Abstract
Objectives
The aim is to demonstrate significant polymerization shrinkage stress reduction in model resins through incorporation of addition–fragmentation chain transfer moieties that promote network stress accommodation by molecular rearrangement. Monomers containing allyl sulfide linkages are incorporated to affect the shrinkage stress that arises during photopolymerization of model resins that contain an initiator and dimethacrylates. Radical-mediated allyl sulfide addition–fragmentation is enabled during polymerization. We hypothesize that allyl sulfide incorporation into methacrylate polymerizations promotes stress relaxation by enabling network adaptation.
Methods
A 1:2 mixture of tetrathiol and allyl sulfide-containing divinyl ethers is formulated with glass-forming dimethacrylates and compared to controls where the allyl sulfide is replaced with a propyl sulfide that is incapable of undergoing addition–fragmentation. Simultaneous shrinkage stress and functional group conversion measurements are performed. The T g is determined by DMA.
Results
Increasing allyl sulfide concentration reduces the relative stress by up to 75% in the resins containing the maximum amount of allyl sulfide. In glassy systems, at much lower allyl sulfide concentrations, the stress is reduced by up to 20% as compared to propyl sulfide-containing systems incapable of undergoing addition–fragmentation chain transfer.
Significance
Shrinkage stress reduction, typically accompanying free-radical polymerization, is a primary focus in dental materials research and new product development. Allyl sulfide addition–fragmentation chain transfer is utilized as a novel approach to reduce stress in ternary thiol-ene-methacrylate polymerizations. The stress reduction effect depends directly on the allyl sulfide concentration in the given ternary systems, with stress reduction observed even in systems possessing super-ambient T g s and low allyl sulfide concentrations.
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Introduction
Polymer-based dental restorative composites possess several advantageous properties such as rapid reaction, high strength, and durability. These composites are considered a favorable alternative to the more traditional amalgams as the clinical use of mercury is avoided. Moreover, the matching of esthetically pleasing natural tooth colors is readily accomplished using polymer composite materials. Nevertheless, the shrinkage stress that accompanies the polymerization of these composite materials is a primary concern for dental clinicians since it can initiate microcracking of restorative materials, leading to bacterial microleakage and secondary caries . While dimethacrylate-based composites are the most common material used as dental restoratives, they are accompanied by substantial polymerization-induced shrinkage stress .
Traditional dimethacrylate-based resins undergo a chain-growth polymerization mechanism, typically gelling at low conversion before vitrifying at higher conversions and thus developing shrinkage stress early on and throughout the polymerization . Conversely, molecular weights and polymer structure build geometrically in a step-growth polymerization, leading to delayed gelation and reduced polymerization shrinkage stress. Thiol-ene photopolymerizations follow a step-growth mechanism and are proposed to reduce polymerization-induced shrinkage stress in dental materials . Although organic thiols are frequently malodorous, high molecular weight thiols such as pentaerythritol tetra(3-mercaptopropionate) (PETMP) exhibit very low volatility, alleviating odor concerns. Additionally, thiol-ene resins have been examined in multiple studies as dental restorative materials since they exhibit rapid photopolymerization, necessary for clinical applicability. However, thiol-ene networks typically exhibit relatively low moduli and glass transition temperatures ( T g ) when compared with methacrylate-based systems. Ternary mixtures of thiol-ene and methacrylate-based monomers yield a synergistic combination of attributes, blending enhanced mechanical properties with lowered shrinkage stress .
Recently, a novel mechanism for stress relaxation in a polymer network has been shown via photo-induced bond rearrangement of a polymeric network , i.e., the formation of a “covalent adaptable network” (CAN). The mechanism for this stress relaxation utilizes radical-mediated addition–fragmentation chain transfer events through allyl sulfide functionalities incorporated within the crosslinking strands of the polymer network, allowing crosslinking strand rearrangement in the presence of radicals ( Fig. 1 ). The addition–fragmentation reaction preserves the concentration of both the allyl sulfide and the radical reactants, which subsequently participate in further chain transfer events . This cascading rearrangement of the network connectivity effects a global reduction in stress without any concomitant degradation of mechanical properties. When applied to thiol-ene networks, the allyl sulfide addition–fragmentation reduced shrinkage stress relative to an analogous network incapable of addition–fragmentation ; however, the low T g of that material (∼−20 °C) precludes it from use as a dental restorative material. Here, we examine the effect of incorporating a dimethacrylate monomer into an allyl sulfide-containing thiol-ene system and determine its effects on the thermomechanical properties and polymerization-induced shrinkage stress. The effects of allyl sulfide concentration and polymer mobility are evaluated to ascertain under which conditions the addition–fragmentation mechanism is capable of reducing stress in model resins.
