Volumetric effects in the degradation of dimethacrylate-based polymer/filler nanocomposites: A positron annihilation study

The applicability of positron annihilation lifetime (PAL) spectroscopy to identify volumetric changes in nanostructured polymer/filler substances is first verified at the example of commercially available dimethacrylate-based dental restorative composites Charisma® (Heraeus Kulzer GmbH, Germany) affected to light-curing photopolymerization and one-year dry physical ageing. Principal channel governing PAL spectra in these nanocomposites is recognized as mixed positron-Ps trapping, where positronium Ps (i.e. bound positron-electron state) decay occurs in free-volume holes localized preferentially in polymer matrix, while free positron self-annihilation is character for interfacial voids at the surface of filler-particles assemblies in the filler and/or mixed filler-polymer environment. Methodological algorithm for experimental PAL spectra refinement referred to as the x3-x2-CDA (coupling decomposition algorithm) is validated for these nanocomposites, obeying changes in atomic-deficient structure due to direct conversion between positron-trapping and Ps-decaying sites. Straightforward parameterization of the reconstructed PAL spectra allows meaningful identification of free-volume entities responsible for nanostructurization-driven transformations.