A physico-chemical explanation of the post-polymerization shrinkage in dental resins

The main problem of a methacrylated dental resin's photopolymerization is the shrinkage phenomenon. This occurs, as expected, during light irradiation but also, unexpectedly, during about 24 h after photopolymerization (i.e. during the so-called ‘post-polymerization’ stage). During this period, the conversion degree does not change significantly (no more initiation, very limited, if any, propagation reaction) but free radicals concentration decreases. To better understand what happens during the 24 h after the photopolymerization, a thermal study of these resins is investigated at first and an explanation is then discussed. In this paper, the glass transition temperatures ( T g) are measured at 0 and 24 h by DMA. The post-shrinkage phenomenon is observed by TMA. Conversion degree (DC) is followed by Raman and free radical decay by ESR spectroscopy. T g increases significantly during post-polymerization (55–80 °C). The same samples were studied by TMA at room temperature and shrinkage is observed. The fact that the degree of conversion (DC) does not increase significantly and that the ‘post-shrinkage’ occurs at T< T g leads to the hypothesis that a physical phenomenon should occur which can, in turn, be responsible for a secondary chemical phenomenon (post-polymerization). The proposed explanation is that, as photopolymerization of dental resins is very fast, a large excess of free volume is trapped in non-equilibrated samples. As they have no time to return to an equilibrium state, free volume should decrease below T g and samples do physically shrink during the first 24 h. As a consequence, free radicals can come into ‘contact’ and undergo limited propagation but significant termination justifying the decrease in overall radical concentration.