OBJECTIVE: The aim of this work was to assess the influence of the bis-EMA content on the degree of conversion (DC) and its effect on the water sorption and solubility.
MATERIALS AND METHODS: In a polytetrafluorethylene (PTFE) mould, 30 samples (Ø = 5 mm, height = 2 mm) of four experimental dental composite resins were cured for 10 s, 20 s and 40 s. The DC was analysed by Fourier Transform (FT)-Raman spectroscopy. To analyse sorption and solubility, six samples (Ø = 15 mm and thickness = 1 mm) of each composite (n = 72) were stored in water at 37°C for different storage periods: 24 h, 7 days and 30 days.
RESULTS: When cured for 20 or 40 s the DC increased with the increasing content of bis-EMA. However, the presence of 15 wt% of bis-GMA did not affect the DC, except when cured with 10 s irradiation time. This study also found a correlation between the content of bis-EMA and the reduced values for sorption and solubility, for all storage times used, when the materials were cured with 20 s.
CONCLUSIONS: The DC of mixtures with higher content of bis-EMA is affected by the presence of bis-GMA at lower energy density delivered from the curing device, suggesting that the restrictions caused by the presence of hydrogen bonds is dependent of the irradiation time used.

The influence of bis-EMA vs bis GMA on the degree of conversion and water susceptibility of experimental composite materials
Cornelio RB, Wikant A, Mjøsund H, Kopperud HBM, Haasum J, Gedde UW, Ørtengren UT.
Acta Odontol Scand. 2014 Aug;72(6):440-7. doi: 10.3109/00016357.2013.856467.