Abstract
The inherent degradation property of most dental resins in the mouth leads to the long-term release of degradation by-products at the adhesive/tooth interface. The by-products increase the virulence of cariogenic bacteria, provoking a degradative positive-feedback loop that leads to physicochemical and mechanical failure. Photoinduced free-radical polymerization and sol‒gel reactions have been coupled to produce a novel autonomous-strengthening adhesive with enhanced hydrolytic stability. This paper investigates the effect of network structure on time-dependent mechanical properties in adhesives with and without autonomous strengthening. Stress relaxation was conducted under 0.2% strain for 8 h followed by 40 h recovery in water. The stress‒time relationship is analyzed by nonlinear least-squares data-fitting. The fitted Prony series predicts the sample’s history under monotonic loading. Results showed that the control failed after the first loading‒unloading‒recovery cycle with permanent deformation. While for the experimental sample, the displacement was almost completely recovered and the Young’s modulus increased significantly after the first test cycle. The experimental polymer exhibited higher degree of conversion, lower leachate, and time-dependent stiffening characteristics. The autonomous-strengthening reaction persists in the aqueous environment leading to a network with enhanced resistance to deformation. The results illustrate a rational approach for tuning the viscoelasticity of durable dental adhesives.
Highlights
Hybrid organic-inorganic polymers have been explored extensively by both industrial and academic research communities [1,2,3]
Based on the polymerization kinetics, solubility, and mechanical properties, we proposed the following mechanism for the autonomous strengthening that was recorded for these novel polymers [8,9]
When the liquid resin is irradiated by visible-light, the polymethacrylate-based network is formed by free-radical polymerization of comonomers, for example, 2-hydroxyethyl methacrylate (HEMA) and bisphenol A glycerolate dimethacrylate (BisGMA)
Summary
Hybrid organic-inorganic polymers have been explored extensively by both industrial and academic research communities [1,2,3] These hybrid materials offer several advantages including the ability to tune the properties through composition, ratio, and manufacturing process, such as chemical catalysis, photo-induced reaction, post-curing reactions, solvents, etc. Hybrid materials with unique properties have been developed using the photoacid-induced sol-gel reaction and UV irradiation [7]. This approach combines free-radical polymerization, cationic polymerization, Polymers 2020, 12, 2076; doi:10.3390/polym12092076 www.mdpi.com/journal/polymers. Our group developed hybrid polymers that coupled visible light-induced sol–gel reaction with free radical polymerization. When the liquid resin is irradiated by visible-light, the polymethacrylate-based network is formed by free-radical polymerization of comonomers, for example, 2-hydroxyethyl methacrylate (HEMA) and bisphenol A glycerolate dimethacrylate (BisGMA). The autonomous-strengthening adhesive provides enhanced hydrolytic stability, increased mechanical properties, and decreased degradant release [11,12]
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