Objectives: The mechanical properties of dental resin-based-composite (RBC) restorative materials can be highly sensitive to photo-polymerisation variables. To date much research has focussed on understanding the degree of monomer to polymer conversion ignoring the concept that polymers with similar conversions may have different structures when considered on atomic length-scales. This novel enquiry aimed to understand the evolution of structure in RBC resin-matrices according to photo-polymerisation variables.
Methods: A series of different weight % blends of Bis-GMA and TEGDMA were prepared with either Lucirin TPO (TPO) or Camphorquinone (CQ) as the photo-initiator. 1mm thickness samples were photo-polymerised using different light intensities (to introduce a range of polymerisation rates) ensuring correct spectral overlap for the photo-initiator species (470nm (CQ) and 381nm (TPO)). Simultaneous synchrotron small-angle X-ray scattering (SAXS) (15KeV, 3-63Å range) provided dynamic measurements of polymer structural evolution. Data fitting and statistical analysis was undertaken using a Gaussian model to reveal changes in cross-linking distances and relative orders.
Results: SAXS measurements revealed a broad-peak feature at q=1.4Å-1 which shifted to lower q during polymerisation, indicative of molecular extension (stretching). TPO initiated systems polymerised the most rapidly and displayed greater extension than CQ samples. However, rapidly cured systems exhibited a subsequent relaxation to shorter lengths not evident in most CQ resins. CQ systems demonstrated the greatest relative order within the matrix in addition to a profile feature at 30Å, thought to correspond to the Bis-GMA cross-link distance with TEGDMA which was more prominent in CQ systems during and post cure.
Conclusion: Resins cured with low polymerisation rates generated smaller initial strains and greater subsequent atomic order which would be considered as being structurally more favourable. We demonstrate for the first time that polymerisation rate can introduce structural differences that will influence residual strains, mechanical properties and ultimately clinical performance of RBC restorative materials.