Structure–property relationships for wet dentin adhesive polymers

Dentin adhesive systems for composite tooth restorations are composed of hydrophilic/hydrophobic monomers, solvents, and photoinitiators. The adhesives undergo phase separation and concomitant compositional change during their application in the wet oral environment; phase separation compromises the quality of the hybrid layer in the adhesive/dentin interface. In this work, the adhesive composition in the hybrid layer can be represented using the phase boundaries of a ternary phase diagram for the hydrophobic monomer/hydrophilic monomer/water system. The polymer phases, previously unaccounted for, play an important role in determining the mechanical behavior of the bulk adhesive, and the chemomechanical properties of the phases are intimately related to the effects produced by differences in the hydrophobic–hydrophilic composition. As the composition of the polymer phases varies from hydrophobic-rich to hydrophilic-rich, the amount of the adsorbed water and the nature of polymer–water interaction vary nonlinearly and strongly correlate with the change in elastic moduli under wet conditions. The failure strain, loss modulus, and glass transition temperature vary nonmonotonically with composition and are explained based upon primary and secondary transitions observed in dynamic mechanical testing. Due to the variability in composition, the assignment of mechanical properties and the choice of suitable constitutive models for polymer phases in the hybrid layer are not straightforward. This work investigates the relationship between composition and chemomechanical properties of the polymer phases formed on the wateradhesive phase boundary using quasistatic and dynamic mechanical testing, mass transfer experiments, and vibrational spectroscopy.Research Grant Nos.R01DE022054 (PI: Spencer) and R01DE025476 (Spencer/Tamerler

Effect of initiator on photopolymerization of acidic, aqueous dental model adhesives

This study evaluated different initiator systems in self-etching model adhesives, in which camphorquinone (CQ) or [3-(3,4-dimethyl-9-oxo-9H-thioxanthen-2-yloxy)-2-hydroxylpropy] trimethylammonium chloride (QTX) was employed as a photoinitiator (dye). N-phenylglycine (NPG), ethyl 4-dimethylaminobenzoate (4E) or 2-(dimethylamino) ethyl methacrylate (DMAEMA) was used as the coinitiator (CI). The role of diphenyliodonium hexafluorophosphate (DPIHP) in the polymerization process was also studied. The concentrations of dye, CI, and DPIHP in model adhesives were all maintained at 0.022 mmol per gram monomer. The model adhesive contained two monomers: (bis[2-(methacryloyloxy)ethyl] phosphate) (2MP) and 2-hydroxyethyl methacrylate (HEMA) whose mass ratio was 1:1, thus representing an acidic and hydrophilic formulation. The polymerization rate and the degree of conversion (DC) of the model adhesives with 5, 15, or 25% water content were determined using FTIR/ATR with a time-based spectrum analysis. The results indicated that with CQ as the photoinitiator, 4E appeared to be the most efficient CI, whereas the CQ-DMAEMA combination led to very low radical generation efficiency (DC < 5%). DPIHP exhibited little effect on the polymerization process. With QTX as the photoinitiator, however, DPIHP played an essential role. Without DPIHP, all three QTX-CI systems failed to initiate polymerization (DC < 5%). The QTX-DPIHP combination, on the other hand, was found to be a viable initiator system. The above results provide the critical information for the development of self-etching adhesive systems

Compositional design and optimization of dentin adhesive with neutralization capability

Objectives The objective of this work was to investigate the polymerization behavior, neutralization capability, and mechanical properties of dentin adhesive formulations with the addition of the tertiary amine co-monomer, 2-N-morpholinoethyl methacrylate (MEMA). Methods A co-monomer mixture based on HEMA/BisGMA (45/55, w/w) was used as a control adhesive. Compared with the control formulation, the MEMA-containing adhesive formulations were characterized comprehensively with regard to water miscibility of liquid resin, water sorption and solubility of cured polymer, real-time photopolymerization kinetics, dynamic mechanical analysis (DMA), and modulated differential scanning calorimetry (MDSC). The neutralization capacity was characterized by monitoring the pH shift of 1 mM lactic acid (LA) solution, in which the adhesive polymers were soaked. Results With increasing MEMA concentrations, experimental copolymers showed higher water sorption, lower glass transition temperature and lower crosslinking density compared to the control. The pH values of LA solution gradually increased from 3.5 to about 6.0–6.5 after 90 days. With the increase in crosslinking density of the copolymers, the neutralization rate was depressed. The optimal MEMA concentration was between 20 and 40 wt%. Conclusions As compared to the control, the results indicated that the MEMA-functionalized copolymer showed neutralization capability. The crosslinking density of the copolymer networks influenced the neutralization rate

Physicochemical interactions at the dentin/adhesive interface using FTIR chemical imaging

This is the published version.To date, much of our understanding of dentin bonding has been based on investigations performed on sound, healthy dentin. This is not the substrate generally encountered in clinical practice, rather dentists must frequently bond to caries-affected dentin. Because of the extreme complexity and variability of the caries-affected dentin substrate, conventional characterization techniques do not provide adequate information for defining those factors that impact bond formation. Using Fourier-transform infrared imaging, we characterized the inhomogeneities and compositional differences across the length and breadth of the caries-affected dentin/adhesive interface. Differences in mineral/matrix ratio, crystallinity, and collagen organization were noted in the comparison of caries-affected and healthy dentin. As compared to healthy dentin, there were striking differences in depth of demineralization, adhesive infiltration, and degree of conversion at the interface with caries-affected dentin

Scanning Acoustic Microscopy Investigation of Frequency-Dependent Reflectance of Acid-Etched Human Dentin Using Homotopic Measurements

Composite restorations in modern restorative dentistry rely on the bond formed in the adhesive-infiltrated acid-etched dentin. The physical characteristics of etched dentin are, therefore, of paramount interest. However, characterization of the acid-etched zone in its natural state is fraught with problems stemming from a variety of sources including its narrow size, the presence of water, heterogeneity, and spatial scale dependency. We have developed a novel homotopic (same location) measurement methodology utilizing scanning acoustic microscopy (SAM). Homotopic measurements with SAM overcome the problems encountered by other characterization/ imaging methods. These measurements provide us with acoustic reflectance at the same location of both the pre- and post-etched dentin in its natural state. We have applied this methodology for in vitro measurements on dentin samples. Fourier spectra from acid-etched dentin showed amplitude reduction and shifts of the central frequency that were location dependent. Through calibration, the acoustic reflectance of acid-etched dentin was found to have complex and non-monotonic frequency dependence. These data suggest that acid-etching of dentin results in a near-surface graded layer of varying thickness and property gradations. The measurement methodology described in this paper can be applied to systematically characterize mechanical properties of heterogeneous soft layers and interfaces in biological materials

Determination of neutralization capacity and stability of a basic methacrylate monomer using NMR

This is an Accepted Manuscript of an article published by Taylor & Francis in International Journal of Polymeric Materials and Polymeric Biomaterials in 2012, available online: http://www.tandfonline.com/10.1080/00914037.2011.574660.The durability of dental resin depends on the stability of the polymer. The neutralizing capacity of a basic methacrylate monomer and its chemical stability were measured using nuclear magnetic resonance (NMR) spectroscopy. Lactic acid solution was titrated with 2-(dimethylamino)ethylmethacrylate (DMAEMA) or 2-hydroxyethylmethacrylate (HEMA) and its chemical shifts monitored. Addition of DMAEMA alters the chemical shift proportionally to pH neutralization, whereas HEMA has no impact. Chemical shifts were used to quantify both the change in pH and monomer stability. The results demonstrate that neutralization by basic monomer can be achieved and that this can be measured using an NMR assay

Effect of Photo-Initiators on the In Vitro Performance of a Dentin Adhesive Exposed to Simulated Oral Environment

Our previous study showed poor mechanical durability and nano-sized heterogeneities in cross-linked dentin adhesives cured in the presence of water. To further explore the relationship between nano-scale heterogeneities and the long-term mechanical properties of dentin adhesives, the properties of model dentin adhesives polymerized using hydrophilic photoinitiators were compared with those of adhesives polymerized using hydrophobic camphorquinone-based photoinitiators. There was a continuous decline of mechanical properties for the specimens cured in the presence of water during 3 months aqueous storage, especially for the specimens that contained hydrophobic photoinitiators. The multi-component systems containing hydrophilic photoinitiators were shown to produce superior model dental adhesives when these materials are cured in the presence of water

Tris(trimethylsilyl)silane as a co-initiator for dental adhesive: Photo-polymerization kinetics and dynamic mechanical property

Objectives The purpose of this study was to evaluate the polymerization behavior of a model dentin adhesive with tris(trimethylsilyl)silane (TTMSS) as a co-initiator, and to investigate the polymerization kinetics and mechanical properties of copolymers in dry and wet conditions. Methods A co-monomer mixture based on HEMA/BisGMA (45/55, w/w) was used as a model dentin adhesive. The photoinitiator system included camphorquinone (CQ) as the photosensitizer and the co-initiator was ethyl-4-(dimethylamino) benzoate (EDMAB) or TTMSS. Iodonium salt, diphenyliodonium hexafluorophosphate (DPIHP) serving as a catalyst, was selectively added into the adhesive formulations. The control and the experimental formulations were characterized with regard to the degree of conversion (DC) and dynamic mechanical properties under dry and wet conditions. Results In two-component photoinitiator system (CQ/TTMSS), with an increase of TTMSS concentration, the polymerization rate and DC of C═C double bond increased, and showed a dependence on the irradiation time and curing light intensity. The copolymers that contained the three-component photoinitiator system (CQ/TTMSS/DPIHP) showed similar dynamic mechanical properties, under both dry and wet conditions, to the EDMAB-containing system. Significance The DC of formulations using TTMSS as co-initiator showed a strong dependence on irradiation time. With the addition of TTMSS, the maximum polymerization rate can be adjusted and the network structure became more homogenous. The results indicated that the TTMSS could be used as a substitute for amine-type co-initiator in visible-light induced free radical polymerization of methacrylate-based dentin adhesives

Autonomous-Strengthening Adhesive Provides Hydrolysis-Resistance and Enhanced Mechanical Properties in Wet Conditions

The low-viscosity adhesive that is used to bond composite restorative materials to the tooth is readily damaged by acids, enzymes, and oral fluids. Bacteria infiltrate the resulting gaps at the composite/tooth interface, demineralize the tooth, and further erode the adhesive. This paper presents the preparation and characterization of a low-crosslink-density hydrophilic adhesive that capitalizes on sol-gel reactions and free-radical polymerization to resist hydrolysis and provide enhanced mechanical properties in wet environments. Polymerization behavior, water sorption, and leachates were investigated. Dynamic mechanical analyses (DMA) were conducted using water-saturated adhesives to mimic load transfer in wet conditions. Data from all tests were analyzed using appropriate statistical tests (α = 0.05). The degree of conversion was comparable for experimental and control adhesives at 88.3 and 84.3%, respectively. HEMA leachate was significantly lower for the experimental (2.9 wt%) compared to control (7.2 wt%). After 3 days of aqueous aging, the storage and rubbery moduli and the glass transition temperature of the experimental adhesive (57.5MPa, 12.8MPa, and 38.7 °C, respectively) were significantly higher than control (7.4MPa, 4.3 MPa, and 25.9 °C, respectively). The results indicated that the autonomic sol-gel reaction continues in the wet environment, leading to intrinsic reinforcement of the polymer network, improved hydrolytic stability, and enhanced mechanical properties

The influence of chemical structure on the properties in methacrylate-based dentin adhesives

Objectives The objective of this study was to investigate the influence of the chemical structure of methacrylate monomers used in dentin adhesives on degree of conversion (DC), water sorption, and dynamic mechanical properties. Materials and methods Experimental adhesives containing 2,2-bis[4-(2-hydroxy-3-methacryloxypropoxy) phenyl]-propane (BisGMA), 2-hydroxyethyl methacrylate (HEMA), and co-monomer, 30/45/25 w/w were photo-polymerized. Ethyleneglycol dimethacrylate (EGDM), diethyleneglycol dimethacrylate (DEGDM), triethyleneglycol dimethacrylate (TEGDMA), 1,3-glycerol dimethacrylate (GDM), and glycerol trimethacrylate (GTM) were used as a co-monomer. The adhesives were characterized with regard to DC, water sorption, and dynamic mechanical analysis and compared to control adhesive [HEMA/BisGMA, 45/55 w/w]. Results DC and water sorption increased with an increase in the number of ethylene glycol units in the monomer. Experimental adhesive containing GDM showed significantly higher storage moduli (p < 0.05) in both dry and wet samples than experimental adhesives containing EGDM or DEGDM. The rubbery moduli of adhesives containing GDM and GTM were found to be significantly greater (p < 0.05) than that of the control. Adhesives containing GTM exhibited the widest tanδ curves, indicating the greatest structural heterogeneity. Significance The hydrophilicity, functionality and size of monomers in dentin adhesives affected the water sorption, solubility, crosslink density and heterogeneity of the polymer network. The experimental adhesives containing GDM and GTM showed higher rubbery moduli, indicating higher crosslink density accompanied by a decrease in the homogeneity of the polymer network structure