Study and evaluation of fracture toughness, flexural and dynamic mechanical properties, and crosslink density of ternary thiol–ene–methacrylate systems and comparison with corresponding conventional methacrylate system were considered in the present study.
Urethane tetra allyl ether monomer (UTAE) was synthesized as ene monomer. Different formulations were prepared based on combination of UTAE, BisGMA/TEGDMA and a tetrathiol monomer (PETMP). The photocuring reaction was conducted under visible light using BD/CQ combination as photoinitiator system. Mechanical properties were evaluated via measuring flexural strength, flexural modulus and fracture toughness. Scanning electron microscopy (SEM) was utilized to study the morphology of the fractured specimen’s cross section. Viscoelastic properties of the samples were also determined by dynamic mechanical thermal analysis (DMTA). The same study was performed on a conventional methacrylate system. The data were analyzed and compared by ANOVA and Tukey HSD tests (significance level = 0.05).
The results showed improvement in fracture toughness of the specimens containing thiol–ene moieties. DMTA revealed a lower glass transition temperature and more homogenous structure for thiol–ene containing specimens in comparison to the system containing merely methacrylate monomer. The flexural modulus and flexural strength of the specimens with higher thiol–ene content were lower than the neat methacrylate system. The SEM micrographs of the fractured surface of specimens with higher methacrylate content were smooth and mirror-like (shiny) which represent brittle fracture.
The thiol–ene–methacrylate system can be used as resin matrix of dental composites with enhanced fracture toughness in comparison to the methacrylate analogous.
Resin-based composites are currently utilized to overcome the shortcomings which esthetically associated with dental amalgam, although their longevity is still inferior to that of the amalgams . The methacrylate-based restorative materials suffer from some drawbacks e.g. low degree of conversion, oxygen inhibition , shrinkage stress , and shrinkage strain during their photochemical polymerization. Polymerization shrinkage is the result of conversion of intermolecular van der Waals distances of the resin monomers to the covalent bond lengths during light curing . Since the restorative materials are bonded to the tooth cavity walls, shrinkage occurs under confinement results in shrinkage and residual stress . Shrinkage stress and strain cause a failure and gap formation between the composite and tooth structure that would be the place for bacterial activity and secondary decay which result in composite failure .
Failures of the resin-based composite restorations have been attributed to marginal, surface and/or bulk cracks due to the shrinkage stress and strain, trimming and polishing, degradation of matrix and fillers, masticatory loads, and water uptake of composites . In this regards improvement in fracture toughness which describes the ability of material to resist brittle fracture would be expected to increase the dental materials’ longevity and performance .
Thiol–ene photopolymerization that proceed via a step-growth mechanism, in which propagation and chain transfer alternate mechanism, has been introduced for circumventing the conventional methacrylate system problems . The thiol–ene systems are rapidly photopolymerized to high functional group conversion without oxygen inhibition . The step growths addition mechanism results in lower stress shrinkage and uniform polymer networks with narrow glass transition regions . In thiol–ene systems gelation occurs later as compared with conventional methacrylate systems. Since the shrinkage stress is developed mainly before gelation point, thiol–ene systems exhibit significant reduction in shrinkage stress due to the delayed gelation . The systems, however, suffer from the lack of adequate mechanical properties for dental applications . Combination of the thiol–ene systems with methacrylates provides the advantages of both systems. Methacrylate–thiol–ene systems are demonstrated to exhibit cure time and mechanical properties equivalent to those of BisGMA/TEGDMA, while achieving increased levels of conversion and exhibiting dramatic reductions in shrinkage stress and strain .
To our knowledge, in few studies the fracture toughness of binary thiol/ene system has been reported . There is no detailed investigation on parameters may affect the fracture toughness in ternary thiol–ene–methacrylate system as the resin phase of dental restorative composites.
Hence, in this study a urethane-based allyl ether monomer (UTAE), as ene monomer, was synthesized and combined with a conventional methacrylate system (BisGMA/TEGDMA), as well as a tetrathiol monomer. The fracture toughness, flexural, and dynamic mechanical properties of the thiol–UTAE–methacrylate system were then studied and the results were interpreted based on chemical composition of the ternary system.
Isophorone diisocyanate (IPDI), and trimethylol propane diallyl ether (DAE), dibutyltin dilaurate (DBTDL), as catalyst, and camphorquinone (CQ) were purchased from Merck (Germany). Tetrahydrofuran (THF) was purchased from Merck (Germany) and dried via distillation over sodium wire. N,N-Dimethyl para-toluidine was obtained from Fluka (Germany). 2,2-Bis-(2-hydroxy-3-methacryloxypropoxy) phenyl] propane (Bis-GMA) and triethyleneglycol dimethacrylate (TEGDMA) were kindly donated by Evonik (Germany). 2, 3-Butanedione (BD) and pentaerythritol tetra (3-mercaptopropionate) (PETMP) were supplied by Aldrich (Germany).
Synthesis and characterization of UTAE monomer
Urethane tetra allyl ether monomer (UTAE) was synthesized by condensation reaction of isophorone diisocyanate (IPDI) and trimethylolpropane diallyl ether (DAE) in stoichiometric ratio of isocyanate and hydroxyl functional groups according to the procedure reported in .
Briefly, under mild stream of demoisturized N 2 gas, DAE (23.33 g, 0.108 mol), dibuthyltin dilaurate (0.1 wt.% of total weight of monomers) and dried THF were placed into a100 ml three-necked round-bottomed flask equipped with a condenser, a dropping funnel, oil bath and magnetic stirrer. IPDI (10.89 g, 0.049 mol) was diluted in THF and dropped into the flask during 30 min. The reaction temperature was adjusted to 55 °C and the reaction continued until no NCO group peak at 2270 cm −1 was detected in the FTIR spectra of the samples collected from the reactor. The product as a viscous liquid was obtained after removing the solvent using a rotary evaporator under reduced pressure. The chemical structure of UTAE was evaluated by spectroscopic analysis. 1 H NMR spectrum of the synthesized monomer was recorded in deuterated DMSO as solvent on an AVANCE 400 MHz spectrometer (Bruker, Germany) and FTIR spectrum was evaluated by an Equinox 55, instrument (Bruker, Germany).
Flexural strength of the samples was measured according to the 3-point bending method carried out with a universal test machine (STM-20, Santam, Iran) at a cross-head speed of 1 mm min −1 . The bar specimens were prepared in dimensions of 2 mm × 2 mm × 25 mm according to ISO 4049 . The specimens were irradiated for 200 s on both sides with a light-curing unit (Optilux 501, Kerr, USA) at an intensity of ca. 550 mW cm −2 . The specimens were stored in distilled water at 37 °C for 24 h prior to testing. The flexural strength (FS) in MPa was then calculated as:
where p stands for load at fracture ( N ), L is the span length (20 mm), and b and d are the width and thickness of the specimens in mm, respectively. The elastic modulus was also determined from the slope of the initial linear part of stress–strain curve.
The energy of fracture, U (ε) , was also calculated as the area under stress–strain curve:
where σ and ε denote stress and strain, respectively, and ε b is the strain at break.
Dynamic mechanical analysis
Viscoelastic properties of the specimens were determined by dynamic mechanical analysis (DMA) using a UK Polymer Lab model MK-II analyzer in bending mode over the temperature range from −100 to 200 °C at a heating rate of 5 °C min −1 and frequency of 1 Hz (also 0.1 and 10 Hz for determining activation energy). The loss and storage modulus, and loss tangent were recorded as a function of temperature. Glass transition temperature ( T g ) was taken of the maximum of loss tangent curve. The crosslink density ( ν c ) of cured resins was calculated from DMTA data using rubber elasticity theory according to the following equation :
where E is storage modulus at rubbery plateau region, and R and T are gas constant and absolute temperature, respectively.
T g1/2width was also measured from the half peak width of tan δ curves.
Fracture toughness of a material is determined from the stress intensity factor ( K ) during crack propagation. To determine the fracture toughness (FT), 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 bending fracture test was performed at a cross-head speed of 0.1 mm min −1 using the universal test machine and the fracture toughness (critical stress intensity factor, 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