A new method for quantifying the intensity of the C=C band of dimethacrylate dental monomers in their FTIR and Raman spectra

The degree of conversion (DC) of methacrylate dental resins is typically determined by spectroscopically measuring the decrease of the vinyl (C=C) stretching band at 1640 cm−1, ratioed before and after polymerization to an internal standard (aromatic ring quadrant stretching vibration (Ph)) at around 1609 cm−1. While standard methods exist for measuring the intensity of the C=C and Ph peaks from the FTIR or Raman spectrum, these methods either fail under certain circumstances, or lack a physical basis, being purely based on spectral features. In this study, we present a rigorous method (named rotational isomerism method) for determining the intensity of the vinyl and aromatic bands from the FTIR and Raman spectra of dental monomer mixtures, and compare it to a standard baseline method (SBM) and a standard curve fitting method. Model triethyleneglycol dimethacrylate (TEGDMA)/2,2-bis(4-hydroxyphenyl)propane (Bisphenol-A) and TEGDMA/2,2-bis(4-(2-hydroxy-3-methacryloxypropoxy)phenyl)propane (Bis-GMA) mixtures with a series of known C=C/Ph molar ratios were prepared in order to simulate the effect of curing. The accuracy of the new method for measuring the DC was found to be as good as the commonly used SBM. The standard curve fitting method was shown to be inappropriate for measuring C=C/Ph ratios by FTIR spectroscopy due to its inability to realistically simulate the features of the spectra. The insight gained through the use of this new method may be useful for the characterization of other methacrylate biomaterials.