The degree of conversion (DC) is a measurement of the curing of a resin-based material. Fourier Transform Infrared spectroscopy (FTIR) may be used to determine the DC. The method calculates the conversion of reactive methacrylate groups in the material. The calculation is based on ratios of area or height of specific peaks in the FTIR-spectrum of the material.
A FTIR instrument may have different sampling techniques, generally transmission or reflection, using liquid, powder or solid samples. Attenuated total reflection (ATR) is a technique where the material is positioned directly on the top of a crystal (often a diamond) and the IR beam penetrates through the crystal into the lower surface of the material and reflects back for detection. ATR may be used to measure a wide variety of samples without any complex sample preparation.
The ATR technique may be used to measure DC at different depths of a material, for instance a cylinder may be used to prepare samples of varying thickness. This method is particularly relevant for light-curing composite materials, where the layering of the material in the restoration is an issue. The material may then be cured from the top surface using an ordinary curing unit in a setup resembling the in situ curing. The method can be used for evaluation of bulk-fill materials for which thicker layers (e.g. 4 mm) are used, compared to conventional composites.
A well-documented method for restorative composites uses the peak of the aliphatic C=C double bonds at 1638 cm-1 of the unreacted methacrylate groups versus the stable peak of the aromatic C=C double bond at 1608 cm-1 found in the Bis-GMA monomer, see equation. The method also requires a reference ratio to be measured before curing, as the degree of conversion is given as a percentage.
Equation for calculating the DC:
NIOM may perform measurements of degree of conversion of restorative materials and other polymer-based biomaterials using our FTIR instrument. We have equipment for measuring transmission and reflection (ATR) as well as a DRIFT accessory (diffuse reflectance) for powder samples.