Abstract
Objectives
To produce a reduced stress dental restorative material while simultaneously maintaining excellent mechanical properties, we have incorporated an allyl sulfide functional group into norbornene–methacrylate comonomer resins. We hypothesize that the addition-fragmentation chain transfer (AFCT) enabled by the presence of the allyl sulfide relieves stress in these methacrylate-based systems while retaining excellent mechanical properties owing to the high glass transition temperature of norbornene-containing resins.
Methods
An allyl sulfide-containing dinorbornene was stoichiometrically formulated with a ring-containing allyl sulfide-possessing methacrylate. To evaluate the stress relaxation effect as a function of the allyl sulfide concentration, a propyl sulfide-based dinorbornene, not capable of addition-fragmentation, was also formulated with the methacrylate monomer. Shrinkage stress, the glass transition temperature and the elastic modulus were all measured. The composite flexural strength and modulus were also measured. ANOVA (CI 95%) was conducted to determine differences between the means.
Results
Increasing the allyl sulfide content in the resin dramatically reduces the final stress in the norbornene–methacrylate systems. Both norbornene–methacrylate resins demonstrated almost zero stress (more than 96% stress reduction) compared with the conventional BisGMA/TEGDMA 70/30 wt% control. Mechanical properties of the allyl sulfide-based dental composites were improved to the point of being statistically indistinguishable from the control BisGMA–TEGDMA by changing the molar ratio between the methacrylate and norbornene functionalities.
Significance
The allyl sulfide-containing norbornene–methacrylate networks possessed super-ambient T g , and demonstrated significantly lower shrinkage stress when compared with the control (BisGMA/TEGDMA 70–30 wt%). Although additional development remains, these low stress materials exhibit excellent mechanical properties which are appropriate for use as dental restorative materials.
1
Introduction
Modern dental restorative materials have favored photoactivated polymer-based composites over the more traditional mercury-based amalgams due to several advantages including on-demand and rapid curing as well as the formation of a material that possesses an esthetically desirable natural tooth color. Unfortunately, the volume shrinkage which occurs during photopolymerization results in a stress that has the potential to cause adhesive failure, initiate microleakage and cause recurrent caries . Polymerization stress in commercial dimethacrylate-based composites is especially problematic due to the early gelation exhibited in chain-growth polymerizations . Many researchers have attempted to mitigate this stress by minimizing volume shrinkage via novel polymerization schemes , such as thiol-ene polymerizations , ring-opening polymerizations , and polymerization-induced phase separation . In the present work, we utilize addition-fragmentation chain transfer (AFCT) to create adaptable polymer networks with radical-mediated bond rearrangement that occurs throughout the polymerization, leading to stress relaxation and the formation of a low stress material.
Addition-fragmentation chain transfer (AFCT) enables the relaxation of polymerization shrinkage-induced stress within a forming polymer network by promoting the rearrangement of the network connectivity without any associated reduction in the crosslink density or the mechanical properties of the ultimate materials . In previous studies, the presence of the allyl sulfide functional group was shown to reduce the final stress in thiol-ene and thiol-yne resins via AFCT. The AFCT mechanism is a non-degradative process of recurring chain scission and recombination which enables the network to maintain its chemical and mechanical properties before and after AFCT is activated. In the previous thiol-ene and thiol-yne studies, the thiyl radical would react with the allyl sulfide bond to induce AFCT, while simultaneously regenerating a thiyl radical and thereby inducing a cascade of reactions ( Fig. 1 (A) ).
The incorporation of the AFCT mechanism into a methacrylate-based resin was recently performed by formulating a bisphenyl-a-dimethacrylate monomer with thiol-ene monomers that contained allyl sulfide moieties . A dramatic reduction in stress was observed upon increasing the allyl sulfide-containing thiol-ene fraction of the resin; however, this reduction was accompanied by reduced mechanical properties (i.e., glass transition temperature and modulus were significantly reduced). Nevertheless, this work demonstrated that allyl sulfide-mediated AFCT was capable of reducing the stress in glassy, methacrylate-based materials.
In an effort to produce a low stress material while also achieving mechanical properties appropriate for a dental restorative, we have incorporated an allyl sulfide functional group into a norbornene–methacrylate monomer system. We hypothesize that AFCT mediated by the allyl sulfide would relieve stress in this methacrylate-based system while retaining the desirable mechanical properties associated with the bulky molecular structure of the norbornene which increases the glass transition temperature of the ultimate resin and composite. The desired curing behavior of the resin was achieved by using 470 nm wavelength light in combination with Ge-based photoinitiators. The mechanical properties and shrinkage stress of these systems were evaluated and compared with a control formulation comprised of 70/30 wt% BisGMA/TEGDMA. The flexural strength and modulus as well as the glass transition temperature of the composites were examined, with the composites containing 75 wt% glass filler.
2
Materials and methods
2.1
Materials
The materials used in this study are shown in Fig. 2 . 2-Methylene-propane-1,3-di(norbornene sulfide) (MDNS) was designed and synthesized to form a polymer network that simultaneously achieves a high glass transition temperature associated with the norbornene groups and lower stress through AFCT of the allyl sulfide. As a negative control, 2-methyl-propane-1,3-di(norbornene sulfide) (MeDNS), which is analogous to MDNS though not capable of undergoing AFCT, was also synthesized. MDNS and MeDNS were synthesized from 5-bromomethyl norbornene with 3-mercapto-2-(mercaptomethyl)-1-propene and 1,3-dimercapto-2-methylpropane, respectively, according to the method described in the literature . A waxy/hazy solid (MDNS) and viscous/colorless liquid (MeDNS) were received after purification. To synthesize 5-bromomethyl norbornene, a reaction of 1,3-dicyclopentadiene (16 g, 0.12 mol, ACROS) with allyl bromide (35 g, 0.29 mol, Sigma–Aldrich) was performed in a pressure vessel heated at 170 °C for 12 h . It was distilled at 175 °C at 1 atmosphere pressure prior to use. Additionally, 2-(methacryloyloxyethyl) 7-methylene-1,5-dithiocan-3-yl phthalate (PAS, phthalate allyl sulfide, synthesized by 3M ) was also evaluated. This monomer has the stress-relieving benefits of an allyl sulfide moiety (in the ring) as well as the enhanced crosslinking density associated with the methacrylate chain-growth mechanism. Bisphenylglycidyl dimethacrylate (BisGMA, provided by Esstech) and triethylene glycol dimethacrylate (TEGDMA, provided by Esstech) were used as received. A Ge-based photoinitiator (dibenzoyl diethyl germane, synthesized) and phosphine oxide, phenyl bis(2,4,6-trimethyl benzoyl) (BAPO, Ciba Specialty Chemicals), visible light-active photoinitiator, were utilized at 3 wt%. The Ge-based photoinitiator (dibenzoyl diethyl germane, synthesized) was prepared by a synthesis from diethyldi(2-phenyl-1,3-dithian-2-yl)germane (8.27 g, 15.9 mmol, synthesized ), CaCO 3 (25 g, 247.4 mmol, Sigma–Aldrich), and iodine (48.3 g, 190.3 mmol, Fisher Chemical) following a procedure in the literature . A surface treated microparticle silica filler (size range: 0.01–3.6 μm, average size 0.6 μm) was obtained from 3M. Composites were prepared by mixing 75 wt% of silica filler and 25 wt% of resin which includes 3 wt% of photoinitiator in a Flacktek at 30 rpm for 2 min. While many of the composites include sulfur-containing compounds, there is no noticeably distinct odor that emanates from these composites beyond that of conventional methacrylate composites.
2.2
Methods
The vinyl functional group conversion in each resin was determined using Fourier transform infrared (FTIR) spectroscopy (Nicolet 750) during photopolymerization. Samples (50 μm thickness) were irradiated for 5 min at 10 mW/cm 2 intensity by one of two visible-light dental lamps (G-light equipped with a 470 nm band-pass filter (GC America Inc.) and 430 nm dental LED light (Model 5560 ALZ, 3M)). The conversions of the methacrylate, norbornene, and allyl sulfide were determined by monitoring infrared absorption peaks centered at 3105 cm −1 (C C H stretching), 3058 cm −1 (C C H stretching), and 3077 cm −1 (C C H stretching), respectively. Gaussian fitting was used to deconvolute the peak areas as the methacrylate peak is overlapped with the norbornene and allyl sulfide peaks .
The shrinkage stress was monitored using tensometry (Paffenbarger Research Center, American Dental Association Health Foundation) during the photopolymerization of each resin or composite. A tensometer utilizes the cantilever beam deflection theory to evaluate how much the beam deflects for a given stress level. In this study, three different beam configurations were necessary to measure the model and control resins due to the limitations of each beam configuration in regards to accurate measurement of the stress level. Since the targeted resin possesses a stress level that is insufficient to deflect the lower compliance beam (stiffer) beam which was used for the control resin, a more deflective beam configuration had to be used to measure stress for the model resins. To adjust compliance for the model system with the same beam type, a shorter beam length was utilized, which was previously reported to lead to increased measured stress levels as the beam length decreases . Therefore, the stress value of the model resin, which was obtained with the shorter beam configuration, would be expected to be even lower if it could be measured accurately at the same length and compliance as the control. Therefore, the difference between the model and control systems are even larger than that which is reported here, and what is reported here represents a conservative estimate of the difference between the sample and control. The experiments were performed by irradiating samples (6 mm in diameter and 1 mm thick) for 5 min at 10 mW/cm 2 intensity using the G-light dental lamp (equipped with a 470 nm filter).
The elastic modulus ( E ′) and glass transition temperature ( T g ) of the polymerized samples were determined by dynamic mechanical analysis (DMA) (TA Instruments Q800). Rectangular samples (length × width × thickness: ∼8 mm × 5 mm × 1 mm) were prepared by sandwiching the material between 2 glass slides with 1 mm thickness spacers and cured under identical irradiation conditions to those used in the tensometer experiments. Experiments were performed at a strain and frequency of 0.1% and 1 Hz, respectively, and scanning the temperature twice at a ramp rate of 1 °C/min. The T g was assigned as the temperature at the tan delta peak maximum of the second scan. Flexural strength and modulus were measured by using a Mechanical Test System (MTS, The 858 Mini Bionix II Test System) using a 3-point bending test procedure with 20 mm span and 1 mm/min rate. For these tests, 2 mm samples were prepared by the same method as described for the DMA sample preparation. All experiments were repeated three times. ANOVA (CI 95%) was conducted to determine statistically significant differences between the means for all the reported results at the 95% confidence level.
2
Materials and methods
2.1
Materials
The materials used in this study are shown in Fig. 2 . 2-Methylene-propane-1,3-di(norbornene sulfide) (MDNS) was designed and synthesized to form a polymer network that simultaneously achieves a high glass transition temperature associated with the norbornene groups and lower stress through AFCT of the allyl sulfide. As a negative control, 2-methyl-propane-1,3-di(norbornene sulfide) (MeDNS), which is analogous to MDNS though not capable of undergoing AFCT, was also synthesized. MDNS and MeDNS were synthesized from 5-bromomethyl norbornene with 3-mercapto-2-(mercaptomethyl)-1-propene and 1,3-dimercapto-2-methylpropane, respectively, according to the method described in the literature . A waxy/hazy solid (MDNS) and viscous/colorless liquid (MeDNS) were received after purification. To synthesize 5-bromomethyl norbornene, a reaction of 1,3-dicyclopentadiene (16 g, 0.12 mol, ACROS) with allyl bromide (35 g, 0.29 mol, Sigma–Aldrich) was performed in a pressure vessel heated at 170 °C for 12 h . It was distilled at 175 °C at 1 atmosphere pressure prior to use. Additionally, 2-(methacryloyloxyethyl) 7-methylene-1,5-dithiocan-3-yl phthalate (PAS, phthalate allyl sulfide, synthesized by 3M ) was also evaluated. This monomer has the stress-relieving benefits of an allyl sulfide moiety (in the ring) as well as the enhanced crosslinking density associated with the methacrylate chain-growth mechanism. Bisphenylglycidyl dimethacrylate (BisGMA, provided by Esstech) and triethylene glycol dimethacrylate (TEGDMA, provided by Esstech) were used as received. A Ge-based photoinitiator (dibenzoyl diethyl germane, synthesized) and phosphine oxide, phenyl bis(2,4,6-trimethyl benzoyl) (BAPO, Ciba Specialty Chemicals), visible light-active photoinitiator, were utilized at 3 wt%. The Ge-based photoinitiator (dibenzoyl diethyl germane, synthesized) was prepared by a synthesis from diethyldi(2-phenyl-1,3-dithian-2-yl)germane (8.27 g, 15.9 mmol, synthesized ), CaCO 3 (25 g, 247.4 mmol, Sigma–Aldrich), and iodine (48.3 g, 190.3 mmol, Fisher Chemical) following a procedure in the literature . A surface treated microparticle silica filler (size range: 0.01–3.6 μm, average size 0.6 μm) was obtained from 3M. Composites were prepared by mixing 75 wt% of silica filler and 25 wt% of resin which includes 3 wt% of photoinitiator in a Flacktek at 30 rpm for 2 min. While many of the composites include sulfur-containing compounds, there is no noticeably distinct odor that emanates from these composites beyond that of conventional methacrylate composites.
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