Comparative PALS-Tracking of Photopolymerization Volumetric Shrinkage in Polymer-Filler Dental Restorative Nanocomposites

Photopolymerization volumetric shrinkage in acrylic polymer-filler composites is compared for commercially available dental restoratives of different filler loading, the densely-packed ESTA-3® and loosely-packed Charisma®. Changes in atomic-deficient structure of these composites are probed by positron annihilation lifetime (PAL) spectroscopy assuming positron-Ps conversion with average lifetime as estimate of shrinkage. Under light curing, fragmentation of Ps-hosting holes prevails in polymer matrix predetermining polymerization-driven inverse positron-to-Ps trapping conversion. The Ps-hosting holes get to be more fragmented in cured composites due to cross-linking of monomer chains merged into large number of small holes. Meaningful description of these changes is developed within the Positronics approach employing the x3-x2-CDA (coupling decomposition algorithm). In densely-packed composites (ESTA-3®), photopolymerization shrinkage prefers by disappearing free-volume space in junctions between filler particles, while in loosely-packed composites (Charisma®) this process dominates at the interfaces between assemblages of filler particles and polymer matrix.