Evolution of Network Structure and Mechanical Properties in Autonomous-Strengthening Dental Adhesive

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.

Autonomous-Strengthening Adhesive Provides Hydrolysis-Resistance and Enhanced Mechanical Properties in Wet Conditions

“…Figure 3 demonstrates the representative plots of dynamic mechanical analysis in wet condition for C1 and E1 copolymers that were aged in water (aqueous aging) for 2, 3, and 4 days. In contrast to the earlier work, which studied the mechanical response of the dry copolymers [ 18 ], this study devoted particular attention to investigating the mechanical properties of the copolymers when they are completely saturated with water. This is critical, as it mimics the load transfer in the wet oral environment [ 21 ].…”

“…Our research group has developed methacrylate-based adhesives that capitalize on free-radical polymerization (FRP) and sol-gel reaction to provide adhesives with autonomic strengthening properties [ 6 , 17 , 18 ]. This category of novel dental adhesive demonstrates a persistent, intrinsic network reinforcement, leading to enhanced hydrolytic resistance (i.e., chemical durability against degradation due to the reaction with water) [ 19 , 20 ].…”

“…While our group has confirmed [ 6 , 17 , 19 ] the beneficial effects of the self-strengthening approach, there are critical aspects that remain unresolved. Our previous work [ 18 ] investigated formulations with relatively higher crosslink densities and hydrophobicity, whereas the objective of the current investigation is to uncover the potential to maximize the FRP and sol-gel reactions by designing a hydrophilic formulation with a relatively lower crosslink density. It is hypothesized that this approach can provide benefits such as hydrophilicity, which can facilitate infiltration of the wet, demineralized dentin matrix; water delivery to the trialkoxysilyl groups in the adhesive, to enable a more effective sol-gel reaction and self-strengthening; and a relatively lower crosslink density, which can prevent early stiffening of the copolymer network (early stiffening may compromise chain mobility, free radical polymerization, and sol-gel reaction).…”

“…It is hypothesized that this approach can provide benefits such as hydrophilicity, which can facilitate infiltration of the wet, demineralized dentin matrix; water delivery to the trialkoxysilyl groups in the adhesive, to enable a more effective sol-gel reaction and self-strengthening; and a relatively lower crosslink density, which can prevent early stiffening of the copolymer network (early stiffening may compromise chain mobility, free radical polymerization, and sol-gel reaction). Further, in contrast to the earlier work that studied the mechanical response of mostly the dry copolymers [ 18 ], in the current work, the mechanical properties are studied with copolymers that are completely saturated with water. It is anticipated that water saturation is more representative of the conditions that the copolymer will experience in the wet, oral environment.…”

Influence of Dimethacrylate Monomer on the Polymerization Efficacy of Resin-Based Dental Cements—FTIR Analysis

“…The degree of polymerization efficacy of resin-based cementitious materials during polymerization is important for the longevity and quality of the restorative procedure, thus determining its long-term clinical success [21,30,31]. An Inadequate polymerization reaction efficacy of resin-based cement materials can undermine their mechanical and adhesive performance [2,32].…”