Quantifying Monomer Conversion in Dental Sealants

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: $$\mathrm{DC}(\%)=\frac{100\left({\mathrm{B}}_0-{\mathrm{B}}_{\mathrm{t}}\right)}{{\mathrm{B}}_0}$$
where B₀ and B_t 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)].