Polymer structure of a light-cured resin composite in relation to distance from the surface

The aim of the investigation was to study the polymer structure of a light-cured resin composite in relation to the distance from the irradiated surface. Ten cylinders (4 x 8 mm) of composite were light-cured in a mold at 580 mW cm-2 for 40 s. The cylinders were expressed from the mold and, after 1 wk of dry storage at 37 degrees C, embedded in dental stone. Grinding parallel to the long axis of the cylinders on carborundum paper exposed the resin composite. The Wallace hardness in relation to distance from the irradiated surface was measured before and after 1 d of ethanol storage. Before ethanol storage and at distances from the surface of 0.5-3.5 mm, no difference in hardness was recorded. The ethanol storage gave rise to a softening of the resin composite. The softening recorded at 3.0-3.5 mm was significantly more pronounced than the softening taking place at 0.5-2.5 mm. This result was explained by a slower polymerization occurring at the greater distances from the irradiated surface, resulting in a polymer with reduced crosslink density.