ranging from 8% to 26% which were significantly lower than [G1] and [G2] (p<0.5). After 24 h, the RDB increased significantly in all groups and was in the range of 52-62%, [G2] having the highest values (p<0.05). After 72 h of storage there were no significant differences between groups with different monomer concentrations (p>0.05). RDB values in all groups were between 75% and 80% and were significantly different to initial and 24 h values (p<0.05).
Conclusions: The highest initial RDB was found in [G2]. Decreasing BisGMA and increasing HEMA concentrations resulted in lower initial RDB. Post-polymerisation phenomenon compensated for the lower initial RDB. Less variation was found after 24 h whilst after 72 h of storage, the RDB was comparable for all mixtures.