Dynamics of hydrogen-rich species in light-cured dental materials

Dental resin composites consist mainly of (di)methacrylate resins and an inorganic filler, and their polymerization reaction is initiated by light activation. Over the years, the physical properties of these light-curing materials have improved, and currently, the polymerization kinetics are well established. However, the influence of hydrogen bond formation and the restriction of molecular mobility during polymer network development is not fully elucidated. Here, we were able to explore the dynamics of hydrogen-rich species in a light-cured model unfilled resin and in a resin-modified polyalkenoate cement by means of a unique combination of light-activation and quasielastic neutron spectroscopy, which was used for modelling of the elastic incoherent structure factor (EISF). This experimental study not only confirms a substantial reduction in the nanoscale mobility of the monomers under light activation but also describes changes to the materials’ local structure. In brief, data from both light-cured samples (resin and cement) are described in terms of lone isotropic rotations, indicating that on a time average, the molecules have no preferred orientation. However, before light activation, more complex dynamical descriptions, including conformational jumps, were required to explain the nanoscale mobility of the unpolymerized materials. Clearly, the obtained emerging details on the evolution of the nanostructure in light-cured dental materials open possibilities for further advanced research in the field.