Abstract
Objectives
To assess the performance of thiol–ene dental composites based on selected ester-free thiol–ene formulations. Further, to point out the benefits/drawback of having a hydrolytically stable thiol–ene matrix within a glass filled composite.
Methods
Composite samples containing 50–65 wt% of functionalized glass microparticles were prepared and photopolymerized in the presence of a suitable visible light photoinitiator. Shrinkage stress measurements were conducted as a function of the irradiation time. Degrees of conversion were measured by FT-IR analysis by comparing the double bond signals before and after photopolymerization. Mechanical tests were carried out on specimens after curing as well as after extended aging in water. Dynamic mechanical analysis was employed to track the changes in storage modulus near body temperature. The properties of the thiol–ene composites were compared with those of the BisGMA/TEGDMA control.
Results
Depending on the resin type, similar or higher conversions were achieved in thiol–ene composites when compared to the dimethacrylate controls. At comparable conversions, lower shrinkage stress values were achieved. Although exhibiting lower initial elastic moduli, the thiol–ene composites’ flexural strengths were found to be comparable with the controls. Contrary to the control, the mechanical properties of the ester-free thiol–ene composites were shown to be unaffected by extensive aging in water and at least equaled that of the control after aging in water for just five weeks.
Significance
Employing non-degradable step-growth networks as organic matrices in dental composites will provide structurally uniform, tough materials with extended service time.
1
Introduction
Most commercial dental restorative materials are based on dimethacrylate resins, which undergo rapid crosslinking polymerizations to form mechanically strong, functional glasses . Such in situ formed glassy matrices are perfect to bind inorganic filling particles to result in restorative composites whose sole purpose is to match the aesthetic and mechanical characteristics of enamel. Impressively, this has not been without success since these composites have been continually commercialized for over 40 years. Despite tremendous progress made in the development of dimethacrylate dental formulations, some of the attributes of chain-growth crosslinking polymerizations that lead to the composite’s failure cannot be entirely eliminated . Although it is advantageous from the clinical standpoint, the instantaneous transition from liquid to solid state as associated with early gelation and the subsequent onset of reaction diffusion controlled network formation hinder the viscous flow at later stages of polymerization, which contributes to stress development at bonded interfaces . Additionally, free volume reduction during covalent linking between monomers causes high contraction, which is proportional to the functional group conversion and strongly coupled to the desired modulus increase . Ideally, low shrinkage, little or no internal stress, homogenous structure, quantitative conversions, and high elastic moduli would all be desired in a dental composite. As many of these parameters are interrelated in a contradictive manner , a compromise has to be identified to maximize the performance and service life of a composite restoration. It should be mentioned, that it is not just the initial mechanics that are essential but also the composite’s long-term resistance to a moist environment that will impact its suitability and performance. Swelling, leaching, and degradation over time have always been a significant concern in any composite evaluation process . A multitude of approaches have been undertaken to improve subsequent generations of dimethacrylate composites. Urethane dimethacrylates and other low-shrinkage resins have been developed by synthesis of high molecular weight monomers and oligomers, or alternatively by employing a ring-opening polymerization mechanism, to reduce the shrinkage . In recent years chain transfer reactions have been implemented within methacrylate polymerizations by thiol and ene inclusion or specifically designed addition fragmentation monomers have been considered .
This study, however, focuses on neat thiol–ene composite mixtures, which have been previously considered for dental applications . Since their first consideration as dental materials 10 years ago, significant progress has been made in the development of thiol–ene dental resins . Despite all the benefits of step-growth crosslinking reactions such as high conversion, low shrinkage/shrinkage stress, and uniform network structure, pure thiol–ene materials have for the most part been impractical for dental restorative materials owing to insufficient mechanical integrity caused by the presence of soft thioether moieties.
Recently, it has been shown that the mechanics of the flexible step-growth polymer backbone containing soft sulfide linkages can be enhanced significantly in poly(thioether) networks by eliminating the presence of esters . Achieving that, and combining other benefits of step-growth processes has led us toward revisiting the thiol–ene concept for dental composite applications. In an initial investigation we aimed to elucidate the structure–property relations within ester-free thiol–ene mixtures of varied functional group concentrations and viscosities. We detailed the resin development process concentrating on resin polymerization kinetics/control over it, limiting conversions, and mechanical properties. Interestingly, we showed a three-fold reduction in water uptake of the unfilled polymers, whose apparent hydrophobicity should aid immensely in preserving the composite properties in a moist environment. We also pointed out that high functional group concentrations in the resin consisting of low molecular weight monomers of functionalities 3 and higher can indeed lead to high modulus glasses, which at the same time may exhibit high shrinkage stress. Sometimes this shrinkage stress even exceeds that of conventional the BisGMA/TEGDMA dimethacrylate control, albeit at significantly higher functional group conversions. Again a compromise has to be reached to fine-tune the resin composition and by that the final composite properties. Most importantly, out of a number of tested resins, promising candidates were preselected to be evaluated as dental composites loaded with inorganic filler particles.
Herein, in the second part of our investigation, we conducted studies on ester-free thiol–ene composite materials focusing on composite property development such as toughness, elastic modulus, shrinkage stress, limiting functional group conversion, etc. We referenced our findings to a dimethacrylate control composed with the same filler loading as the thiol–ene systems, and cured at the same curing conditions. We detailed the procedures for adequate filler silanization with thiol, and ene functionalities to enhance the bonding at the filler-resin interface in thiol–ene composites. Finally, extended water treatment studies were undertaken to assess the long-term effects of water-induced swelling on the composite’s mechanical response.
2
Materials and methods
2.1
Materials
2,2-Bis[4-(2-hydroxy-3-methacrylyloxypropoxy)phenyl] propane (Bis-GMA) and triethylene glycol dimethacrylate (TEGDMA) (Esstech, Essington, PA, USA) were purchased from Esstech (Essington, PA, USA) as a premixed monomer mixture in 70:30 mass ratio. Triallyl-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione (TTT), hexamethylene diisocyanate (HMDI), and triethylamine (TEA) were purchased from Sigma–Aldrich. Divinyl sulfone (DVS) was purchased from Oakwood Chemicals, and Irgacure 819 (bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide) was obtained from BASF. Schott glass (mean particle size 0.4 μm) untreated as well as surface treated with a coating of γ-methacryloxypropyltrimethoxysilane, were used as the inorganic fillers (Esstech). Prior to implementation and as described later, these fillers were subsequently functionalized with thiol and/or allyl groups for inclusion and copolymerization in the composite. Tetra(2-mercaptoethyl)silane (SiTSH) and the urethane-based tetraallylether monomer (TENE) were both synthesized according to previously reported procedures . The structures of DVS-activated thiol monomers as well as the other monomers and solvents used are detailed elsewhere .
2.2
Filler functionalization
A typical procedure for glass particles silanization is as follows: 40 g of silica particles (Schott, 4.0 μm) were first taken in a glass tube and heated at 165 °C under vacuum using a Buchi heater/condenser for 3 h. The dried microparticles were then transferred to a 250 ml bottom rounded flask containing 800 ml of dry toluene supplemented with 12 ml of either 3-mercaptopropyltrimethoxysilane or allyltrimethoxysilane pre-reacted for 2 h with 0.8 g of n -propylamine. The reaction mixture was then left under stirring (24 h) for silanization. After particle functionalization, the liquid suspension was centrifuged and the solid pellets collected thoroughly, and washed with toluene (4×) and methylene chloride (3×) in two separate washing/centrifugation cycles. Finally, the washed filler particles were dried under vacuum overnight at 70 °C. The thiol and allyl functionalized fillers were analyzed by FT-IR spectroscopy and thermogravimetry (TG). The 0.5 wt% mass loss difference between silanized and unfunctionalized fillers suggests successful functional group grafting on the surface of glass particles in each case (see supporting information Fig. S1). Also, the DRIFT FT-IR characterization provides evidence of silanol group disappearance around 3745 cm −1 , implying successful surface modification (Fig. S2). Appropriately silanized fillers and resins were blended in a speedmixer (DAC 150 FVZ, Flakteck) to ensure homogenous formulations at 50/50 and 35/65 resin/filler wt% ratios.
2.3
Flexural tests and conversion analysis
Flexural properties of the composites were assessed in a three-point bending configuration (MTS 858 Mini Bionix II). Composite sample dimensions were 2/2.5/10 mm ( n = 5). Samples sandwiched between two glass slides were irradiated on both sides (5 min on each side) to ascertain uniform conversions throughout the sample thickness. All composite formulations were irradiated with visible light in the range of 400–500 nm, and at an irradiance of 30–50 mW/cm 2 . Conversions were analyzed in near-IR experiments by comparing the double bond peak area before and immediately after curing. In each case the curing was performed in the presence of 1 wt% of visible light photoinitiator, which was IR 819. Flexural tests were performed one week after curing, and/or after additional seven days during which time the specimen were treated with deionized water at 37–38 °C.
2.4
Shrinkage stress measurement
Polymerization shrinkage stress was measured with a tensometer device manufactured at the Paffenberger Research Center of the American Dental Association Health Foundation (ADAHF-PRC, Gaithersburg, MD, USA). This device is based on cantilever beam theory and measures the tensile force generated by the shrinking sample, which causes the cantilever beam to deflect. Shrinkage stress is obtained by dividing the shrinkage force by the cross-sectional area of the disk-shaped sample (1.0 mm thick by 6.0 mm diameter). As the specimens tested were the particle-filled resins, the changes in opacity of the 6 mm in diameter discs during reaction did not allow for real time monitoring of the polymerization conversion. Therefore, the shrinkage stress is plotted as a function of time. The values of ultimate conversions were obtained on MTS samples analyzed before and after irradiation by utilizing near-infrared (NIR) spectroscopy coupled with a fiber optic remote sensing technique. In the methodology of shrinkage stress measurements a composite sample was placed between two cylindrical glass rods that usually are treated beforehand with a methacrylate functional silane to promote bonding at the glass surface/resin interface. However, for the thiol–ene composites the glass rods were treated with mercaptoproplytrimethoxysilane/n-propylamine mixture in a 10:1 weight ratio to promote sulfide formation and bonding at the rod surface/thiol–ene resin interface. This was necessary because methacrylate silanization did not provide sufficient bonding and delamination occurred, preventing an accurate shrinkage stress measurement from being completed. For each composition, experiments were performed in triplicate.
2.5
Dynamic mechanical analysis
Dynamic mechanical analysis (DMA) was performed on a TA Q800 instrument. The conditions for sample curing were the same as for flexural testing. The composite samples were of rectangular shape (2/2.5/6 mm). Temperature scans were performed over 10–160/200 °C with a heating rate of 3 °C/min at a frequency of 1 Hz. The loss and storage moduli were recorded as a function of temperature for the first and second heating ramp. Glass transition was taken at the maximum of tan delta versus temperature curve.
2.6
Statistical analysis
The experimental results were analyzed in a one-way analysis of variance (ANOVA) based on n -number of specimens: FT-IR ( n = 5), shrinkage stress ( n = 3), flexural modulus and strength testing ( n = 5), DMA ( n = 3). Multiple pair-wise comparisons were further conducted using Tukey’s test with a significance level of 0.05.
2
Materials and methods
2.1
Materials
2,2-Bis[4-(2-hydroxy-3-methacrylyloxypropoxy)phenyl] propane (Bis-GMA) and triethylene glycol dimethacrylate (TEGDMA) (Esstech, Essington, PA, USA) were purchased from Esstech (Essington, PA, USA) as a premixed monomer mixture in 70:30 mass ratio. Triallyl-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione (TTT), hexamethylene diisocyanate (HMDI), and triethylamine (TEA) were purchased from Sigma–Aldrich. Divinyl sulfone (DVS) was purchased from Oakwood Chemicals, and Irgacure 819 (bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide) was obtained from BASF. Schott glass (mean particle size 0.4 μm) untreated as well as surface treated with a coating of γ-methacryloxypropyltrimethoxysilane, were used as the inorganic fillers (Esstech). Prior to implementation and as described later, these fillers were subsequently functionalized with thiol and/or allyl groups for inclusion and copolymerization in the composite. Tetra(2-mercaptoethyl)silane (SiTSH) and the urethane-based tetraallylether monomer (TENE) were both synthesized according to previously reported procedures . The structures of DVS-activated thiol monomers as well as the other monomers and solvents used are detailed elsewhere .
2.2
Filler functionalization
A typical procedure for glass particles silanization is as follows: 40 g of silica particles (Schott, 4.0 μm) were first taken in a glass tube and heated at 165 °C under vacuum using a Buchi heater/condenser for 3 h. The dried microparticles were then transferred to a 250 ml bottom rounded flask containing 800 ml of dry toluene supplemented with 12 ml of either 3-mercaptopropyltrimethoxysilane or allyltrimethoxysilane pre-reacted for 2 h with 0.8 g of n -propylamine. The reaction mixture was then left under stirring (24 h) for silanization. After particle functionalization, the liquid suspension was centrifuged and the solid pellets collected thoroughly, and washed with toluene (4×) and methylene chloride (3×) in two separate washing/centrifugation cycles. Finally, the washed filler particles were dried under vacuum overnight at 70 °C. The thiol and allyl functionalized fillers were analyzed by FT-IR spectroscopy and thermogravimetry (TG). The 0.5 wt% mass loss difference between silanized and unfunctionalized fillers suggests successful functional group grafting on the surface of glass particles in each case (see supporting information Fig. S1). Also, the DRIFT FT-IR characterization provides evidence of silanol group disappearance around 3745 cm −1 , implying successful surface modification (Fig. S2). Appropriately silanized fillers and resins were blended in a speedmixer (DAC 150 FVZ, Flakteck) to ensure homogenous formulations at 50/50 and 35/65 resin/filler wt% ratios.
2.3
Flexural tests and conversion analysis
Flexural properties of the composites were assessed in a three-point bending configuration (MTS 858 Mini Bionix II). Composite sample dimensions were 2/2.5/10 mm ( n = 5). Samples sandwiched between two glass slides were irradiated on both sides (5 min on each side) to ascertain uniform conversions throughout the sample thickness. All composite formulations were irradiated with visible light in the range of 400–500 nm, and at an irradiance of 30–50 mW/cm 2 . Conversions were analyzed in near-IR experiments by comparing the double bond peak area before and immediately after curing. In each case the curing was performed in the presence of 1 wt% of visible light photoinitiator, which was IR 819. Flexural tests were performed one week after curing, and/or after additional seven days during which time the specimen were treated with deionized water at 37–38 °C.
2.4
Shrinkage stress measurement
Polymerization shrinkage stress was measured with a tensometer device manufactured at the Paffenberger Research Center of the American Dental Association Health Foundation (ADAHF-PRC, Gaithersburg, MD, USA). This device is based on cantilever beam theory and measures the tensile force generated by the shrinking sample, which causes the cantilever beam to deflect. Shrinkage stress is obtained by dividing the shrinkage force by the cross-sectional area of the disk-shaped sample (1.0 mm thick by 6.0 mm diameter). As the specimens tested were the particle-filled resins, the changes in opacity of the 6 mm in diameter discs during reaction did not allow for real time monitoring of the polymerization conversion. Therefore, the shrinkage stress is plotted as a function of time. The values of ultimate conversions were obtained on MTS samples analyzed before and after irradiation by utilizing near-infrared (NIR) spectroscopy coupled with a fiber optic remote sensing technique. In the methodology of shrinkage stress measurements a composite sample was placed between two cylindrical glass rods that usually are treated beforehand with a methacrylate functional silane to promote bonding at the glass surface/resin interface. However, for the thiol–ene composites the glass rods were treated with mercaptoproplytrimethoxysilane/n-propylamine mixture in a 10:1 weight ratio to promote sulfide formation and bonding at the rod surface/thiol–ene resin interface. This was necessary because methacrylate silanization did not provide sufficient bonding and delamination occurred, preventing an accurate shrinkage stress measurement from being completed. For each composition, experiments were performed in triplicate.
2.5
Dynamic mechanical analysis
Dynamic mechanical analysis (DMA) was performed on a TA Q800 instrument. The conditions for sample curing were the same as for flexural testing. The composite samples were of rectangular shape (2/2.5/6 mm). Temperature scans were performed over 10–160/200 °C with a heating rate of 3 °C/min at a frequency of 1 Hz. The loss and storage moduli were recorded as a function of temperature for the first and second heating ramp. Glass transition was taken at the maximum of tan delta versus temperature curve.
2.6
Statistical analysis
The experimental results were analyzed in a one-way analysis of variance (ANOVA) based on n -number of specimens: FT-IR ( n = 5), shrinkage stress ( n = 3), flexural modulus and strength testing ( n = 5), DMA ( n = 3). Multiple pair-wise comparisons were further conducted using Tukey’s test with a significance level of 0.05.
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