Optical characterization of bisphenol-A-glycidyldimethacrylate-triethyleneglycoldimethacrylate (BisGMA/TEGDMA) monomers and copolymer.

Objective: The fundamental optical characteristics of dental polymers as a function of curing time are exiguously reported in the literature. In this work the refractive index [1] and the extinction coefficient [2] of bisphenol-A-glycidyldimethacrylate-triethyleneglycoldimethacrylate (BisGMA/TEGDMA)-based polymers of different volume fractions have been studied as a function of the curing time. A comparison between the measured index of refraction and theoretical calculation based on molar refraction is made. Also the temporal behavior of both the refractive index and extinction coefficient is compared.

Methods: Index of refraction: BisGMA/TEGDMA monomer samples were cured with a commercial light-curing unit by using camphorquinone and DMAEMA as the photoinitiator system. Curing times of 0, 40, 60, 300, 900 and 1800 s were used. The refractive indices were determined with a refractometer at 470 nm wavelength, where the peak absorption of the camphorquinone photoinitiator is located. Extinction coefficient: BisGMA/TEGDMA samples having different lengths were cured and the relative attenuation of light at 470 nm was measured as a function of the curing time (0, 40, 60, 300, 900, 1800 s).

Results: The results show that in general the index of refraction increases during the photopolymerization process in a time frame from 0 to 40s and then approaches a steady-state value, which depends on the original composition of the admixture. The detailed refractive index change depends on the volume fractions of the constituent substances. The extinction coefficient decreases in a time frame from 60 to 900 s and reaches a steady-state value thereafter.

Significance: Knowledge of the fundamental optical characteristics of commonly used dental polymers is essential in the development of advanced light-initiated curing methods.