Increase in degree of conversion and reduction in <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='Rpmax’>RpmaxRpmax
R p max
Delay in gelation and reduction in polymerization stress.
Increase in mechanical properties.
More homogenous networks.
Thio-urethane oligomers have been shown to reduce stress and increase toughness in highly filled composite materials. This study evaluated the influence of thio-urethane backbone structure on rheological and mechanical properties of resin cements modified with a fixed concentration of the oligomers.
Thio-urethane oligomers (TU) were synthesized by combining thiols – pentaerythritol tetra-3-mercaptopropionate (PETMP) or trimethylol-tris-3-mercaptopropionate (TMP) – with isocyanates – 1,6-hexanediol-diissocyante (HDDI) (aliphatic) or 1,3-bis(1-isocyanato-1-methylethyl)benzene (BDI) (aromatic) or dicyclohexylmethane 4,4′-diisocyanate (HMDI) (cyclic), at 1:2 isocyanate:thiol, leaving pendant thiols. 20 wt% TU were added to BisGMA-UDMA-TEGDMA (5:3:2). 60 wt% silanated inorganic fillers were added. Near-IR was used to follow methacrylate conversion and rate of polymerization ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='Rpmax’>RpmaxRpmax
R p max
). Mechanical properties were evaluated in three-point bending (ISO 4049) for flexural strength/modulus (FS/FM, and toughness), and notched specimens (ASTM Standard E399-90) for fracture toughness ( K IC ). PS was measured on the Bioman. Viscosity ( V ) and gel-points (defined as the crossover between storage and loss shear moduli ( G ′/ G ″)) were obtained with rheometry. Glass transition temperature ( T g ), cross-link density and homogeneity of the network were obtained with dynamic mechanical analysis. Film-thickness was evaluated according to ISO 4049.
DC and mechanical properties increased and <SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='Rpmax’>RpmaxRpmax
R p max
and PS decreased with the addition of TUs. Gelation ( G ′/ G ″) was delayed and DC at G ′/ G ″ increased in TU groups. T g and cross-link density dropped in TU groups, while oligomers let to more homogenous networks. An increase in V was observed, with no effect on film-thickness. Significant reductions in PS were achieved at the same time conversion and mechanical properties increased.
The addition of thio-urethane oligomers proved successful in improving several key properties of resin cements, without disrupting the procedures dentists use to polymerize the material. This approach has potential to be translated to commercial materials very readily.
Composite cements are extensively used in adhesive dentistry because of their ability to chemically bond to the restorative material and, in some cases, also to the tooth structure. They are very useful in a plethora of clinical applications, such as veneers, onlays, inlays, crowns, fixed partial bridges and intra-canal posts . The adhesive bonding of materials with composite cements promotes higher bond strength and lower solubility in the oral environment than other luting agents. Higher mechanical properties of composite cements are also important to extend the stability of the bonding interface constantly subjected to tensile, compressive and oblique loads, as well as to strengthen the restorative material .
The vast majority of contemporary composite cements is methacrylate-based, which achieve vitrification at very early stages of polymerization. This, allied with the volumetric shrinkage intrinsic to the polymerization process, leads to the development of polymerization stress in the bulk of the material and bonding interfaces. Current commercially available composite cements have shown volumetric shrinkage ranging from 1.7 to 5.3% .
For years, studies have focused on increasing the mechanical properties and reducing the polymerization stress of methacrylate-based materials. Oligomers based on thiol-ene “click-reactions” have been suggested to increase degree of conversion, mechanical properties and homogeneity of methacrylate networks . Previous work conducted in our group has presented encouraging proof of concept results in regards to the use of thio-urethane oligomers to improve properties of composite cements, with increase in degree of conversion, general mechanical properties and reduction in shrinkage and stress of polymerization . The general mechanism for improved conversion and decreased polymerization stress stems from the chain-transfer reactions of the thiol functionalities pending from the thio-urethane oligomer, which delay gelation, as previously described for small molecule thiols . The increase in toughness and fracture toughness is a well-known advantage of the thio-urethane bond . One additional advantage of using these oligomers in dental materials applications is that the inclusion of the thio-urethanes does not require any change in the basic curing chemistry or clinical procedures the dentist uses to polymerize the materials.
Therefore, the objectives of this study were to expand on previously reported work and synthesize thiol-terminated thio-urethane oligomers with different backbone structures and to assess the properties of methacrylate-based composite cements modified with such oligomers. The oligomer concentration was kept at 20 wt% as this was previously determined to be the best compromise between stress reduction and improvement in mechanical properties. The hypotheses of this study were that the use of thio-urethanes would improve general mechanical properties, increase the degree of conversion, and reduce polymerization stress of composite cements.
Materials and methods
The experimental composite cement formulated for the study (BUT) was composed of Bis-phenol A diglycidyl dimethacrylate (Bis-GMA; Esstech, Essington, PA, USA), urethane dimethacrylate (UDMA; Esstech) and tri-ethylene glycol dimethacrylate (TEGDMA; Esstech) in a 50:30:20 mass ratio. Photoinitiators were added to the monomers as follows: 0.6 wt% of a tertiary amine (EDMAB – ethyl 4-dimethylaminobenzoate; Avocado, Heysham, England), 0.2 wt% of dl -camphoroquinone (Polysciences Inc., Warrington, PA, USA), and 0.5 wt% inhibitor (BHT – 2,6-di-tert-butyl-4-methylphenol; Sigma–Aldrich, St. Louis, MO, USA).
Six oligomers were synthesized in solution in the presence of catalytic amounts of triethylamine. Multi-functional thiols – pentaerythritol tetra-3-mercaptopropionate (PETMP) or trimethylol-tris-3-mercaptopropionate (TMP) – were combined with di-functional isocyanates – 1,6-hexanediol-diissocyante (HDDI) (aliphatic) or 1,3-bis(1-isocyanato-1-methylethyl)benzene (BDI) (aromatic) or dicyclohexylmethane 4,4′-diisocyanate (HMDI) (cyclic) in 4 × volume of dichloromethane (very diluted solution). In addition, the isocyanate:thiol ratio was kept at 1:2 (by mol) to avoid gelation of the oligomer during reaction, according to the Flory–Stockmayer theory , leaving pendant thiols. Oligomers were purified by precipitation in hexanes and rotaevaporation, and then characterized by 1 H NMR and mid-IR spectroscopy . The disappearance of the isocyanate peak at 2270 cm −1 and the appearance of resonance signals at 3.70 ppm were used as evidence for completion of isocyanate reaction and thio-urethane bond formation, respectively . The thiol group (SH) concentration for each oligomer was determined using a titration method with Ellman’s reagent well established in the literature . Thio-urethane oligomers were added to the methacrylate organic phase in proportion of 20 wt%, as defined in our previous investigation . The final oligomer product presented as a viscous liquid at room temperature, and was completely miscible with methacrylate monomers.
Filler was introduced at 60 wt% (barium glass 0.7 μm, density 3.0 g/ml, refractive index 1.553 – V117 4107, Esstech), with the aid of a mechanical mixer (DAC 150 Speed Mixer, Flacktek, Landrum, SC, USA) for 5 min at 2400 rpm. All procedures were carried out under yellow lights.
Photopolymerization reaction kinetics and degree of conversion
The degree of conversion (DC) was obtained using near-infrared (NIR) spectroscopy in specimens of 10 mm in diameter and 0.8 mm thick laminated between two glass slides. The methacrylate CH 2 absorption at 6165 cm −1 was recorded before and after 60 s of irradiation at 700 mW/cm 2 (Bluephase, Ivoclar Vivadent, Liechtenstein) with the light source in direct contact with the glass slide mold. Real-time monitoring of the polymerization kinetics was carried out in specimens of the same size at 2 scans per spectrum with 4 cm −1 resolution, which provides a greater than 2 Hz data acquisition rate. Kinetic data was collected continuously for 5 min. Samples ( n = 5) were irradiated for 60 s at an incident irradiance of 550 mW/cm 2 . The light attenuation in this case was due to a distance of 2 cm separating the tip of the light guide and the surface of the specimen.
Flexural strength, elastic modulus and toughness
The 3-point bending method was used to access the flexural strength. Tests were carried out with a universal test machine (Q-test, MTS, Eden Prairie, WI) at a cross-head speed of 0.5 mm min −1 . The bar specimens were prepared in dimensions of 2 mm × 2 mm × 25 mm according to ISO 4049 . The specimens ( n = 10) were fabricated between glass slides and photopolymerized with three overlapping 60 s exposures at 700 mW/cm 2 . Specimens were stored dry for one week in dark containers at room temperature. The flexural strength (FS) in MPa was then calculated as:
FS ( σ ) = 3 F l 2 b h 2
where F stands for load at fracture ( N ), l is the span length (20 mm), and b and h are the width and thickness of the specimens in mm, respectively.
The elastic modulus was determined from the slope of the initial linear part of stress–strain curve.
E = F l 3 4 b h 3 d
where F is the load at some point on the linear region of the stress–strain curve, d the slack compensated deflection at load F , and l , b , and h are as defined above.
Toughness was calculated in MPa from the integration of the stress × strain curve using software (Origin 9.1, OriginLab Corporation, Northampton, MA, USA).
The Bioman apparatus was used to access the polymerization stress in real time. This system consists of a cantilever load cell whose extremity contains a 5-mm diameter, 0.5-mm tall steel rod. The surface of the rod was treated with a thin layer of Metal primer (Z-prime plus, Bisco, Schaumburg, IL, USA). The opposite surface was a rigid fused silica glass plate of 3 mm thickness, treated with a thin layer of silane ceramic primer (3M ESPE, St. Paul, MN, USA). The cement ( n = 5) was then inserted into the 0.5-mm gap between the upper rod and the lower glass slide and shaped into a cylinder, providing a C-factor of 4. The specimens were photoactivated through the glass during 60 s at an incident irradiance of 700 mW/cm 2 (Bluephase 2) and the stress followed for 500 s. The load signal from the cantilever cell was amplified and acquired by a computer.
The method utilized to determine fracture toughness ( K IC ) was based on the evaluation of pre-cracked specimens under fatigue in linear-elastic, plane-strain conditions . Single-edge notch beam (SENB) specimens were fabricated according to ASTM Standard E399-90 in a 5 mm × 2 mm × 25 mm split steel mold with a razor blade providing a 2.5 mm notch in the middle of the specimens. The cement was photoactivated during 60 s at an incidence of 700 mW/cm 2 . The bending fracture test was performed using a universal test machine (Q-test) at a cross-head speed of 0.5 mm min −1 and K IC was calculated according the following equation:
K IC = 3 P L 2 B W 3 / 2 1.93 a W 1 / 2 − 3.07 a W 3 / 2 + 14.53 a W 5 / 2 − 25.11 a W 7 / 2 + 25.8 a W 9 / 2