The degree of monomer conversion was assessed using ATR–FTIR (Nicolet IS5, Thermo Fisher Scientific, Waltham, MA, USA). The test was performed at room temperature (25 ± 1 °C). The materials were injected into a metal ring (1 mm in thickness and 10 mm in diameter) (n = 5) placed around the ATR diamond. They were covered with an acetate sheet and light cured for 20 s or 40 s using an LED light-curing unit (1250 mW/cm2, Smartlite Focus, DENTSPLY Sirona, York, PA, USA) [20 (link)]. The curing-light position was fixed at ~2 mm from the surface of the specimen. The FTIR spectra of the specimens between 700 and 4000 cm−1, before and after curing, were recorded. The degree of monomer conversion (DC) of the sealants was calculated using the following equation:
where B0 and Bt are the absorbance of the C-O peak (1320 cm−1) [21 (link)] above the background level at 1335 cm−1 initially and after time t. This C-O bond lengthens and shifts to lower wavenumbers (1250 cm−1) when the adjacent C=C group reacts. The C-O peak was used instead of the C=C peak at 1640 cm−1 as it has been shown to provide more reproducible results [18 (link)].
where B0 and Bt are the absorbance of the C-O peak (1320 cm−1) [21 (link)] above the background level at 1335 cm−1 initially and after time t. This C-O bond lengthens and shifts to lower wavenumbers (1250 cm−1) when the adjacent C=C group reacts. The C-O peak was used instead of the C=C peak at 1640 cm−1 as it has been shown to provide more reproducible results [18 (link)].