Neil B. Cramer

Recent Advances and Developments in Composite Dental Restorative Materials

Composite dental restorations represent a unique class of biomaterials with severe restrictions on biocompatibility, curing behavior, esthetics, and ultimate material properties. These materials are presently limited by shrinkage and polymerization-induced shrinkage stress, limited toughness, the presence of unreacted monomer that remains following the polymerization, and several other factors. Fortunately, these materials have been the focus of a great deal of research in recent years with the goal of improving restoration performance by changing the initiation system, monomers, and fillers and their coupling agents, and by developing novel polymerization strategies. Here, we review the general characteristics of the polymerization reaction and recent approaches that have been taken to improve composite restorative performance.

Investigation of thiol-ene and thiol-ene-methacrylate based resins as dental restorative materials

OBJECTIVES The objective of this work was to evaluate thiol-norbornene and thiol-ene-methacrylate systems as the resin phase of dental restorative materials and demonstrate their superior performance as compared to dimethacrylate materials. METHODS Polymerization kinetics and overall functional group conversions were determined by Fourier transform infrared spectroscopy (FTIR). Flexural strength and modulus were determined with a 3-point flexural test. Polymerization-induced shrinkage stress was measured with a tensometer. RESULTS Thiol-ene polymer systems were demonstrated to exhibit advantageous properties for dental restorative materials in regards to rapid curing kinetics, high conversion, and low shrinkage and stress. However, both the thiol-norbornene and thiol-allyl ether systems studied here exhibit significant reductions in flexural strength and modulus relative to BisGMA/TEGDMA. By utilizing the thiol-ene component as the reactive diluent in dimethacrylate systems, high flexural modulus and strength are achieved while dramatically reducing the polymerization shrinkage stress. The methacrylate-thiol-allyl ether and methacrylate-thiol-norbornene systems both exhibited equivalent flexural modulus (2.1+/-0.1 GPa) and slightly reduced flexural strength (95+/-1 and 101+/-3 MPa, respectively) relative to BisGMA/TEGDMA (flexural modulus; 2.2+0.1 GPa and flexural strength; 112+/-3 MPa). Both the methacrylate-thiol-allyl ether and methacrylate-thiol-norbornene systems exhibited dramatic reductions in shrinkage stress (1.1+/-0.1 and 1.1+/-0.2 MPa, respectively) relative to BisGMA/TEGDMA (2.6+/-0.2 MPa). SIGNIFICANCE The improved polymerization kinetics and overall functional group conversion, coupled with reductions in shrinkage stress while maintaining equivalent flexural modulus, result in a superior overall dental restorative material as compared to traditional bulk dimethacrylate resins.

Properties of methacrylate–thiol–ene formulations as dental restorative materials

OBJECTIVES The objective of this study was to evaluate ternary methacrylate-thiol-ene systems, with varying thiol-ene content and thiol:ene stoichiometry, as dental restorative resin materials. It was hypothesized that an off-stoichiometric thiol-ene component would enhance interactions between the methacrylate and thiol-ene processes to reduce shrinkage stress while maintaining equivalent mechanical properties. METHODS Polymerization kinetics and functional group conversions were determined by Fourier transform infrared spectroscopy (FTIR). Cured resin mechanical properties were evaluated using a three-point flexural test, carried out with a hydraulic universal test system. Polymerization shrinkage stress was measured with a tensometer coupled with simultaneous real-time conversion monitoring. RESULTS The incorporation of thiol-ene mixtures as reactive diluents into conventional dimethacrylate resins previously was shown to combine synergistically advantageous methacrylate mechanical properties with the improved polymerization kinetics and reduced shrinkage stress of thiol-ene systems. In these systems, due to thiol consumption resultant from both the thiol-ene reaction and chain transfer involving the methacrylate polymerization, the optimum thiol:ene stoichiometry deviates from the traditional 1:1 ratio. Increasing the thiol:ene stoichiometry up to 3:1 results in systems with equivalent flexural modulus, 6-20% reduced flexural strength, and 5-33% reduced shrinkage stress relative to 1:1 stoichiometric thiol:ene systems. SIGNIFICANCE Due to their improved overall functional group conversion, and shrinkage stress reduction while maintaining equivalent flexural modulus, methacrylate-thiol-ene resins, particularly those with excess thiol, beyond the conventional 1:1 thiol:ene molar ratio, yield superior dental restorative materials compared with purely dimethacrylate resins.

Thiol–ene–methacrylate composites as dental restorative materials

OBJECTIVES The objective of this study was to evaluate composite methacrylate-thiol-ene formulations with varying thiol:ene stoichiometry relative to composite dimethacrylate control formulations. It was hypothesized that the methacrylate-thiol-ene systems would exhibit superior properties relative to the dimethacrylate control resins and that excess thiol relative to ene would further enhance shrinkage and conversion associated properties. METHODS Polymerization kinetics and functional group conversions were determined by Fourier transform infrared spectroscopy (FTIR). Volume shrinkage was measured with a linometer and shrinkage stress was measured with a tensometer. Flexural modulus and strength, depth of cure, water sorption and solubility tests were all performed according to ISO 4049. RESULTS All of the methacrylate-thiol-ene systems exhibited improvements in methacrylate conversion, flexural strength, shrinkage stress, depth of cure, and water solubility, while maintaining equivalent flexural modulus and water sorption relative to the dimethacrylate control systems. Increasing the thiol to ene stoichiometry resulted in further increased methacrylate functional group conversion and decreased volume shrinkage. Flexural modulus and strength, shrinkage stress, depth of cure, water sorption and solubility did not exhibit statistically significant changes with excess thiol. SIGNIFICANCE Due to their improved overall functional group conversion and reduced water sorption, the methacrylate-thiol-ene formulations are expected to exhibit improved biocompatibility relative to the dimethacrylate control systems. Improvements in flexural strength and reduced shrinkage stress may be expected to result in composite restorations with superior longevity and performance.

The reciprocity law concerning light dose relationships applied to BisGMA/TEGDMA photopolymers: Theoretical analysis and experimental characterization

OBJECTIVES A model BisGMA/TEGDMA unfilled resin was utilized to investigate the effect of varied irradiation intensity on the photopolymerization kinetics and shrinkage stress evolution, as a means for evaluation of the reciprocity relationship. METHODS Functional group conversion was determined by FTIR spectroscopy and polymerization shrinkage stress was obtained by a tensometer. Samples were polymerized with UV light from an EXFO Acticure with 0.1wt% photoinitiator. A one-dimensional kinetic model was utilized to predict the conversion-dose relationship. RESULTS As irradiation intensity increased, conversion decreased at a constant irradiation dose and the overall dose required to achieve full conversion increased. Methacrylate conversion ranged from 64±2% at 3mW/cm(2) to 78±1% at 24mW/cm(2) while the final shrinkage stress varied from 2.4±0.1MPa to 3.0±0.1MPa. The ultimate conversion and shrinkage stress levels achieved were dependent not only upon dose but also the irradiation intensity, in contrast to an idealized reciprocity relationship. A kinetic model was utilized to analyze this behavior and provide theoretical conversion profiles versus irradiation time and dose. SIGNIFICANCE Analysis of the experimental and modeling results demonstrated that the polymerization kinetics do not and should not be expected to follow the reciprocity law behavior. As irradiation intensity is increased, the overall dose required to achieve full conversion also increased. Further, the ultimate conversion and shrinkage stress that are achieved are not dependent only upon dose but rather upon the irradiation intensity and corresponding polymerization rate.

Evaluation of highly reactive mono-methacrylates as reactive diluents for BisGMA-based dental composites.

OBJECTIVES This study evaluates the performance of highly reactive novel monomethacrylates characterized by various secondary moieties as reactive diluent alternatives to TEGDMA in BisGMA filled dental resins. We hypothesize that these monomers improve material properties and kinetics over TEGDMA because of their unique polymerization behavior. METHODS The cure rates and final double bond conversion of the resins were measured using real-time FTIR spectroscopy. The glass transition temperature and storage modulus of the formed polymers were measured using dynamic mechanical analysis. Flexural modulus and flexural strength values were obtained using a three-point bending flexural test. RESULTS Polymerization kinetics and polymer mechanical properties were evaluated for the novel resin composites. It was observed that upon the use of novel monomethacrylates as reactive diluents, polymerization kinetics increased by up to 3-fold accompanied by increases in the extent of cure from 5% to 13% as compared to the BisGMA/TEGDMA control. Polymer composites formed from resins of BisGMA/novel monomethacrylates exhibited comparable T(g) values to the control, along with 27-37% reductions in the glass transition half widths indicating the formation of more homogeneous polymeric networks. The BisGMA/monomethacrylate formulations exhibited equivalent flexural modulus and flexural strength values relative to BisGMA/TEGDMA. SIGNIFICANCE Formulations containing novel monovinyl methacrylates exhibit dramatically increased curing rates while also exhibiting superior or at least comparable composite polymer mechanical properties. Thus, these types of materials are attractive for use as reactive diluent alternatives to TEGDMA in dental formulations.

Using hyperbranched oligomer functionalized glass fillers to reduce shrinkage stress

OBJECTIVE Fillers are widely utilized to enhance the mechanical properties of polymer resins. However, polymerization stress has the potential to increase due to the higher elastic modulus achieved upon filler addition. Here, we demonstrate a hyperbranched oligomer functionalized glass filler UV curable resin composite which is able to reduce the shrinkage stress without sacrificing mechanical properties. METHODS A 16-functional alkene-terminated hyperbranched oligomer is synthesized by thiol-acrylate and thiol-yne reactions and the product structure is analyzed by (1)H NMR, mass spectroscopy, and gel permeation chromatography. Surface functionalization of the glass filler is measured by thermogravimetric analysis. Reaction kinetics, mechanical properties and shrinkage stress are studied via Fourier transform infrared spectroscopy, dynamic mechanical analysis and a tensometer, respectively. RESULTS Silica nanoparticles are functionalized with a flexible 16-functional alkene-terminated hyperbranched oligomer which is synthesized by multistage thiol-ene/yne reactions. 93% of the particle surface was covered by this oligomer and an interfacial layer ranging from 0.7 nm to 4.5 nm thickness is generated. A composite system with these functionalized silica nanoparticles incorporated into the thiol-yne-methacrylate resin demonstrates 30% reduction of shrinkage stress (from 0.9 MPa to 0.6 MPa) without sacrificing the modulus (3100 ± 300 MPa) or glass transition temperature (62 ± 3°C). Moreover, the shrinkage stress of the composite system builds up at much later stages of the polymerization as compared to the control system. SIGNIFICANCE Due to the capability of reducing shrinkage stress without sacrificing mechanical properties, this composite system will be a great candidate for dental composite applications.

Mechanistic Modelling and Network Properties of Ternary Thiol–Vinyl Photopolymerizations

Ternary thiol–vinyl polymerizations offer a unique platform for improved control over polymerization kinetics and network properties as compared to both binary thiol–vinyl systems and traditional (meth)acrylic systems. Therefore, this study seeks to improve the fundamental understanding of the complex ternary thiol–vinyl systems to enable enhanced control over polymerization kinetics, network evolution, and, ultimately, network properties. The polymerization kinetics and material properties afforded by thiol–triazine–methacrylate systems are investigated. The ternary kinetics are successfully predicted by understanding the reaction mechanisms of the corresponding binary components. In ternary thiol–ene–(meth)acrylate systems, the variation in stoichiometric ratios of thiol and ene does not significantly impact material properties as in thiol–ene- or thiol–(meth)acrylate systems. Further, the ternary systems also provide unique polymer properties such as high glass transition temperature with narrow transition widths.

Monolithic integration of optical waveguide and fluidic channel structures in a thiol-ene/methacrylate photopolymer

We present a thiol-ene/methacrylate-based photopolymer capable of creating coplanar physical features (e.g. micro-fluidic channels) and optical index features (e.g. waveguides) using standard mask-based lithography techniques. This new photopolymer consists of two monomer species that polymerize at different rates. By selectively exposing different areas of a device for various amounts of time, we can select the state of the polymer (i.e. liquid, rubbery, or glassy) to create fluid channels or optical index structures such as waveguides. Using only three exposure steps and two masks, we demonstrate an integrated refractometer with a 90° channel-waveguide crossing to illustrate the fabrication process and the ability to create lithographically aligned waveguides across a gap.

Formation of a host nanostructure for ferroelectric liquid crystals using thiol-ene polymers

Liquid crystal displays are a subject of intense research interest because of their application to high definition display devices. Recently, polymer stabilized ferroelectric liquid crystals (PSFLCs) have been investigated due to the enhanced electro-optic properties of FLCs. We have utilized thiol-ene photopolymerizations to form a PSFLC system. Thiol-ene photopolymerizations are radical reactions, which proceed via a step growth reaction mechanism. During the polymerization, the polymer network structure is trapped into place due to the rapid transition from low molecular mass monomers and oligomers to high molecular mass polymer. This aspect is evidenced by phase transition data for the FLC, which indicates that the monomer and polymer are not phase separated from the FLC. Infrared dichroism shows that both monomer and polymer are ordered in parallel with the smectic layers of the FLC. Small angle X-ray scattering (SAXS) data show that both monomer and polymer are swelling the smectic layers. Thus, a polymer nanostructure is produced that serves as an ordered, stabilizing host for the FLC.