| Summary: | Photo-initiated dimethacrylate polymers are of great biomedical importance, in orthopaedics and particularly in dentistry where they are used as a matrix to form resin-based-composite (RBC) filling materials. However, the physiochemical properties of these materials are highly sensitive to photo-polymerisation variables, particularly the polymerisation rate, and as a consequence clinical performance is not ideal. Despite extensive mechanical and Infra-red spectroscopic characterisation of the cross-linked polymer resin matrix, the impact of polymerisation rate on the resultant structure is unknown. Using novel synchrotron FTIR, X-ray and neutron scattering techniques, this study aimed to investigate the effect that polymerisation rate has on the polymer structure, at atomic to micron length scales, of experimental RBCs based on common dimethacrylate resins. Kinetic X-ray scattering measurements demonstrated that polymerisation generates chain segment extension and changes in short range order within the structures of the constituent resin monomers. Accelerating polymerisation confers greater chain extension and reduced short range order, which at extremely fast polymerisation rates may store residual strains within the system. Neutron scattering confirmed that this relationship between polymerisation rate and structure occurs at longer length scales within the polymer, specifically at cross-linking distances. Synchrotron FTIR micro-spectroscopy showed that the introduction of filler particles into a resin matrix produced local heterogeneity with respect to the degree of monomer-polymer conversion and chemical aromatic bond strain. At nearest neighbour distances, heterogeneity is believed to originate from a lower converted and reduced bond strain boundary layer surrounding each filler particle, whilst light attenuation and likely, relative monomer mobility effects dominate at longer length scales within the resin matrix This study demonstrates that advanced synchrotron and neutron based characterisation techniques, seldom used in the field of dental materials research, can provide new insight into how the polymerisation rate may impact on the structure of the polymer matrix components of dental RBCs.
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