The ratio of the double-bond content of monomer to polymer, i.e. degree of conversion (DC) has been used frequently as a convenient means of comparing the behavior and properties of dental composites and adhesives. The purpose of this investigation was to study the relationship of photopolymerization processes, bulk properties, and structure using model dentin adhesives cured in the presence of different ethanol content as an example. There was little difference in the DC of model BisGMA-based adhesives cured in the presence of ethanol concentrations ranging from 0 to 40 wt %, but there were substantial differences in the mechanical properties. Ultimate tensile strength (UTS) and modulus of elasticity decreased with an increase in ethanol content. Polymer structure was revealed by thermal behavior in the glass transition temperature (T g ) region; these measurements were obtained by modulated temperature differential scanning calorimetry (MTDSC) technology, which removes the competing irreversible effects associated with release of volatiles and residual curing. Glass transition temperature of model adhesives decreased substantially with an increase in ethanol content. The DC based on the quantity of remaining double bond has been used extensively to characterize and provide a relative assessment of the quality of dentin adhesives and dental composites. Since polymers differing in linearity, and therefore crosslink density, may have a similar degree of conversion, the measurement of monomer/polymer conversion does not necessarily provide complete representation of the quality or durability of the polymer structure.
Under in vivo conditions, there is little control over the amount of water left on the tooth during dentin bonding. As a result, it is possible to leave the dentin surface so wet that the adhesive actually undergoes physical separation into hydrophobic-and hydrophilic-rich phases. Using tapping mode atomic force microscopy/PhaseImaging technique, nanosized phases with worm-like features were found on the surface of model HEMA/BisGMA dentin adhesives cured in the presence of varying concentrations of water. The phase contrast became evident with the increase of water concentration in the initial adhesive formulation and varied with the ratio of hydrophilic/hydrophobic composition. Oversaturated water droplets of variable sizes may accumulate as micro-voids within the hydrophilic and hydrophobic polymer phases. The phase domains were also identified following ethanol-etching in combination with SEM/AFM techniques.
Our previous study showed poor mechanical durability and nano-sized heterogeneities in crosslinked 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.
It was reported previously that, in the presence of water, a commercially available light-activated BisGMA/HEMA adhesive underwent physical separation into solid BisGMA-rich particles and a fluid-like HEMA-rich phase. The HEMA-rich phase exhibited limited monomer conversion suggesting that the photoinitiator is localized to the hydrophobic phase or that the photoinitiator is not compatible with the hydrophilic HEMA. The objective of the present study was to identify photoinitiators that are compatible with the hydrophilic HEMA-rich phase, when the mixtures are prepared without and with water added. The photoinitiator was camphoquinone (CQ, 0.5 mol %), and the coinitiators (0.5 mol %) were 2,2′-dihydroxyethyl-para-toluidine (DHEPT), dimethylaminoethyl methacrylate (DMAEMA), and N-phenylgly-cine (NPG), and (1 wt %) diphenyliodonium chloride (DPIC). Reactivities were evaluated using photodifferential scanning calorimetry, at 37°C, using visible light (>418 nm), with the parameters determined being enthalpy (ΔH), the induction time (herein defined as the time for 1% of the photopolymerization to be complete), and the time at which the maximum exotherm occurred. The degree of monomer conversion was measured using micro-Raman spectroscopy. It was shown that the reactivity ranking (based on time to exotherm peak maximum and total enthalpy) was HEMA/CQ/DHEPT < HEMA/ CQ/DMAEMA < HEMA/CQ/NPG. Reactivity was dramatically increased for CQ/DMAEMA and CQ/NPG in the presence of DPIC, but not for CQ/DHEPT. Water has a major effect on HEMA conversion. At 10% of water, the conversion level of HEMA formulated with CQ/DMEMA dropped from ~100% to 86%. In comparison, the conversion in 10% of water increased to nearly 96% when DPIC was used. The results suggest that DHEPT, which is commonly used in commercial adhesives, is not compatible with HEMA. Both NPG and DMAEMA appear compatible with the HEMA. The ionic hydrophilic iodonium salt, DPIC, enhances the polymerization of HEMA, even in the presence of water. Future studies on water-compatible photoinitiators should be performed to address the detrimental effects of water on dentin adhesive systems.
Aims-The purpose of this study was to evaluate the effects of photoinitiator type and water content on the polymerization rate (R p ) and degree of conversion (DC) of a model BisGMA/HEMA-based resin.Materials and methods-The comonomer mixture consisted of BisGMA/HEMA (60/40 by weight). Different two-or three-component photoinitiator systems were incorporated. Twocomponent systems were 0.5% CQ (camphorquinone) and 0.5% DMAEMA (2-(dimethylamino) ethyl methacrylate) or 0.5% CQ and 0.5% 4E (Ethyl 4-dimethylaminobenzoate). The threecomponent systems were added 1% DPIHP (diphenyliodonium hexafluorophosphate) to the above systems. Each system was tested as made, or after addition of 5, 10, 15wt% water. When cured under a conventional dental light, the R p and DC of each formulation was determined using time-resolved attenuated total reflection (ATR)-Fourier transform infrared (FTIR) spectroscopy.Results-For mixtures containing two-component initiator systems, when the hydrophobic initiator CQ was used in combination with hydrophilic DMAEMA, R p s and DCs were dramatically decreased as a function of water content. The R p s and DCs of the hydrophobic CQ/4E system were higher than those of the CQ/DMAEMA system in the presence of water. For three-component initiator systems, incorporation of DPIHP enhanced the polymerization of all mixtures in the presence of water compared to their counterpart two-component initiators. Interestingly, the CQ/DMAEMA caused greater DC and Rp when DPIHP was used.Significance-The hydrophobicity/hydrophilicity of photoinitiator components significantly affects both the DC as well as R p when in the presence of water. The results indicate that formulation of photoinitiator components should be based on the effectiveness of the bonding systems under both dry and wet conditions.
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