Analysis of monomer ratios in a dimethacrylate mixture using positron annihilation spectroscopy

Dimethacrylate based samples suitable for photopolymerization are usually used in mixtures with different ratios. In the case of the monomers D₃MA and UDMA, the ideal monomer ratio of 1:1 was reported in the literature and designed as 2 M. Dimethacrylate-based photopolymers are highly cross-linked polymer networks, indispensable in applications requiring rapid polymerization processes, such as light-curable materials for dental applications. Different ratios of the starting monomers D₃MA and UDMA in a mixture with the addition of a 0.1 mol% photoinitiator was used in near-UV light photopolymerization. The lifetime of the ortho-positronium was monitored by PALS and the local free volume were determined. NIR was used to follow the double bond transformation in the dimethacrylates region. Bulk density was determined using Archimedes' law. The phase transitions and the glass transition temperatures (Tg´s) were determined using the dynamic mechanical thermal analysis (DMTA). Based on the free volume size, double bonds and density of the material, the most suitable ratio of monomers used in dentistry and other application was determined. Correlations between the local free volume, density, and chemical bonding of the studied samples with different ratios are discussed.