PALS probing of photopolymerization shrinkage in densely packed acrylate-type dental restorative composites.

Background: Using positron annihilation lifetime spectroscopy (PALS), microstructural changes in commercial dental restorative composites under light-curing polymerization were identified as a modification in mixed positron/Ps trapping, where the decay of positronium (Ps; the bound state of positrons and electrons) is caused by free-volume holes mainly in the polymer matrix, and positron trapping is defined by interfacial free-volume holes in a mixed filler-polymer environment. In loosely packed composites with a filler content of <70-75%, this process was related to the conversion of Ps-to-positron trapping.

Objectives: To disclose such peculiarities in densely packed composites using the example of he commercially available acrylate-based composite ESTA-3® (ESTA Ltd., Kiev, Ukraine), which boasts a polymerization volumetric shrinkage of only 1.5%.

Material and methods: ESTA‑3® was used as a commercially available acrylate-based dental restorative composite. A fast-fast coincidence system of 230‑ps resolution based on 2 photomultiplier tubes coupled to a BaF2 detector and ORTEC® electronics was used to register lifetime spectra in normal-measurement statistics. The raw PAL spectra were treated using x3-x2-CDA (coupling decomposition algorithm).

Results: The annihilation process in the densely packed dental restorative composites (DRCs), as exemplified by the commercially available acrylate-based composite ESTA‑3®, is identified as mixed positron/ Ps trapping, where o-Ps decay is caused by free-volume holes in the polymer matrix and interfacial filler-polymer regions, and free positron annihilation is defined by free-volume holes between filler particles. The most adequate model-independent estimation of the polymerization volumetric shrinkage can be done using averaged positron annihilation lifetime. A meaningful description of the transformations in Psand positron-trapping sites under light curing can be developed on the basis of a semiempirical model exploring x3‑x2‑CDA. There is a strong monolithization of agglomerated filler nanoparticles in these composites, caused by the photo-induced disappearing of positron traps at the cost of Ps-decaying holes.

Conclusions: Governing the polymerization void-evolution process in densely packed DRC ESTA‑3® occurs mainly in the filler sub-system as positron-to-Ps trapping conversion, which is the reason for the low corresponding volumetric shrinkage.