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ABSTRACT
Oxygen inhibits free radical polymerization and yields polymers with uncured surfaces. This is a concern when thin layers of resin are being polymerized, or in circumstances where conventional means of eliminating inhibition are inappropriate. In this study, we tested the hypothesis that viscosity, filler content, and polymerization temperature modify oxygen diffusion in the resin or the reactivity of radical species, and affect the degree of conversion near the surface. Confocal Raman micro-spectroscopy was used to measure monomer conversion from the surface to the bulk of cured resins. Increased viscosity was shown to limit oxygen diffusion and increase conversion near the surface, without necessarily modifying the depth of inhibition. The filler material was shown to increase, simultaneously, oxygen diffusivity and the viscosity of the resin, which have opposite effects on conversion. Polymerization at a temperature above ~ 110°C was shown to eliminate oxygen inhibition.
KEY WORDS: oxygen inhibition, confocal Raman spectroscopy, visible-light cure, filler content, viscosity.
INTRODUCTION
Typically, commercial dental composites are random copolymers of 2,2-bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl]propane (Bis-GMA) and triethyleneglycol dimethacrylate (TEGDMA), filled with various types of inorganic particles. Bis-GMA and TEGDMA are bi-functional methacrylate monomers that harden following a free-radical-induced polymerization reaction. While this type of reaction has several distinct advantages (fast reaction rates, high degrees of monomer conversion, and absence of solvents), it is strongly inhibited by free-radical scavengers such as oxygen (Xia and Cook, 2003). The inhibition resulting from oxygen diffusing from the atmosphere into curing resins is responsible for the inhibited surface layers commonly found on freshly polymerized unfilled resins (Finger et al., 1996; Vallittu, 1999; Yatabe et al., 2001). This is due to the oxidation of radicals into stable species known as peroxides (Reaction 1) (Andrzejewska et al., 1998; Schulze and Vogel, 1998), which have low reactivity toward monomers.
It is hypothesized that intrinsic resin parameters such as viscosity, filler content, and polymerization temperature may modify how oxygen diffuses into the resin and therefore affect oxygen inhibition.
Since inhibition is proportional to the quantity of oxygen present in the resin during curing, the degree of conversion can therefore be used to quantify inhibition. In the past, Raman spectroscopy has been successfully used to quantify the conversion of the methacrylate group during polymerization (Shin et al., 1993; Claybourn et al.,...