Microstructural study of different thick dimethacrylate-based samples using different amounts of photoinitiator

Abstract

A commercial photopolymer mixture of two dimethacrylate (2M) samples was studied using positron annihilation spectroscopy (PALS), near infrared spectroscopy (NIR), and bulk density measurements with a focus on microstructural free volume, which is known to affect material properties. First, the changes in the local free volume during the crosslinking of discontinuous irradiated 2M samples with two different thicknesses in the presence of 0.1 mol% or 0.5 mol% of photoinitiator (PI) were investigated by PALS. The PALS data of a series of 2M samples obtained from discontinuous irradiation was used for the reconstruction of microstructural changes for the continuous irradiated 2M samples. The evolution of free volume changes during photopolymerization of the 2M samples was characterized using time parameters. Here, it was found that 0.5 mol% of PI is sufficient to reach comparable microstructures for the 2 mm and 5 mm-thick 2M samples. Finally, the impact of both the parameters, i.e., the sample thickness and amount of PI, on the free volume characteristics of the final cured poly2M samples is correlated with the bulk density and the conversion of monomers, as well as the H bonded interactions determined from NIR. However, the spatial graded microstructural study of poly2M samples with different thicknesses and amounts of photoinitiator (PI) revealed that an increased amount of PI and a reduction in thickness led to more pronounced microstructural differences. Spatial graded free volume characteristics appear to be important factors influencing the final material properties.