Abstract
Objectives
To reduce polymerization-induced shrinkage stress while maintaining mechanical properties, reversible addition-fragmentation chain transfer (RAFT)-capable functional groups were incorporated into a photopolymerizable dimethacrylate-based dental composite. We hypothesize that the incorporation of trithiocarbonate-based RAFT functional groups into conventional dimethacrylate dental resins will reduce polymerization stress.
Methods
A trithiocarbonate dimethacrylate (TTCDMA) monomer, capable of undergoing radical-mediated RAFT, is mixed with 70 wt% BisGMA (bisphenylglycidyl dimethacrylate) and compared to a conventional dental resin comprised of TEGDMA (triethylene glycol dimethacrylate) and 70 wt% BisGMA. The shrinkage stress and methacrylate conversion were simultaneously measured during polymerization. The fracture toughness and elastic modulus were measured to evaluate the effect of the TTCDMA monomer on the mechanical properties. All the materials used herein were evaluated as a composite, including 75 wt% silica fillers. ANOVA (CI 95%) was conducted to assess the differences between the means.
Results
The TTCDMA composite exhibited a 65% stress reduction compared with TEGDMA–BisGMA though the reaction rate was slower than the conventional dental composite, owing to the additional RAFT reaction. The fracture toughness and elastic modulus of the TTCDMA-based composite were not significantly different than in the TEGDMA-based composite, while the T g was decreased by 30 °C to 155 ± 2 °C.
Significance
Despite only replacing the reactive-diluent, significant and dramatic stress reduction was observed while maintaining the elastic modulus and fracture toughness. This new RAFT-capable monomer shows great promise to replace the reactive diluent in BisGMA-based dental materials. Formulation optimization and further exploration of other RAFT-capable functional groups will provide further stress reduction in dental materials.
1
Introduction
Polymer-based composites have been vastly improved over the past few decades; however, volume shrinkage and the associated stress that evolve during curing remain a critical drawback, as they leads to microcracking, microleakage, and secondary caries . During the polymerization of dimethacrylate-based resins, which represent the primary polymerizable component of dental composites, a large stress arises due to the nature of the monomer-polymer densification process. Once the resin undergoes a transition from a liquid-like to a solid-like material (i.e., sol-to-gel transition), the shrinkage stress begins to build throughout the remainder of the polymerization due to the decreasing of molecular spacing. Methacrylate-based polymerizations are particularly susceptible to stress development owing to both the low gel-point conversion as well as the large amount of volume shrinkage per double bond that occurs .
There have been several approaches to reduce stress that utilize different polymerizable functional groups and/or different polymerization mechanisms to reduce the volume shrinkage and/or delay the gel point, including thiol-ene and thiol-yne reactions , polymerization-induced phase separation , and ring-opening polymerizations . We have previously reported the incorporation of a reversible covalent bond, which undergoes addition-fragmentation chain transfer (AFCT – e.g., see Fig. 1 A), into a polymerizable material that leads to significant stress reduction . The AFCT functional groups facilitates stress relaxation throughout the polymerization by undergoing a bond breaking and reforming exactly the same chemical structure before the bond breaking with leading to rearrangement of network strands ; thus, this approach is focused on stress dissipation rather than reducing volume shrinkage. The allyl sulfide AFCT functional group was incorporated in both thiol-ene and thiol-yne reactions to yield significant polymerization stress reduction. Unfortunately, the effect is reduced in non-thiol containing resins, such as methacrylates, since the complete reversibility of the allyl sulfide AFCT mechanism requires the presence of thiyl radicals (see Fig. 1 A). While allyl sulfide incorporation into a methacrylate-based system does lead to reduced stress, the effect is significantly diminished with increasing methacrylate content . The presence of the carbon-centered radical in methacrylate homopolymerizations leads to irreversible AFCT of the allyl sulfide group (see Fig. 1 B).
The trithiocarbonate functional group is frequently used as a reversible addition-fragmentation chain transfer (RAFT) agent to synthesize polymers having low polydispersity . Unlike the allyl sulfide functional group, the trithiocarbonate functional group is capable of fully reversible AFCT when reacting with a carbon-centered radical, such as those present in a methacrylate polymerization ( Fig. 1 C). We hypothesize that the incorporation of a trithiocarbonate-based dimethacrylate monomer will significantly lower the polymerization stress in a conventional dental resin compared with an otherwise similar dimethacrylate monomer formulation. Here, the effect of adding a trithiocarbonate dimethacrylate monomer to reduce stress was evaluated by replacing the reactive diluent, TEGDMA, within a conventional BisGMA–TEGDMA (bisphenylglycidyl dimethacrylate/triethylene glycol dimethacrylate) dental composite. In addition, fracture toughness and elastic modulus were measured to compare the mechanical properties. All experiments were performed on the formulated composite, which includes 75 wt% silica fillers.
2
Materials and methods
2.1
Materials
The monomers and photoinitiator used in this study are shown in Fig. 2 . S,S′-bis[α,α′-Dimethyl-α″-(acetyloxy)ethyl 2-methyl-2-propenoate]-trithiocarbonate (TTCDMA, trithio carbonate dimethacrylate) was synthesized from S,S′-bis(α,α′-dimethyl-α″-acetyl chloride)-trithiocarbonate and 2-hydroxyethyl methacrylate (HEMA) following the procedure in the literature . The crude oil was purified by dissolving it in a 9:1 hexanes:ethyl acetate mixture and subsequently filtering the insoluble impurities. Column chromatography was performed using an 8:2 hexanes/ethyl acetate solution. S,S′-bis(α,α′-Dimethyl-α″-acetyl chloride)-trithiocarbonate product was made by the chlorination of the S,S′-bis(α,α′-dimethyl-α″-acetic acid)-trithiocarbonate with thionyl chloride . S,S′-bis(α,α′-dimethyl-α″-acetic acid)-trithiocarbonate was prepared according to a previously published procedure . Bisphenylglycidyl dimethacrylate (BisGMA, provided by Esstech) and triethylene glycol dimethacrylate (TEGDMA, provided by Esstech) were used as received. Resins were composed of 70 wt% BisGMA and 30 wt% of either TEGDMA or TTCDMA. A phosphine oxide, phenyl bis(2,4,6-trimethyl benzoyl) (BAPO, BASF Corp.), was utilized at 1.5 wt% in the resins as a visible light-active photoinitiator. 75 wt% of barium glass filler (0.4 μm, Schott(Elmsford, NY)), no surface treated) was used to comprise the composite.
2.2
Methods
Composite samples (2 mm thickness) were irradiated at 70 mW/cm 2 intensity with 400–500 nm light (Acticure 4000) for 20 min to observe the conversion of the methacrylate functional group during polymerization. The methacrylate conversion was determined by monitoring the infrared absorption peak centered at 6166 cm −1 (C C H stretching, overtone) using Fourier transform infrared (FTIR) spectroscopy (Nicolet 750). Specimens for fracture toughness test were prepared by photocuring composites at the same irradiation condition with the IR experiments in a cuboid mold (5.5 mm length × 25 mm width × 2.5 mm thickness) having a razor blade (2 mm length × 0.2 mm width × 2.5 mm thickness) which is positioned in the top-center of the mold by vertically across the cuboid to form the initial crack in the test specimen. After the specimens are separated from the mold, the specimen surface was ground with sandpaper to create uniform dimension and remove defects. Fracture toughness was measured by using a Mechanical Test System (MTS, The 858 Mini Bionix II Test System) using a 3-point bending test procedure (the force was applied from the opposite side of the initial crack of the specimen, specimen dimension; 25 mm length × 2.5 mm width × 5.5 mm thickness) with 20 mm span and 1 mm/min rate. The elastic moduli ( E ′) and glass transition temperature ( T g ) of the polymerized samples were determined by dynamic mechanical analysis (DMA) (TA Instruments Q800). DMA experiments were performed at a strain and frequency of 0.1% and 1 Hz, respectively, and scanning the temperature twice at ramp rate of 2 °C/min. The T g was assigned as the temperature at the tan delta peak maximum of the second heating scan. This methodology does not measure the T g of the as cured sample due to changes in conversion that occur during the first thermal scan. Rather, the measurement is indicative of the maximum T g achieved under these conditions . Specimens used for DMA experiments were prepared by sandwiching the uncured composite in a rectangular mold (2 mm gap) and irradiating under the same conditions used for the FTIR experiments. The shrinkage stress was monitored using tensometry (Paffenbarger Research Center, American Dental Association Health Foundation) , which was equipped with optical fibers which enable simultaneous monitoring of the reaction progression via FTIR spectroscopy. Uncured composite was injected between two glass rods which are positioned in a 9 cm beam length of the stainless steel beam. Samples were covered with a plastic sheath to prevent oxygen inhibition of the methacrylate during polymerization. ANOVA (CI 95%) was conducted to determine differences between the means for all the reported results.
2
Materials and methods
2.1
Materials
The monomers and photoinitiator used in this study are shown in Fig. 2 . S,S′-bis[α,α′-Dimethyl-α″-(acetyloxy)ethyl 2-methyl-2-propenoate]-trithiocarbonate (TTCDMA, trithio carbonate dimethacrylate) was synthesized from S,S′-bis(α,α′-dimethyl-α″-acetyl chloride)-trithiocarbonate and 2-hydroxyethyl methacrylate (HEMA) following the procedure in the literature . The crude oil was purified by dissolving it in a 9:1 hexanes:ethyl acetate mixture and subsequently filtering the insoluble impurities. Column chromatography was performed using an 8:2 hexanes/ethyl acetate solution. S,S′-bis(α,α′-Dimethyl-α″-acetyl chloride)-trithiocarbonate product was made by the chlorination of the S,S′-bis(α,α′-dimethyl-α″-acetic acid)-trithiocarbonate with thionyl chloride . S,S′-bis(α,α′-dimethyl-α″-acetic acid)-trithiocarbonate was prepared according to a previously published procedure . Bisphenylglycidyl dimethacrylate (BisGMA, provided by Esstech) and triethylene glycol dimethacrylate (TEGDMA, provided by Esstech) were used as received. Resins were composed of 70 wt% BisGMA and 30 wt% of either TEGDMA or TTCDMA. A phosphine oxide, phenyl bis(2,4,6-trimethyl benzoyl) (BAPO, BASF Corp.), was utilized at 1.5 wt% in the resins as a visible light-active photoinitiator. 75 wt% of barium glass filler (0.4 μm, Schott(Elmsford, NY)), no surface treated) was used to comprise the composite.
