J.W. Stansbury

Synthesis, characterization and evaluation of urethane derivatives of Bis-GMA.

OBJECTIVES 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. METHODS 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. RESULTS 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. SIGNIFICANCE 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.

Spatial and Temporal Control of Thiol-Michael Addition via Photocaged Superbase in Photopatterning and Two-Stage Polymer Networks Formation.

Photochemical processes enable spatial and temporal control of reactions, which can be implemented as an accurate external control approach in both polymer synthesis and materials applications. “Click” reactions have also been employed as efficient tools in the same field. Herein, we combined photochemical processes and thiol-Michael “click” reactions to achieve a “photo-click” reaction that can be used in surface patterning and controlled polymer network formation, owing to the ease of spatial and temporal control through use of photolabile amines as appropriate catalysts.

Academy of Dental Materials guidance—Resin composites: Part I—Mechanical properties

OBJECTIVE The objective of this project, which was initiated from the Academy of Dental Materials, was to review and critically appraise methods to determine fracture, deformation and wear resistance of dental resin composites, in an attempt to provide guidance for investigators endeavoring to study these properties for these materials. METHODS Test methods have been ranked in the priority of the specific property being tested, as well as of the specific test methods for evaluating that property. Focus was placed on the tests that are considered to be of the highest priority in terms of being the most useful, applicable, supported by the literature, and which show a correlation with clinical findings. Others are mentioned briefly for the purpose of being inclusive. When a standard test method exists, including those used in other fields, these have been identified in the beginning of each section. Also, some examples from the resin composite literature are included for each test method. RESULTS The properties for evaluating resin composites were ranked in the priority of measurement as following: (1) Strength, Elastic Modulus, Fracture toughness, Fatigue, Indentation Hardness, Wear-abrasion (third body) and Wear-attrition (contact/two body), (2) Toughness, Edge strength (chipping) and (3) Wear determined by toothbrush. SIGNIFICANCE The following guidance is meant to aid the researcher in choosing the proper method to assess key properties of dental resin composites with regard to their fracture, deformation and wear resistance.

Academy of Dental Materials guidance-Resin composites: Part II-Technique sensitivity (handling, polymerization, dimensional changes)

OBJECTIVE The objective of this work, commissioned by the Academy of Dental Materials, was to review and critically appraise test methods to characterize properties related to critical issues for dental resin composites, including technique sensitivity and handling, polymerization, and dimensional stability, in order to provide specific guidance to investigators planning studies of these properties. METHODS The properties that relate to each of the main clinical issues identified were ranked in terms of their priority for testing, and the specific test methods within each property were ranked. An attempt was made to focus on the tests and methods likely to be the most useful, applicable, and supported by the literature, and where possible, those showing a correlation with clinical outcomes. Certain methods are only briefly mentioned to be all-inclusive. When a standard test method exists, whether from dentistry or another field, this test has been identified. Specific examples from the literature are included for each test method. RESULTS The properties for evaluating resin composites were ranked in the priority of measurement as follows: (1) porosity, radiopacity, sensitivity to ambient light, degree of conversion, polymerization kinetics, depth of cure, polymerization shrinkage and rate, polymerization stress, and hygroscopic expansion; (2) stickiness, slump resistance, and viscosity; and (3) thermal expansion. SIGNIFICANCE The following guidance is meant to aid the researcher in choosing the most appropriate test methods when planning studies designed to assess certain key properties and characteristics of dental resin composites, specifically technique sensitivity and handling during placement, polymerization, and dimensional stability.

Dental Restorative Materials Based on Thiol-Michael Photopolymerization:

Step-growth thiol-Michael photopolymerizable resins, constituting an alternative chemistry to the current methacrylate-based chain-growth polymerizations, were developed and evaluated for use as dental restorative materials. The beneficial features inherent to anion-mediated thiol-Michael polymerizations were explored, such as rapid photocuring, low stress generation, ester content tunability, and improved mechanical performance in a moist environment. An ester-free tetrafunctional thiol and a ultraviolet-sensitive photobase generator were implemented to facilitate thiol-Michael photopolymerization. Thiol-Michael resins of varied ester content were fabricated under suitable light activation. Polymerization kinetics and shrinkage stress were determined with Fourier-transform infrared spectroscopy coupled with tensometery measurements. Thermomechanical properties of new materials were evaluated by dynamic mechanical analysis and in 3-point bending stress-strain experiments. Photopolymerization kinetics, polymerization shrinkage stress, glass transition temperature, flexural modulus, flexural toughness, and water sorption/solubility were compared between different thiol-Michael systems and the BisGMA/TEGDMA control. Furthermore, the mechanical performance of 2 thiol-Michael composites and a control composite were compared before and after extensive conditioning in water. All photobase-catalyzed thiol-Michael polymerization matrices achieved >90% conversion with a dramatic reduction in shrinkage stress as compared with the unfilled dimethacrylate control. One prototype of ester-free thiol-Michael formulations had significantly better water uptake properties than the BisGMA/TEGDMA control system. Although exhibiting relatively lower Young’s modulus and glass transition temperatures, highly uniform thiol-Michael materials achieved much higher toughness than the BisGMA/TEGDMA control. Moreover, low-ester thiol-Michael composite systems show stable mechanical performance even after extensive water treatment. Although further resin/curing methodology optimization is required, the photopolymerized thiol-Michael prototype resins can now be recognized as promising candidates for implementation in composite dental restorative materials.

Nanogels as a Basis for Network Construction

Reactive nanogels with diverse chemical and physical compositions were synthesized using solution free radical polymerization. Conventional monomer resins show a reduction in polymerization shrinkage and stress as well as improved bond strength when nanogels are used as a filler. Nanogels dispersed in inert solvent at appropriate concentrations form macroscopic networks with chemical compositions that are not directly available from the starting monomers.

Simultaneous measurement of fluorescence, conversion and physical/mechanical properties for monitoring bulk and localized photopolymerization reactions in heterogeneous systems

An FT-NIR spectrometer, rheometer and fluorescence spectrophotometer were coupled for the real-time monitoring of polymerization reactions, allowing the simultaneous tracking of polymerization kinetics, storage modulus as well as fluorescence. In this study, a methacrylate functionalized dansyl chromophore (DANSMA) was synthesized and two different nanogels were made from urethane dimethacrylate and isobornyl methacrylate. Two series of resin formulations were prepared using the DANSMA probe, ethoxylated bisphenol A dimethacrylate as the matrix monomer, Irgacure® 651 as the initiator and the dispersed, monomer-swollen nanogels to give clear UV-curable resins. Placement of the fluorescent probe either throughout the resin or linked into the nanogel before its dispersion in the matrix provides a tool to study how the nanogel structure affects local network development by means of fluorescence from the DANSMA probe. We demonstrate the potential of this new technique using a composite as the two phase system (resin and polymerizable nanogel) including a dansyl derivative as a polymerizable probe to follow the reactions that are taking places in both phases.

Evaporation of low-volatility components in polymeric dental resins.

OBJECTIVES This study provides measurement of the volatility of selected photoinitiators and monomers used in dental adhesive resins. A detailed determination of the spatial and temporal character of camphorquinone (CQ) volatilization with respect to air flow conditions as well as media viscosity is assessed to gauge the effect of evaporative loss on the photopolymerization process and the photopolymers formed. METHODS Vapor pressures of materials are measured by thermogravimetric analysis. A quantitative model assuming one-dimensional Fickian diffusion with surface evaporation is presented and compared with measured photoinitiator volatilization from viscous and non-viscous resin samples, obtained by spectrophotometry and confocal microscopy. Model resins are prepared and subject to airthinning followed by photocuring, monitored in real-time by Fourier transform infrared spectrometry. RESULTS Vapor pressure measurements of the individual components of the adhesive resin span nearly four orders of magnitude, with the photoinitiator CQ near the middle (0.6Pa) and the monomer HEMA at the upper end (10Pa). We see depth-averaged CQ loss from non-viscous open films, while depthresolved measurements of viscous droplets show strong surface-localized CQ depletion. Good agreement is observed between measurements and the model. Finally, air-thinning of samples prepared with more-volatile photoinitiator and monomer is shown to cause longer induction times, slower early-stage polymerization rates and lower late-stage degree of conversion. SIGNIFICANCE Widely used compounds with vapor pressures as low as 0.6Pa (0.001Torr) undergo significant volatilization from samples ventilated under conditions generally representative to clinically used air-thinning procedures, with the potential to adversely affect the photopolymerization of both viscous and non-viscous resins. The inverse relationship between air-thinning and adhesive bond strength, observed elsewhere, may be partially caused by this same effect.