The relationship between the degree of conversion in dental dimethacrylate polymers determined by infrared spectroscopy and polymerization shrinkage

This work aimed to study the relationship between the degree of conversion (DC) in dental dimethacrylate polymer networks determined by two different methods - Fourier transform infrared spectroscopy (FT IR) and polymerization shrinkage. For this purpose, a series of compositions of dental monomers Bis-GMA and TEGDMA were prepared and polymerized. The DC determined by FT IR (DC_IR) was calculated by using the internal standard method, in which the absorption intensity of the double bond vibrations band (located at 1637cm⁻¹) was related to the absorption intensity of the internal standard – the band coming from skeletal stretching vibrations of the C=C aromatic ring (located at 1608 cm⁻¹). The DC determined from the polymerization shrinkage (DC_S) was calculated by measuring the differences in the monomer and polymer densities, resulting from the volumetric contraction of the system due to polymerization. The decrease in molar volume, resulting from the polymerization of one mole of double bond, was taken from the literature, and it was 22.5 cm³. The obtained results revealed that the DC_IR ranged from 55 to 72% and increased as the concentration of Bis-GMA increased and then decreased when the Bis-GMA concentration exceeded 70 wt.%. A similar tendency was observed for the DC_S. However, the obtained values were lower. DC_S ranged from 46 to 62%.