Network structure of Bis-GMA- and UDMA-based resin systems
Introduction
2,2-Bis[p-(2′-hydroxy-3′-methacryloxypropoxy)phenylene]propane (Bis-GMA) and 1,6-bis(methacryloxy-2-ethoxycarbonylamino)-2,4,4-trimethylhexane (UDMA) are commonly used as dental base monomers in composite formulations, but their high viscosity and multiple functional groups require the use of a low viscosity diluent monomer, such as triethylene glycol dimethacrylate (TEGDMA). The structures of the monomers are shown in Fig. 1. When cured, these systems often have incomplete double bond conversion after polymerization [1], [2], [3], [4]. As the polymerization progresses, the network formation decreases the mobility of monomers and oligomeric molecules resulting in pendant methacrylate groups and unreacted monomer trapped in the material. Low levels of conversion affect physical properties of the polymer, decreasing its strength [5].
It has been a common practice to measure the reduction in double bonds due to the polymerization process via infrared (IR) spectroscopy on thin films [6] and in bulk [7], [8]. In crosslinking systems, residual double bonds in the polymer do not necessarily indicate free monomer, but may also result from pendant double bonds that are tied into the polymer network. Crosslinking is important for good network formation and physical properties. The extent of crosslinking, pendant double bonds, and leachable components have been analyzed by extraction of methyl methacrylate and crosslinking dimethacrylates from denture base polymers [9]. Also, crosslink density has been indirectly assessed by softening of polymers after exposure to ethanol [10], [11].
If unreacted free monomer is left in the polymer matrix, the unbound components are able to diffuse out of the polymer system [12], [13], [14], [15], [16], [17]. When this free monomer leaches out of the polymer, it may damage or irritate the surrounding tissue [18], [19]. Pulpal cellular components can also be affected by the other components of the system, such as initiators or radicals [20]. The amount leached out of the polymer may vary with the type of monomers in the system and the degree of cure [21]. The toxicity of dental monomers also varies depending on the type and quantity of monomer [18], [19], [22], [23]. Some monomers have been found to encourage bacteria growth [24]. Estrogenicity or allergenic reactions have been reported [25], [26], however, the estrogenic reaction may have come from impurities or degradation products.
To better understand the dependence of double bond conversion on monomer structure and reactivity of the monomer molecules in the forming network, these parameters were evaluated in a previous study [1] by photo-differential scanning calorimetry (DSC) using resin mixtures of TEGDMA with Bis-GMA or UDMA as base monomers. When compared at similar diluent concentrations, UDMA resins were significantly more reactive than Bis-GMA resins. Optimum reactivities in the UDMA resin system was obtained with the addition of relatively small amounts of TEGDMA, whereas the Bis-GMA/TEGDMA resin system required near-equivalent mole ratios for highest reactivity. Synergistic effects of base and diluent monomer on the polymerization rate and the final conversion were found for both Bis-GMA and UDMA base resins. The structures of the individual monomers, and consequently, the resin viscosities of the comonomer mixtures strongly influence both the rate and the extent of conversion of the photo-polymerization process.
This paper focuses on the network formation in bulk resin specimens of the same monomers, analyzing the leachable monomer fractions, and determining polymer crosslinking and pendant double bonds.
Section snippets
Materials and methods
The monomers Bis-GMA, TEGDMA, and UDMA (Esstech, Essington, PA), the photo- and coinitiators camphorquinone (CQ) and ethyl-4-dimethylaminobenzoate (4-EDMAB), inhibitor 2,6-di-tert-butyl-4-methylphenol (BHT, Aldrich Chemical Co., Milwaukee, WI) and solvents dichloromethane and cyclohexane (J.T. Baker, Phillipsburg, NJ) were used as received.
TEGDMA was mixed with Bis-GMA or UDMA in mole fraction increments of 0.125, creating the series B1 (all TEGDMA) to B9 (all Bis-GMA) and U1 (all TEGDMA) to U9
Results
Generally, the double bond conversion, measured by the decrease of the normalized CCH2 peak area at 6164 cm−1, showed an increase in conversion with a decrease in base monomer. The exception was pure TEGDMA, which had the lowest conversion around 42 % (Fig. 2a). For the Bis-GMA containing polymers, the conversion ranged from 54% to 85%. The UDMA containing polymers conversion ranged from 76% to 87%. The UDMA rich mixtures had significantly higher conversion than the Bis-GMA polymers at mole
Discussion
Measuring the conversion of double bonds in a given resin system has generally been considered a good indicator for the prospective physical properties of such systems. The conversion data reported here were measured by NIR spectroscopy. In a previous study on the same comonomer series, where photo-differential scanning calorimetry was used for more basic analyses of the relationship of diluent and base monomer [1], the same trends of double bond conversion with respect to the resin composition
Disclaimer
Certain commercial materials and equipment are identified in this paper for adequate definition of the experimental procedure. In no instance does such identification imply recommendation or endorsement by the National Institute of Standards and Technology or the ADA Foundation or that the material or equipment identified is necessarily the best available for the purpose.
Acknowledgements
The authors are grateful to Esstech, Essington, PA, for the donation of the monomers and to Dentsply Caulk, Milford, DE, for the supply of the barium oxide glass fillers. This study was supported in part by the American Dental Association Foundation, the National Institute of Standards and Technology, and the National Institute of Dental and Craniofacial Research, Grant No. DE13298.
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