Jeffrey W. Stansbury

Determination of double bond conversion in dental resins by near infrared spectroscopy

OBJECTIVES This study determined the validity and practicality of near infrared (NIR) spectroscopic techniques for measurement of conversion in dental resins. METHODS Conversion measurements by NIR and mid-IR were compared using two techniques: (1) The conversion of 3mm thick photopolymerized Bis-GMA/TEGDMA resin specimens was determined by transmission NIR. Specimens were then ground and reanalyzed in KBr pellet form by mid-IR. (2) As further verification, thin resin films were photocured and analyzed by mid-IR. Multiple thin films were then compressed into a thick pellet for examination by NIR. RESULTS Conversion values obtained by NIR and mid-IR techniques did not differ significantly. A correction for changing specimen thickness due to polymerization shrinkage was applied to NIR conversion measurements since an internal standard reference peak was not employed. Sensitivity of the NIR technique was superior to those based on the mid-IR. SIGNIFICANCE The nondestructive analysis of conversion in dental resins by NIR offers advantages of convenience, practical specimen dimensions and precision compared with standard mid-IR analytical procedures. Because glass is virtually transparent in the NIR spectrum, this technique has excellent potential for use with filled dental resins as well.

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.

The effect of cure rate on the mechanical properties of dental resins

OBJECTIVE This study investigates the effect of cure rate on the mechanical properties of a common dimethacrylate dental resin formulation (75/25 wt% bis-GMA/TEGDMA). METHODS The polymerization rate and final conversion of the exact specimens subsequently used for mechanical testing were monitored by near-infrared (near-IR) spectroscopy. The glass transition temperature (T(g)) and modulus, as a function of temperature, were determined by dynamic mechanical analysis (DMA). Iniferter initiating systems were used to create partially cured networks that did not contain any trapped radicals. By the elimination of trapped radicals from the system, the formed networks can be characterized as a function of both temperature and double bond conversion without inducing additional thermal cure during testing. RESULTS Copolymer specimens were cured with UV and visible light initiating systems, UV light intensities that varied by over four orders of magnitude, and cure temperatures that differed by 60 degrees C. Even though the polymerization rates for these resins were vastly different, similar T(g) and modulus were measured for specimens cured to the same final double bond conversion. SIGNIFICANCE This study shows that highly cross-linked dimethacrylate systems, such as bis-GMA/TEGDMA, exhibit similar network structure and properties as a function of double bond conversion, regardless of the method or rate of cure.

3D printing with polymers: Challenges among expanding options and opportunities.

OBJECTIVES Additive manufacturing, which is more colloquially referred to as 3D printing, is quickly approaching mainstream adoption as a highly flexible processing technique that can be applied to plastic, metal, ceramic, concrete and other building materials. However, taking advantage of the tremendous versatility associated with in situ photopolymerization as well as the ability to select from a variety of preformed processible polymers, 3D printing predominantly targets the production of polymeric parts and models. The goal of this review is to connect the various additive manufacturing techniques with the monomeric and polymeric materials they use while highlighting emerging material-based developments. METHODS Modern additive manufacturing technology was introduced approximately three decades ago but this review compiles recent peer-reviewed literature reports to demonstrate the evolution underway with respect to the various building techniques that differ significantly in approach as well as the new variations in polymer-based materials being employed. RESULTS Recent growth of 3D printing has been dramatic and the ability of the various platform technologies to expand from rapid production prototypic models to the greater volume of readily customizable production of working parts is critical for continued high growth rates. This transition to working part production is highly dependent on adapting materials that deliver not only the requisite design accuracy but also the physical and mechanical properties necessary for the application. SIGNIFICANCE With the weighty distinction of being called the next industrial revolution, 3D printing technologies is already altering many industrial and academic operations including changing models for future healthcare delivery in medicine and dentistry.

Impact of Curing Protocol on Conversion and Shrinkage Stress

Since considerable shrinkage stress develops during the curing of dental composites, various soft-start photocuring protocols, aiming to lower stress but not compromise conversion, have been proposed. We hypothesized that utilizing soft-start photocuring will result in not only reduced stress, but also decreased conversion. We evaluated the impact of 3 protocols (soft-start, pulse, and standard) on the stress development and polymerization extent of an experimental composite. A novel set-up capable of simultaneous shrinkage stress, conversion, and temperature measurements on the same specimen was utilized. Analysis of the data shows that stress rises dramatically as a function of conversion in the vitrified state, and the utilization of soft-start or pulse curing results in specimens with reduced final conversion and shrinkage stress, compared with specimens cured according to the standard full-intensity protocol. Finally, this study demonstrates that the predominant reason for the reduced shrinkage stress attained with soft-start or pulse curing is a modest decrease in final conversion.

Network formation and compositional drift during photo-initiated copolymerization of dimethacrylate monomers

The copolymerization of viscous monomers with a nonviscous diluent comonomer was investigated. Overall, photopolymerization kinetics was compared for equimolar mixtures of triethylene glycol dimethacrylate (TEGDMA), a low viscosity monomer, with either a rigid aromatic dimethacrylate (Bis-GMA) or a flexible aliphatic urethane dimethacrylate (UDMA), as examples of viscous comonomers. Sol fraction analysis from partial cure polymerizations provided individual monomer reactivities as a function of conversion. The resin containing UDMA reached greater maximum polymerization rate and final conversion compared with the resin based on Bis-GMA. The viscous monomers (Bis-GMA or UDMA) were found to be less reactive than the diluent comonomer (TEGDMA) at nearly all stages of the photopolymerization process. The reactivity differential was especially dramatic at the latter stages of the polymerization.

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.

Dimethacrylate network formation and polymer property evolution as determined by the selection of monomers and curing conditions

OBJECTIVES This overview is intended to highlight connections between monomer structure and the development of highly crosslinked photopolymer networks including the conversion dependent properties of shrinkage, modulus and stress. METHODS A review is provided that combines the polymer science and dental materials literature along with examples of relevant experimental results, which include measurements of reaction kinetics, photorheology as well as polymerization shrinkage and stress. RESULTS While new monomers are continually under development for dental materials applications, mixtures of dimethacrylate monomers persist as the most common form of dental resins used on composite restorative materials. Monomer viscosity and reaction potential is derived from molecular structure and by employing real-time near-infrared spectroscopic techniques, the development of macromolecular networks is linked to the evolution of polymerization shrinkage (measured by linometer), modulus (measured by photorheometer), and stress (measured by tensometer). Relationships between the respective polymer properties are examined. SIGNIFICANCE Through a better understanding of the polymer network formation and property development processes using conventional dimethacrylate monomer formulations, the rational design of improved materials is facilitated with the ultimate goal of achieving dental polymers that deliver enhanced clinical outcomes.

Control of polymerization shrinkage and stress in nanogel-modified monomer and composite materials

OBJECTIVES This study demonstrates the effects of nano-scale prepolymer particles as additives to model dental monomer and composite formulations. METHODS Discrete nanogel particles were prepared by solution photopolymerization of isobornyl methacrylate and urethane dimethacrylate in the presence of a chain transfer agent, which also provided a means to attach reactive groups to the prepolymer. Nanogel was added to triethylene glycol dimethacrylate (TEGDMA) in increments between 5 and 40 wt% with resin viscosity, reaction kinetics, shrinkage, mechanical properties, stress and optical properties evaluated. Maximum loading of barium glass filler was determined as a function of nanogel content and composites with varied nanogel content but uniform filler loading were compared in terms of consistency, conversion, shrinkage and mechanical properties. RESULTS High conversion, high molecular weight internally crosslinked and cyclized nanogel prepolymer was efficiently prepared and redispersed into TEGDMA with an exponential rise in viscosity accompanying nanogel content. Nanogel addition at any level produced no deleterious effects on reaction kinetics, conversion or mechanical properties, as long as reactive nanogels were used. A reduction in polymerization shrinkage and stress was achieved in proportion to nanogel content. Even at high nanogel concentrations, the maximum loading of glass filler was only marginally reduced relative to the control and high strength composite materials with low shrinkage were obtained. SIGNIFICANCE The use of reactive nanogels offers a versatile platform from which resin and composite handling properties can be adjusted while the polymerization shrinkage and stress development that challenge the adhesive bonding of dental restoratives are controllably reduced.

Synthesis and Evaluation of New Oxaspiro Monomers for Double Ring-opening Polymerization

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.