Light-Curing Volumetric Shrinkage in Dimethacrylate-Based Dental Composites by Nanoindentation and PAL Study

In situinvestigation of the kinetics and microstructure during photopolymerization by positron annihilation technique and NIR-photorheology

The microstructural free volume evolution during a photopolymerization process was studied on a commercial photopolymer (SPOT LV) in situ by positron annihilation lifetime spectroscopy (PALS) and concomitant NIR-photorheology. Analysis of the positron lifetime spectra revealed a high sensitivity of the PALS technique to the different phases of photopolymerization associated with different reaction rates as well as to the evolution of microstructural free-volume shrinkage, which was described at the molecular level by the Kohlrausch–Williams–Watts equation. The in situ PALS study of microstructural changes in photopolymerization was related to the vitrification (gel point) accompanied by shrinkage stress registered via NIR-photorheology. The simultaneous NIR measurements yield information on the monomer conversion of SPOT LV, which can be correlated to the occurrence of the gel point and the evolution of the microstructural free volume. This combined study allows us to see deeper into the crosslinking process and its influence on the resulting material characteristics.

Positron annihilation lifetime spectroscopy is a sensitive tool for the in situ study of the microstructural evolution during photopolymerization.