The reciprocity law concerning light dose relationships applied to BisGMA/TEGDMA photopolymers: theoretical analysis and experimental characterization

Abstract

Objectives: A model BisGMA/TEGDMA unfilled resin was utilized to investigate the effect of varied irradiation intensity on the photopolymerization kinetics and shrinkage stress evolution, as a means for evaluation of the reciprocity relationship.

Methods: Functional group conversion was determined by FTIR spectroscopy and polymerization shrinkage stress was obtained by a tensometer. Samples were polymerized with UV light from an EXFO Acticure with 0.1wt% photoinitiator. A one-dimensional kinetic model was utilized to predict the conversion-dose relationship.

Results: As irradiation intensity increased, conversion decreased at a constant irradiation dose and the overall dose required to achieve full conversion increased. Methacrylate conversion ranged from 64±2% at 3mW/cm(2) to 78±1% at 24mW/cm(2) while the final shrinkage stress varied from 2.4±0.1MPa to 3.0±0.1MPa. The ultimate conversion and shrinkage stress levels achieved were dependent not only upon dose but also the irradiation intensity, in contrast to an idealized reciprocity relationship. A kinetic model was utilized to analyze this behavior and provide theoretical conversion profiles versus irradiation time and dose.

Significance: Analysis of the experimental and modeling results demonstrated that the polymerization kinetics do not and should not be expected to follow the reciprocity law behavior. As irradiation intensity is increased, the overall dose required to achieve full conversion also increased. Further, the ultimate conversion and shrinkage stress that are achieved are not dependent only upon dose but rather upon the irradiation intensity and corresponding polymerization rate.

Keywords: Dental restorative materials; Photopolymerization; Reciprocity law.

Figure 1

Conversion versus a) time and b) dose (irradiation intensity*time) for BisGMA/TEGDMA at irradiation intensities of 3 (—), 6 (— —), 12 (– – –), and 24 (---) mW/cm². Samples contain 0.1 wt% DMPA and are irradiated with 365 nm UV light.

Figure 2

Shrinkage stress versus a) time and b) dose (irradiation intensity*time) for BisGMA/TEGDMA at irradiation intensities of 3 (—), 6 (——), 12 (– – –), and 24 (---) mW/cm². Samples contain 0.1 wt% DMPA and are irradiated with 365 nm UV light.

Figure 3

Predicted conversion as a function of time and dose (irradiation intensity*time) for a range of different irradiation intensities: 3 mW/cm² (—), 6 mW/cm² (— —), 12 mW/cm² (– – –), and 24 mW/cm² (---). Kinetic parameters are based on a BisGMA:TEGDMA mixture ^, .

Figure 4

Predicted conversion as a function of time and dose (irradiation intensity*time) for a range of different irradiation intensities: 3 (—), 6 (——), 12 (— —), and 24 (---) mW/cm². Kinetic parameters are based on a BisGMA:TEGDMA mixture ^, with k_t reduced by 10⁴.

Figure 5

Predicted conversion as a function of time and dose (irradiation intensity*time) for a range of different irradiation intensities: 3 (—), 6 (---), 12 (——), and 24 (— —) mW/cm², assuming a unimolecular termination mechanism (R_t=k_t[M•]) to bring about the reciprocity law. Kinetic parameters are based on a BisGMA:TEGDMA mixture ^, .