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)].
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 tan δ curves, indicating the greatest structural heterogeneity.
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.
In 2005, 166 million dental restorations were placed in the United States and clinical studies suggest that more than half were replacements for failed restorations . The emphasis on replacement therapy is expected to increase as concern about mercury release from dental amalgam forces dentists to select alternative materials. Resin composite is the most commonly used alternative , but moderate to large composite restorations have higher failure rates, more recurrent caries and increased frequency of replacement as compared to amalgam . The primary factor in the premature failure of moderate to large composite restorations is recurrent decay at the margins of the restorations. In class II composite restorations, recurrent decay is most often localized gingivally and is linked to failure of the bond between the tooth and composite and increased levels of the cariogenic bacteria, Streptococcus mutans , at the perimeter of these materials .
The composite is too viscous to bond directly to the tooth and thus, a low viscosity adhesive must be used to form a bond between the tooth and composite. Acid-etching provides effective mechanical bonding between enamel and adhesive, but bonding to dentin has been fraught with problems. At the most vulnerable margin, i.e. the gingival margin of class II composite restorations there is very little enamel available and thus, the bond at this margin depends on the integrity of the adhesive seal formed with dentin . In vitro and in vivo studies have suggested that several factors inhibit the formation of a durable adhesive/dentin bond. These factors include: (1) adhesive phase separation; (2) water sorption and hydrolysis of the adhesive polymer; (3) degree of conversion of monomer to polymer; (4) mechanical properties; and (5) polymer network structure.
Current dentin adhesive system typically consists of monomers, initiators, solvents, inhibitors or stabilizers and sometimes inorganic fillers . The monomers are particularly critical since polymerization of the monomers produces a crosslinked matrix that provides chemical/thermal stability and mechanical strength. Monomer selection exerts considerable influence on the properties, durability and behavior of dentin adhesives in the wet, oral environment. Although numerous monomers have been investigated the lack of dentin adhesives that are both effective and durable continues to be a major problem with the use of composites in direct restorative dentistry.
The objective of this study was to investigate the influence of the chemical structure of methacrylate monomers used in dentin adhesives on the following properties: degree of conversion, water sorption, solubility, and dynamic mechanical properties. These results elucidate critical structure/property relationships for methacrylate monomers and provide vital information for future development of durable dentin adhesives.
Materials and methods
Experimental adhesives containing bisphenol-A diglycidyl ether dimethacrylate (BisGMA, Polysciences, Warrington, PA), 2-hydroxyethyl methacrylate (HEMA, Acros Organics, NJ), and co -monomer, at 30/45/25 (w/w) were polymerized with visible light and compared to control adhesives [HEMA/BisGMA, 45/55 (w/w)]. Ethyleneglycol dimethacrylate (EGDM), diethyleneglycol dimethacrylate (DEGDM), triethyleneglycol dimethacrylate (TEGDMA), 1,3-glycerol dimethacrylate (GDM), and glycerol trimethacrylate (GTM) were used as a co -monomer (all co -monomers were from Aldrich, Milwaukee, WI). The chemical structures of monomers used in this study are given in Fig. 1 . The following three-component visible light photoinitiators (all from Aldrich, Milwaukee, WI) were used in this work: camphoroquinone (CQ, 0.5 wt%), ethyl-4-(dimethylamino)benzoate (EDMAB, 0.5 wt%) and diphenyliodonium hexafluorophosphate (DPIHP, 0.5 wt%) without further purification . The concentration of the photoinitiator component is calculated with respect to the total amount of monomer. All materials were used as received.
Sample preparation and degree of conversion
Mixtures of monomers/photoinitiators were prepared in a brown glass vial in the absence of visible light. To achieve a homogeneous mixture, the solutions were stirred at room temperature for 12 h. The prepared resins were injected into a glass-tubing mold ((Fiber Optic Center, Inc., Part #: ST8100, New Bedford, MA)) and light-cured for 10 s at room temperature with a LED light curing unit (LED Curebox, Proto-tech, Portland, OR, USA) . The polymerized samples were stored in the dark at room temperature for 48 h and 1 week in a vacuum oven in the presence of a drying agent at 37 °C. The resultant rectangular beam specimens (1 mm × 1 mm × 15 mm) were used to determine the degree of conversion (DC), water sorption and solubility, and dynamic mechanical properties.
The DC was determined by using a LabRAM ARAMIS Raman spectrometer (LabRAM HORIBA Jobin Yvon, Edison, NJ) with a HeNe laser ( λ = 633 nm, a laser power of 17 mW) as an excitation source . The instrument conditions were: 200 μm confocal hole, 150 μm wide entrance slit, 600 g/mm grating, and 10× objective Olympus lens. Data processing was performed using LabSPEC 5 (HORIBA Jobin Yvon). The samples were mounted on a computer-controlled, high-precision x – y stage. To determine the DC, spectra of the unpolymerized resins and rectangular beam samples were acquired over a range of 700–1800 cm −1 . The change of the band height ratios of the aliphatic C C double bond peak at 1640 cm −1 and the aromatic C C at 1610 cm −1 (phenyl) in both the cured and uncured states was monitored and the DC calculated using the following equation based on the decrease in the intensity band ratios before and after light curing.
DC ( % ) = 1 − R cured R uncured × 100 ,
where R = band height at 1640 cm −1 /band height at 1610 cm −1 . All experiments were carried out in triplicate and the results were averaged.
Mass change, water sorption and solubility
Rectangular beam specimens (1 mm × 1 mm × 15 mm; n = 3 in each group) were stored in a desiccator at 23 °C for 1 h prior to weighing with a calibrated electronic balance (Mettler Toledo, XS 205; resolution of 0.01 mg). This drying cycle was repeated until a constant mass ( m 1 ) was obtained. After drying, the specimens were immersed in distilled water at 37 °C. The specimens were removed at fixed time intervals (3, 5, 24, 48, 96, 168, and 240 h), blotted to remove excess water, weighed ( m 2 ) and returned to the water. All the specimens were then removed from the water and placed in a vacuum oven containing a freshly dried silica gel at 37 °C until a constant weight was achieved ( m 3 ) . The values (%) for mass change ( W mc ), solubility ( W su ), and water sorption ( W sp ) were calculated as
W m c ( % ) = m 2 − m 1 m 1
W s u ( % ) = m 1 − m 3 m 1
W s p ( % ) = W m c ( % ) + W s u ( % )