[show abstract][hide abstract] ABSTRACT: The goal of this work was to investigate the feasibility of formulating novel dental restorative materials that utilize a step-growth thiol-ene photopolymerization. Particularly, we are aiming to significantly reduce the polymerization shrinkage and shrinkage stress while retaining adequate physical properties as compared to current dimethacrylatre-based systems.
The thiol-ene system is composed of a 4:3 molar mixture of triallyl-1,3,5-triazine-2,4,6-trione (TATATO) and pentaerythritol tetramercaptopropionate (PETMP). The simultaneous measurement of shrinkage stress and functional group conversion was performed. Solvent extraction of unreacted monomers and dynamic mechanical analysis on the polymer networks that were formed were also studied. Flexural strength was measured for both filled and unfilled PETMP/TATATO and Bis-GMA/TEGDMA systems.
Photopolymerization of PETMP/TATATO occurs at a much higher rate, with the maximum polymerization rate six times faster, than Bis-GMA/TEGDMA cured under the identical conditions. The results from the simultaneous measurement of shrinkage stress and conversion showed that the onset of shrinkage stress coincides with the delayed gel point conversion, which is predicted to be 41% for the 3:4 stoichiometric PETMP/TATATO resin composition. The maximum shrinkage stress developed for PETMP/TATATO was about 0.4 MPa, which was only approximately 14% of the maximum shrinkage stress of the Bis-GMA/TEGDMA system. Adequate flexural strength and flexural modulus values were obtained for both filled and unfilled PETMP/TATATO systems.
The dramatically reduced shrinkage stress, increased polymerization rate, significance increased functional group conversion, and decreased leachable species are all benefits for the use-of thiol-ene systems as potential dental restorative materials.
[show abstract][hide abstract] ABSTRACT: Although various photo-curing techniques aiming to partially relieve the shrinkage stress by flow have been proposed and investigated, the direct monitoring and observation of stress relaxation behavior received little attention. In this study, the elucidation of stress relaxation behavior and its impact on the overall stress development were investigated.
A novel experimental set-up capable of real-time, simultaneous shrinkage stress and double bond conversion measurements on the same sample was utilized. The coupled shrinkage stress and conversion for specimens of a barium glass filled Bis-GMA/TEGDMA composite were continuously monitored during and after irradiation for various intervals (2, 3, 6, 10 and 60s).
Specimens irradiated for 60s reached the highest conversion (67.9+/-1.2%) and shrinkage stress (2.9+/-0.1 MPa). Shrinkage stress relaxation phenomena were only observed prior to vitrification, which only applied to specimens partially cured for 2 or 3 s; specimens irradiated for 2 s exhibit the largest capability to relieve stress (approximately 40%). However, to achieve this a much longer post-cure stress relaxation time is required as compared to clinically practical time scales. For specimens irradiated for 6, 10 or 60 s, the majority of the shrinkage stress is developed during and after the vitrification stage, and no appreciable stress relaxation can be observed.
Although stress relaxation prior to vitrification stage has been observed, this study revealed that it did not provide a significant benefit towards the reduction of overall shrinkage stress since, to achieve clinically relevant conversion, the majority of the shrinkage stress is developed during and after the vitrification stage, which does not permit stress relaxation on the time scales used in this study.
[show abstract][hide abstract] ABSTRACT: The aims of the study were to synthesize derivatives of Bis-GMA having pendant n-alkyl urethane substituents and to characterize and evaluate their physicochemical properties.
Stoichiometric amounts of Bis-GMA and n-alkyl isocyanates were reacted in dichloromethane with dibutyltin dilaurate as a catalyst. Volumetric shrinkage, water uptake, degree of vinyl conversion, refractive index and viscosity of resulting urethane monomers and those of Bis-GMA were measured. The flexural strengths of their corresponding homopolymers and that of Bis-GMA were also measured.
These types of urethane derivatives of Bis-GMA exhibited lower viscosities and were more hydrophobic than Bis-GMA. Generally, the viscosity of these experimental monomers decreased with increasing chain length of the alkyl urethane substituent. Photopolymerization of the new monomers gave high degrees of vinyl conversion compared to Bis-GMA. The experimental monomers also yielded polymers with lower polymerization shrinkages at equivalent degrees of vinyl conversion, than Bis-GMA. The refractive indices of these urethane derivatives were similar to Bis-GMA, but the flexural strengths of their polymers were lower than that of the Bis-GMA homopolymer, decreasing with increasing chain length of the alkyl urethane substituent.
Because of their excellent overall properties, these new derivatives of Bis-GMA have potential as dental monomers that can improve many properties of resin based dental materials that utilize methacrylate monomer systems.
[show abstract][hide abstract] ABSTRACT: Because of its relatively high solubility in aqueous media and its rapid transformation to hydroxyapatite, amorphous calcium phosphate (ACP) has been utilized as the filler phase of resin-based bioactive composites that have remineralization potential. The objectives of this study were to determine how various methacrylate resins and various types of ACP fillers affect acrylic vinyl conversion and polymerization shrinkage (PS). Several types of photo-crosslinkable resin systems were prepared and admixed with a mass fraction of 40% of either unhybridized, silica- or zirconia-hybridized ACP. After visible light-activated photo-polymerization ACP composites were assessed by near infrared spectroscopy for degree of vinyl conversion and by mercury dilatometry for PS. It was found for these composites that vinyl conversion was independent of filler type but strongly dependent on the type and composition of the resin phase. PS, on the other hand, showed more complex dependence both on the resin type and composition and, in some cases, on the type of ACP. In order to obtain ACP/methacrylate-based composites with maximal vinyl conversion, resin type and composition are of primary importance. However, in order to minimize volume contraction on polymerization it appears necessary to consider both the resin and filler type of these bioactive composites.
[show abstract][hide abstract] ABSTRACT: The properties of dental matrix resins have been improved by synthesis of new monomers. However, except for improvements in water-resistance, monomers with better mechanical properties than Bis-GMA and UDMA could not being synthesized. Changing the point of emphasis, we tried to improve the mechanical properties controlling the matrix resin higher structure using noncovalent bonds. We prepared a matrix resin structured by UDMA, which is a high viscosity base monomer with imino groups, and by a low viscosity acidic monomer with carboxyl groups, which permits noncovalent bonds such as hydrogen bonds or electrostatic interaction with imino groups. The maximal mechanical strength for matrix resins structured by UDMA and an acidic monomer was obtained with a composition of imino groups and carboxyl groups at a ratio of 1:1. This mechanical strength value was higher than those obtained with UDMA resin or with a Bis-GMA/TEGDMA/UDMA resin with typical composition. The improvement in mechanical properties may be due to the complex based on noncovalent bonds, between the imino groups of UDMA and the carboxyl groups of the acidic monomers.
[show abstract][hide abstract] ABSTRACT: There are many unique properties associated with fluorinated polymers that make these materials attractive for use in the challenging oral environment. This study was devised to better define the influence of fluorine content and its structural distribution on properties of fluorinated resins and composites, especially with regard to their water-related and mechanical properties.
A series of fluorinated dimethacrylate monomers was prepared by reaction of aromatic diepoxides with fluoroalcohols and subsequent conversion of the resulting diols to the methacrylates. Composites based on monomer systems comprised of the fluorinated monomers with 1,10-decamethylene dimethacrylate and reinforced with silanized quartz filler were evaluated for conversion, water contact angle, water sorption and diametral tensile strength.
By selection of reactants, fluorine was introduced as trifluoromethyl groups, extended fluoroalkyl pendant chains, or combinations of the two. Photopolymerization conversion among the experimental composites was generally equal to or greater than that of a conventional Bis-GMA/TEGDMA composite. While the water contact angles generally increased with fluorine content, no correlation was obtained between fluorine content and water sorption of the composites. The mechanical strength of the fluorinated composites showed a general decline with increasing fluorine content and consistent variations due to specific structural features.
A versatile route to fluorinated dimethacrylates with diverse structural and fluorine distribution patterns is presented. Composites from these monomers are very hydrophobic but have relatively low mechanical strength. The monomers described can be considered as useful additives to moderate the water sorption of conventional resins. However, the results of this study point to specific fluorinated resin structures that are expected to provide a more optimal balance between hydrophobicity and mechanical strength that will improve the long-term performance of dental composites.
[show abstract][hide abstract] ABSTRACT: alpha-Methylene-gamma-butyrolactone (MBL), which can be described as the cyclic analog of methyl methacrylate, exhibits greater reactivity in free radical polymerizations than conventional methacrylate monomers. Unfilled resin formulations composed of Bis-GMA/MBL or Bis-GMA/TEGDMA/MBL were light-cured. The effect of the more reactive methylene lactone monomer on mechanical properties and the degree of conversion of the polymers was examined. The infrared absorption bands for the carbon-carbon double bonds of MBL and the methacrylate monomers are well resolved and allow the conversion of each component to be calculated individually. The incorporation of a small amount of MBL (5 w/o) to Bis-GMA significantly increased the conversion; however, additional MBL (10 to 30 w/o) did not further increase the Bis-GMA conversion level. This appears to indicate an incompatibility between MBL and the bulky Bis-GMA monomer. Addition of 10 w/o MBL to Bis-GMA/TEGDMA (7:3) resulted in a cured resin with 71% methacrylate and 75% overall conversion efficiencies compared with the 57% conversion of the control formulation. The diametral tensile and the transverse strengths were approximately 10% greater for the MBL resin compared with the Bis-GMA/TEGDMA control; however, these differences were not statistically significant. The synthesis and polymerization of several substituted methylene lactones was also studied.
[show abstract][hide abstract] ABSTRACT: Polymerization with expansion in volume can be achieved with spiro orthocarbonate monomers through a double ring-opening process wherein two bonds are cleaved for each new bond formed. The resulting expansion can be applied to counter the polymerization shrinkage associated with the conventional methacrylate monomers used in dental composites and thereby provide formulations with drastically reduced degrees of shrinkage. New monomers have been prepared that exhibit enhanced reactivities and ring-opening efficiencies compared with earlier free-radical-polymerizable oxaspiro compounds. In dental composite formulations, the monofunctional oxaspiro monomers provided DTS values equivalent to those of the controls under certain curing conditions; however, only modest reductions in polymerization shrinkage were observed. 2,3-Bis(methylene) spiro orthocarbonate monomers with a conjugated diene structure were also synthesized and evaluated. These novel monomers appear to offer significant potential for future development of free-radical ring-opening polymerization. While visible-light-cured formulations of the bis(methylene) compounds with methacrylate comonomers did not yield acceptable composite materials in this initial attempt, the high reactivity and the ability to form rigid, cross-linked polymers make this type of monomer worthy of continued investigation. These properties may allow the bis(methylene) oxaspiro monomers to be used alone or in concert with other ring-opening monomers for special applications.
Journal of Dental Research 08/1992; 71(7):1408-12. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: A new type of multifunctional oligomer was synthesized, and its potential as a base monomer in dental composite formulations was evaluated. The oligomer of ethoxylated bis-phenol A diacrylate (OEBPA) was prepared in good yield by a formaldehyde insertion/condensation reaction. Although double bonds along the oligomer backbone are arranged in pairs such that cyclopolymerization is possible, it is not presently known whether this process plays a significant role in the polymerization. Indirect evidence supporting efficient cyclopolymerization involves the reduced polymerization shrinkage observed for polymerized OEBPA relative to polymers of other monomers used as base resins. Photo-cured composites containing either OEBPA, BIS-GMA, or ethoxylated bis-phenol A dimethacrylate (EBPADM) as base resin and TEGDMA as diluent were compared. While the resulting diametral tensile and transverse strengths did not differ significantly, the values for the energy absorbed to failure indicated that the OEBPA- and EBPADM-based formulations yielded composites with somewhat greater toughness than that of the BIS-GMA material. This multifunctional oligomer offers mechanical strength and conversion values that are comparable with those of existing base resin monomers while providing an approximate 30% reduction in polymerization shrinkage.
Journal of Dental Research 04/1992; 71(3):434-7. · 3.83 Impact Factor