Graphical abstract
Highlights
- •
The maximum polymerization rate of double bond can be adjusted by TTMSS concentration.
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The polymethacrylate network structure became more homogenous with the decreasing of polymerization rate.
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The conversion of neat resin formulated with TTMSS showed a strong dependent on the irradiation time.
Abstract
Objectives
The purpose of this study was to evaluate the polymerization behavior of a model dentin adhesive with tris(trimethylsilyl)silane (TTMSS) as a co-initiator, and to investigate the polymerization kinetics and mechanical properties of copolymers in dry and wet conditions.
Methods
A co-monomer mixture based on HEMA/BisGMA (45/55, w/w) was used as a model dentin adhesive. The photoinitiator system included camphorquinone (CQ) as the photosensitizer and the co-initiator was ethyl-4-(dimethylamino) benzoate (EDMAB) or TTMSS. Iodonium salt, diphenyliodonium hexafluorophosphate (DPIHP) serving as a catalyst, was selectively added into the adhesive formulations. The control and the experimental formulations were characterized with regard to the degree of conversion (DC) and dynamic mechanical properties under dry and wet conditions.
Results
In two-component photoinitiator system (CQ/TTMSS), with an increase of TTMSS concentration, the polymerization rate and DC of C C double bond increased, and showed a dependence on the irradiation time and curing light intensity. The copolymers that contained the three-component photoinitiator system (CQ/TTMSS/DPIHP) showed similar dynamic mechanical properties, under both dry and wet conditions, to the EDMAB-containing system.
Significance
The DC of formulations using TTMSS as co-initiator showed a strong dependence on irradiation time. With the addition of TTMSS, the maximum polymerization rate can be adjusted and the network structure became more homogenous. The results indicated that the TTMSS could be used as a substitute for amine-type co-initiator in visible-light induced free radical polymerization of methacrylate-based dentin adhesives.
1
Introduction
Camphorquinone (CQ)/amine initiation system is the most widely employed system for visible-light curing of methacrylate-based dental restorative materials. CQ itself can photoinitiate polymerization, but at a low reaction rate. Aliphatic or aromatic amines, such as N , N -dimethyl-ptoluidine, 2-ethyl-dimethylbenzoate, N -phenylglycine, ethyl-4-dimethylaminobenzoate (EDMAB), 2-(dimethylamino) ethyl methacrylate (DMAEMA), and many other amine-containing compounds, are widely used as co-initiators for CQ . Among these compounds, EDMAB is a very popular co-initiator in dental restorative materials due to its low basicity and high efficiency.
The limitations of EDMAB include sensitivity to oxygen inhibition , unstable under acidic conditions and in acidic dental resin formulations , and leached EDMAB is potentially cytotoxic . Usually, an acid-base reaction occurs between electron donors and electron acceptors, which may result in a charge transfer complex (CTC). This charge transfer complex may interfere with polymerization. For example, the polymerizable co-initiator DMAEMA was ineffective for acidic monomers because of its strong basicity . The search for new co-initiators remains an important issue in the development of durable dentin adhesives and composites.
Silyl radicals have widespread use in hydrosilylation and reduction reactions in organic chemistry. Tris(trimethylsilyl)silane (TTMSS) was synthesized by Gilman and co-workers in 1965 . Nearly 20 years later, the Chatgilialoglu laboratory discovered that TTMSS could serve as a radical-based agent . Recently, the TTMSS radical has been characterized for applications in photoinitiation systems by Lalevee et al. . Lalavee and colleagues reported that TTMSS had the following attributes: (1) a high inherent reactivity for the addition to double bonds, and (2) a low ionization potential which is associated with an oxidation process and the formation of silylium cations. Newly developed photoinitiator (PI) systems based on TTMSS exhibited a high reactivity both in free radical polymerization (FRP) and free radical promoted cationic polymerization (FRPCP) . The ability to efficiently consume oxygen is a particularly interesting feature of the PI systems based on TTMSS. Potentially the TTMSS-based PI systems can overcome the classical and well known oxygen inhibition of the FRP or FRPCR processes . In addition, TTMSS did not exhibit a toxic response when tested in several biological test systems .
The efficacy of the PI systems depends on the H-atom donor ability of co-initiators and the compatibility of initiator components with resin. The hydrophobicity of CQ and EDMAB has limited their performance in the wet, oral environment. To address this limitation, a third component, iodonium salt, diphenyliodonium hexafluorophosphate (DPIHP), has been adopted into the two-component PI system . DPIHP, with higher solubility in water, acts as an electron acceptor which offers dual roles, i.e., to regenerate the photosensitizer (CQ) and to generate additional active phenyl radicals . Due to this unique ability, the iodonium salt is expected to promote the free radical polymerization within the CQ/TTMSS PI system.
It is well known that the structure and properties of polymeric materials are governed in part by the kinetics of the polymerization reaction. Polymerization kinetics determines the microgel structures, degree of conversion (DC), and many other characteristics . Silanes in the presence of a PI such as benzophenone (BP), isopropylthioxanthone (ITX), or camphorquinone (CQ) are highly reactive and even better than a reference amine co-initiator such as EDMAB . In spite of these advantages, there are no reports on the use of TTMSS as a co-initiator in methacrylate-based dental polymers.
In this work, HEMA/BisGMA (45/55, w/w) was used as a model resin. The polymerization behavior of this model resin when EDMAB or TTMSS was used as a co-initiator for photosensitizer CQ was studied in detail. The objective of this work was to evaluate the efficiency of TTMSS as co-initiator in neat methacrylate-based resin. The overall research hypotheses of this study were: (1) the polymerization behavior of neat resin formulated with TTMSS was comparable to neat resin formulated with EDMAB in the two-component PI system, (2) because of its ability to enhance radical efficacy, the addition of DPIHP will significantly increase the final degree of conversion and maximum polymerization rate, (3) the relative crosslink densities of the polymethacrylate network formulated using TTMSS was similar to EDMAB, and (4) the rate of polymerization with TTMSS will affect the mechanical properties of the neat methacrylate resin under dry and wet conditions.
2
Materials & methods
2.1
Materials
2,2-Bis[4-(2-hydroxy-3-methacryloxypropoxy) phenyl]propane (BisGMA, St. Louis, MO) and 2-hydroxyethyl methacrylate (HEMA, St. Louis, MO) were used as received without further purification as monomers in dentin adhesives. Camphoroquinone (CQ), ethyl-4-(dimethylamino) benzoate (EDMAB), tris(trimethylsilyl)silane (TTMSS) and diphenyliodonium hexafluorophosphate (DPIHP) were obtained from Sigma-Aldrich (St. Louis, MO). All other chemicals were reagent grade and used without further purification.
2.2
Preparation of adhesive formulations
The monomer mixtures were made with 45 wt% HEMA and 55 wt% BisGMA. The formulation containing CQ (0.5 wt%), EDMAB (0.5 wt%) and DPIHP (0.5 wt%) were used as controls . The experimental formulations consisting of EDMAB, TTMSS and DPIHP are listed in Table 1 . The mixtures of monomers/PIs are prepared in brown glass vials under amber light. The preparation of adhesive formulations and their polymer beams have been reported previously . In brief, the solutions containing the monomers/PIs were mixed overnight at 23 ± 2 °C to promote complete dissolution and formation of a homogeneous solution. The prepared resins were injected into a glass-tubing mold (Wilmad, P 1m-1.2m-0-914m) and light-cured for 40 s at 23 ± 2 °C with a LED light curing unit (LED Curebox, 100 mW/cm 2 irradiance, Proto-tech, Portland, OR). The polymerized samples were stored in the dark at 23 ± 2 °C for at least 48 h prior to testing. The resultant round beam specimens (L × D = 15 mm × 1.0 mm) were used to determine dynamic mechanical properties.
| Type | Run a | CQ | EDMAB | TTMSS | DPIHP |
|---|---|---|---|---|---|
| Two-component PI | CE-0.5 b | 0.5 | 0.5 | / | / |
| CE-1.0 | 0.5 | 1.0 | / | / | |
| CT-0.5 | 0.5 | / | 0.5 | / | |
| CT-1.0 | 0.5 | / | 1.0 | / | |
| CT-3.0 | 0.5 | / | 3.0 | / | |
| Three-component PI | CED c | 0.5 | 0.5 | / | 0.5 |
| CTD-0.5 | 0.5 | / | 0.5 | 0.5 | |
| CTD-1.0 | 0.5 | / | 1.0 | 0.5 | |
| CTD-3.0 | 0.5 | / | 3.0 | 0.5 | |
| Binary co-initiator | CET-1.0 | 0.5 | 0.5 | 1.0 | / |
| CET-3.0 | 0.5 | 0.5 | 3.0 | / | |
| CET-0.25 | 0.5 | 0.25 | 0.25 | / | |
a The resin was mixed HEMA/BisGMA in the ratio of 45/55 (w/w).
b The formulation was used as the control in two-component or combined co-initiator PI systems.
c The formulation was used as the control in three-component PI system.
2.3
Real-time double bond conversion and maximal polymerization rate
The DC and polymerization behavior were determined by FTIR as described by our group . Real-time in-situ monitoring of the photopolymerization behavior of the different adhesive formulations was performed using an infrared spectrometer (Spectrum 400 Fourier transform infrared spectrophotometer, Perkin-Elmer, Waltham, MA) at a resolution of 4 cm −1 . One drop of adhesive solution was placed on the zinc selenide (ZnSe) crystal top plate of an attenuated total reflectance (ATR) accessory (PIKE Technologies Gladi-ATR, Madison, WI) and covered with a mylar film to prevent oxygen inhibition of polymerization. A 40 or 120-s exposure to the commercial visible-light-polymerization unit (Spectrum ® 800, Dentsply, Milford, DE) at an intensity of 550 mW/cm 2 was initiated after 50 infrared spectra had been recorded. Real-time IR spectra were continuously recorded for 600 s after light activation began. A time-based spectrum collector (Spectrum TimeBase, Perkin-Elmer) was used for continuous and automatic collection of spectra during polymerization. A minimum of three replicates were obtained for each adhesive formulation. The change of the band ratio profile-1637 cm −1 (C C)/1608 cm −1 (phenyl) was monitored, and DC was calculated using the following equation based on the decrease in the absorption intensity band ratio before and after light curing.
DC = 1 − absorbanc e 1637 cm − 1 sample / absorbanc e 1608 cm − 1 sample absorbanc e 1637 cm − 1 monomer / absorbanc e 1608 cm − 1 monomer × 100 %
The average of the last 50 values of the time-based spectra is reported as the DC value. The maximum polymerization rate ( R p (max) /[ M ]) was determined using the maximum slope of the linear region of the DC-time plots .
To determine the DC as a function of irradiation intensity, samples were irradiated with the commercial visible-light-polymerization unit (Spectrum ® 800, Dentsply, Milford, DE) at intensities of 550, 300, or 50 mW/cm 2 . Irradiation intensity was measured at the sample surface with a visible light curing meter (Cure Rite, Model 644726, Dentsply, Milford, DE).
2.4
Dynamic mechanical analysis (DMA)
Dynamic mechanical analysis (DMA) is a thermal analysis technique that measures the properties of materials as they are deformed under periodic stress. This technique is particularly well suited for characterizing viscoelastic materials. Since the technique measures both elastic and viscous responses, it is considered a valuable tool for obtaining information regarding the crosslink density and structural heterogeneity of polymer networks .
In this study, DMA tests were performed using a TA instruments Q800 DMA (TA Instruments, New Castle, USA) with a three-point bending clamp. The dynamic mechanical properties of methacrylate-based dentin adhesives have been described by our group . A sinusoidal stress is applied and the resultant strain is measured. The properties measured under this oscillating loading are storage modulus, loss modulus, and tan δ . The storage modulus ( E ′) represents the stiffness of a viscoelastic material and is proportional to the energy stored during a loading cycle. The loss modulus ( E ″) is related to the amount of energy lost due to viscous flow. The ratio of loss ( E ″”) to storage modulus ( E ′) is referred to as the mechanical damping, or tan δ . The frequency used to measure the storage modulus is 1 Hz with an amplitude of 15 μm and a preload of 0.01 N . In the dry condition, the storage modulus is measured from 10 to 200 °C with a ramping rate of 3 °C /min. The glass transition temperature ( T g ) is determined as the position of the maximum on the derivate storage modulus vs. temperature plots. Round beam specimens (1.0 mm × 15 mm) prepared as described previously are used for DMA measurements. A minimum of three specimens of each material are measured.
The inverse ratio ( ζ ) of the modulus in the rubbery region to the temperature was used to represent the relative crosslink density . The full-width-at-half-maximum (FWHM) of the tan δ curves was used to represent the heterogeneity of the network.
Wet-condition DMA tests are operated using the three-point submersion clamp . Round beam specimens (1.0 mm × 15 mm) are immersed in water at 23 °C for at least 5 days to be fully hydrated. The test temperature is varied from 10 to 80 °C with a ramping rate of 1.5 °C min −1 .
2.5
Statistical analysis
The results were analyzed statistically using one-way/two-way analysis of variance (ANOVA), together with Tukey’s test at α = 0.05 (Microcal Origin Version 8.0, Microcal Software Inc., Northampton, MA) to identify significant differences in the means or interaction.
2
Materials & methods
2.1
Materials
2,2-Bis[4-(2-hydroxy-3-methacryloxypropoxy) phenyl]propane (BisGMA, St. Louis, MO) and 2-hydroxyethyl methacrylate (HEMA, St. Louis, MO) were used as received without further purification as monomers in dentin adhesives. Camphoroquinone (CQ), ethyl-4-(dimethylamino) benzoate (EDMAB), tris(trimethylsilyl)silane (TTMSS) and diphenyliodonium hexafluorophosphate (DPIHP) were obtained from Sigma-Aldrich (St. Louis, MO). All other chemicals were reagent grade and used without further purification.
2.2
Preparation of adhesive formulations
The monomer mixtures were made with 45 wt% HEMA and 55 wt% BisGMA. The formulation containing CQ (0.5 wt%), EDMAB (0.5 wt%) and DPIHP (0.5 wt%) were used as controls . The experimental formulations consisting of EDMAB, TTMSS and DPIHP are listed in Table 1 . The mixtures of monomers/PIs are prepared in brown glass vials under amber light. The preparation of adhesive formulations and their polymer beams have been reported previously . In brief, the solutions containing the monomers/PIs were mixed overnight at 23 ± 2 °C to promote complete dissolution and formation of a homogeneous solution. The prepared resins were injected into a glass-tubing mold (Wilmad, P 1m-1.2m-0-914m) and light-cured for 40 s at 23 ± 2 °C with a LED light curing unit (LED Curebox, 100 mW/cm 2 irradiance, Proto-tech, Portland, OR). The polymerized samples were stored in the dark at 23 ± 2 °C for at least 48 h prior to testing. The resultant round beam specimens (L × D = 15 mm × 1.0 mm) were used to determine dynamic mechanical properties.
| Type | Run a | CQ | EDMAB | TTMSS | DPIHP |
|---|---|---|---|---|---|
| Two-component PI | CE-0.5 b | 0.5 | 0.5 | / | / |
| CE-1.0 | 0.5 | 1.0 | / | / | |
| CT-0.5 | 0.5 | / | 0.5 | / | |
| CT-1.0 | 0.5 | / | 1.0 | / | |
| CT-3.0 | 0.5 | / | 3.0 | / | |
| Three-component PI | CED c | 0.5 | 0.5 | / | 0.5 |
| CTD-0.5 | 0.5 | / | 0.5 | 0.5 | |
| CTD-1.0 | 0.5 | / | 1.0 | 0.5 | |
| CTD-3.0 | 0.5 | / | 3.0 | 0.5 | |
| Binary co-initiator | CET-1.0 | 0.5 | 0.5 | 1.0 | / |
| CET-3.0 | 0.5 | 0.5 | 3.0 | / | |
| CET-0.25 | 0.5 | 0.25 | 0.25 | / | |
a The resin was mixed HEMA/BisGMA in the ratio of 45/55 (w/w).
b The formulation was used as the control in two-component or combined co-initiator PI systems.
c The formulation was used as the control in three-component PI system.
2.3
Real-time double bond conversion and maximal polymerization rate
The DC and polymerization behavior were determined by FTIR as described by our group . Real-time in-situ monitoring of the photopolymerization behavior of the different adhesive formulations was performed using an infrared spectrometer (Spectrum 400 Fourier transform infrared spectrophotometer, Perkin-Elmer, Waltham, MA) at a resolution of 4 cm −1 . One drop of adhesive solution was placed on the zinc selenide (ZnSe) crystal top plate of an attenuated total reflectance (ATR) accessory (PIKE Technologies Gladi-ATR, Madison, WI) and covered with a mylar film to prevent oxygen inhibition of polymerization. A 40 or 120-s exposure to the commercial visible-light-polymerization unit (Spectrum ® 800, Dentsply, Milford, DE) at an intensity of 550 mW/cm 2 was initiated after 50 infrared spectra had been recorded. Real-time IR spectra were continuously recorded for 600 s after light activation began. A time-based spectrum collector (Spectrum TimeBase, Perkin-Elmer) was used for continuous and automatic collection of spectra during polymerization. A minimum of three replicates were obtained for each adhesive formulation. The change of the band ratio profile-1637 cm −1 (C C)/1608 cm −1 (phenyl) was monitored, and DC was calculated using the following equation based on the decrease in the absorption intensity band ratio before and after light curing.
DC = 1 − absorbanc e 1637 cm − 1 sample / absorbanc e 1608 cm − 1 sample absorbanc e 1637 cm − 1 monomer / absorbanc e 1608 cm − 1 monomer × 100 %
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