2.1

Materials

2,2-Bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl]-propane (BisGMA, Polysciences, Warrington, PA) and 2-hydroxyethylmethacrylate (HEMA, Acros Organics, NJ) were used as received without further purification, as monomers in dentin adhesives. Two siloxane methacrylate monomers (SM1 and SM2) were synthesized in our lab. The control adhesive resin consisted of HEMA and BisGMA with a mass ratio of 45/55. This control was used to compare with the experimental adhesive resins with HEMA/BisGMA/SM = 45/55 − x / x (w/w) ratio. The control and experimental adhesives were also formulated with 12 wt% water to simulate bonding in the mouth. The concentration of water was based on the total final weight of the adhesive resin. Camphorquinone (CQ, 0.5 wt%), ethyl-4-(dimethylamino)benzoate (EDMAB, 0.5 wt%) and diphenyliodonium hexafluorophosphate (DPIHP,1.0 wt%) were obtained from Aldrich (Milwaukee, WI, USA) and used as a three-component-photoinitiator system without further purification. 2,4,6,8-Tetramethyl-2,4,6,8-tetrakis(propyl glycidyl ether)cyclotetrasiloxane (TPGTS), glycerol dimethacrylate (GDMA, assay 85%, mixture of isomers), ethyl acetate, boron trifluoride diethyl etherate (BF ₃ O(C ₂ H ₅ ) ₂ ), anhydrous magnesium sulfate (MgSO ₄ ), and all other chemicals were purchased from Sigma–Aldrich at reagent grade and used without further purification.

2.2

Synthesis of siloxane-methacrylate (SM) monomers

2,4,6,8-Tetramethyl-2,4,6,8-tetrakis(propyl glycidyl ether)cyclotetrasiloxane (TPGTS, 10 g, 0.0143 mol), 2-hydroxyethylmethacrylate (HEMA, 10 g, 0.076 mol) or glycerol dimethacrylate (GDMA, mixture of isomers, 13.75 g, 0.06 mol) and ethyl acetate (EA, 100 mL) were mixed together. Boron trifluoride diethyl etherate (0.2 mL) was added into this solution. The reaction was allowed to continue at room temperature for about 12 h, and it was monitored by thin layer chromatography (mobile phase: dichloromethane/hexane, 50/50, wt%). The product-containing solution was washed with distilled water and dried over anhydrous MgSO ₄ . The solvent was removed with a rotary evaporator at 35–40 °C. The process for the synthesis of the two siloxane methacrylate monomers (SM1 and SM2) is shown in Scheme 1 . The yields of SM1 and SM2, which were colorless and viscous, were 92.4% and 95.0%, respectively. The structure of the synthesized compounds (SM1, SM2) was confirmed using FTIR (Spectrum 400, Perkin-Elmer, Waltham, MA), ¹ H NMR and ¹³ C NMR (FT-400 MHz Bruker Spectrometer, DMSO as solvent) spectroscopies.

Synthesis scheme for new siloxane-methacrylate monomers.

2.3

Preparation of adhesive formulations

The preparation of the control adhesive formulations has been reported . As shown in Table 1 , the control adhesive formulation (C0) consisted of HEMA and BisGMA with a mass ratio of 45/55 which is similar to widely used commercial dentin adhesives. In the experimental adhesive formulations (E-SM), siloxane-methacrylates were used as co-monomers to partially replace the BisGMA ( Table 1 ). The siloxane-methacrylates were included at 5, 15, and 25 wt%. The abbreviations E1, E2, and E3 represent 5, 15, and 25 wt% of either SM1 or SM2. Water at 12 wt% was added into C0, E3-SM1 and E3-SM2 to simulate the moist environment of the mouth. The composition of the three component initiator system was CQ (0.5 wt%), EDMAB (0.5 wt%) and DPIHP (1.0 wt%) with respect to the total amount of monomer . The resin mixtures were prepared in a brown glass vial. The solutions containing the monomers/photoinitiators were mixed for 48 h at room temperature to promote complete dissolution and formation of a homogeneous solution.

Table 1

Adhesive formulation, degree of conversion, water miscibility and water sorption.

^a Significantly ( p < 0.05) different from the corresponding control.

^b Significantly ( p < 0.05) different from the corresponding E2-SM1.

^c Significantly ( p < 0.05) different from the corresponding E3-SM1.

^d (p < 0.05) different from the corresponding E3-SM2.

2.4

Real-time conversion and maximal polymerization rate

Real-time in situ monitoring of the photopolymerization of the 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 diamond crystal top-plate of an attenuated total reflectance (ATR) accessory (Pike, GladiATR, Pike Technology, Madison, WI) and covered with a mylar film. A 40-s-exposure to the commercial visible-light-polymerization unit (Spectrum ^® , Dentsply, Milford, DE), at an intensity of 550 mW cm ⁻² , was initiated after 50 spectra had been recorded. Real-time IR spectra were recorded continuously for 600 s after light curing began. A time-based spectrum collector (Spectrum TimeBase, Perkin-Elmer) was used for continuous and automatic collection of spectra during polymerization. Three replicates were obtained for each adhesive formulation. To determine degree of conversion, heavy water (deuterium oxide, 99.9%, D ₂ O) (Cambridge Isotope Laboratories, Inc., Andover, MA, USA) was used in this study to reduce interference from the overlapping water peak at 1640 cm ⁻¹ . The change of the band ratio profile (1637 cm ⁻¹ (C C)/1608 cm ⁻¹ (phenyl)) was monitored. DC was calculated using the following equation, which is based on the decrease in the absorption intensity band ratios before and after light curing. The average of the last 50 values of time-based data points is reported as the DC value at 10 min.

The maximal polymerization rate was determined using the maximum slope of the linear region of the DC–time plots .

2.5

Viscosity measurement

Brookfield DV-II + Pro viscometer with a cone/plate set up was used to measure the viscosity of the formulations at varying shear rate 45–75 s ⁻¹ . For each formulation viscosity of three samples was measured at 25.0 ± 0.2 °C. The sample volume was kept constant to 0.5 mL.

2.6

Preparation of adhesive polymer specimens for property analyses

The preparation of polymer specimens for property analyses has been reported . In brief, round beams with a diameter of 1 mm and a length of at least 15 mm were prepared by injecting the adhesive formulations into glass molds (Fiber Optic Center, Inc., part no.: ST8100, New Bedford, MA). Fifteen specimens were prepared for each formulation. The samples were light polymerized with an LED light curing unit for 40 s (LED Curebox, 200 mW/cm ² irradiance, Prototech, and Portland, OR). The polymerized samples were stored in the dark at room temperature for two days to provide adequate time for post-cure polymerization. The samples were subsequently extracted from the glass molds. The resultant beam specimens were used for water sorption, dynamic mechanical analysis and microscale morphologic characterization.

2.7

Water miscibility

Water miscibility is the property of the liquid monomer resin to mix with water, forming a homogeneous solution. In principle, the main focus is usually on the solubility of water in different formulations of the monomer resin. About 0.5 g of each neat resin was weighed into a brown vial, and water was added in increments of approximately 0.005 g until the mixture was visually observed to be turbid. The percentage of water in the mixture was noted ( w ₁ ). The mixture was then back-titrated using the neat resin until the turbidity disappeared, and the percentage of water in the mixture was again noted ( w ₂ ). Three samples were tested for each formulation and water miscibility of the liquid formulation was calculated as the average of w ₁ and w ₂ .

2.8

Water sorption

The water sorption protocol has been reported . In brief, water sorption was measured using cylindrical beam specimens (1 mm × 15 mm). Five specimens were prepared for each adhesive formulation.

Prewash . Each beam specimen was weighed in air ( m _1dry ) using an analytical balance with a resolution of 0.01 mg (Mettler Toledo, XS205 dualRange, Greifensee, Switzerland). The specimens were immersed in deionized water and stored at room temperature. The water was changed daily. After five days of prewash, the polymer specimens were allowed to dry in the vacuum chamber at 37 °C until a constant weight ( m _3dry ) was obtained. The solubility during the prewash experiment was calculated as:

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