Photopolymerization shrinkage in dimethacrylate-based dental restorative composites probed by means of positron annihilation lifetime spectroscopy

Peculiarities of microstructure changes in void subsystem under light-curing polymerization in loosely- and densely-packed polymer/filler nanocomposites are studied using positron annihilation lifetime (PAL) spectroscopy, at the example of commercially available dimethacrylate-based dental restorative composites Dipol (R) and (sic)CTA-3 (R). Main process governing volumetric shrinkage in these polymer/filler nanocomposites is identified as positron-to-Ps trapping conversion, initiated by fragmentation of Ps-decay holes due to crosslinking of monomer chains. Meaningful description of photopolymerization changes is developed within x3-x2-CDA (coupling decomposition algorithm). In loosely-packed nanocomposites (such as Dipol (R)), the Ps decay takes place in intrinsic holes of polymer matrix, and free positron trapping occurs in filler sub-system, specifically in the interfacial voids between filler particles themselves and filler particles contacting with surrounding polymer. Under photopolymerization, fragmentation of intrinsic holes in polymer matrix dominates over enhanced trapping of positrons in filler subsystem since filler-particles assemblies get more tightly covered by polymer owing to disappearing of interfacial free-volume voids at their surface. In densely-packed nanocomposites (such as (sic)CTA-3 (R)), where positron trapping in interfacial voids at the surface of agglomerated filler particles takes minor role, the polymerization-driven fragmentation of Ps-decay holes causes stress inside monolith filler-particles assemblies, resulting in disappearing of some interparticulate traps (positron-trapping voids between filler particles).