Free Volume Structure of Acrylic-Type Dental Nanocomposites Tested with Annihilating Positrons

Light-curing effects in acrylic-type dental nanocomposites probed by annihilating positrons: the case of loosely monolith Dipol® restoratives

The possibility of application of positron annihilation lifetime (PAL) spectroscopy to commercially available dimethacrylate-based dental restorative composites Dipol® (Oksomat-AN Ltd, Ukraine) subjected to photopolymerization due to light curing is analyzed. The governing annihilation process in these composites is identified as mixed positron (e+)-positronium (Ps) trapping, where Ps decaying is caused entirely by input from free-volume holes in polymer matrix superimposed on free e+-trapping contribution from interfacial free-volume holes between filler nanoparticles and surrounded polymer matrix. Photopolymerization shrinkage is revealed through decrease in the average lifetime of annihilating positrons due to opposite changes in Ps-decaying and e+-trapping channels. The growing light-activated polymerization is characteristic of both intensities related to the second and third components in the unconstrained ×3-term decomposed PAL spectra, accompanied by decrease in the corresponding lifetimes. This process resulted in enhanced trapping rate in the defects and depressed fractional free-volume saturation with light curing. Light exposure causes smaller voids in composites owing to free-volume fragmentation in Ps- and e+-trapping sites. The microstructure scenario for these transformations includes photo-induced cross-linking of structural chains in the polymer matrix, followed by conversion of o-Ps traps in interfacial free-volume voids near agglomerated filler nanoparticles. A meaningful description of this process is developed on the basis of the semi-empirical model exploring the  ×3– ×2-coupling decomposition algorithm.