Synthesis and evaluation of modified urethane dimethacrylate resins with reduced water sorption and solubility
Introduction
The most preferred dimethacrylate resin monomers for use in dental composite materials today include the aromatic Bis-phenol A derivative, Bis-GMA, and the aliphatic urethane dimethacrylate UDMA [1]. The Bis-GMA monomer contains pendant hydroxyl groups within its molecular backbone. Because of these polar groups, polymers made with this monomer tend to be somewhat hydrophilic and susceptible to increased water sorption [2]. Various methods have been employed in an effort to reduce the hydrophilicity of Bis-GMA based systems. Some of these methods include capping the Bis-GMA hydroxyl groups with more hydrophobic substituents [3], synthesis of novel fluorinated aliphatic and aromatic dimethacrylates [4], [5] and the incorporation of methacrylate terminated butadiene terpolymers [6]. Bis-EMA, a non-hydroxylated analogue of Bis-GMA, has also been utilized [7] (Fig. 1). Compared to Bis-GMA, the Bis-EMA monomer is less hydrophilic and exhibits a reduced viscosity [7].
UDMA, 1,6-bis(methacrylyloxy-2-ethoxycarbonylamino)-2,4,4-trimethylhexane, was first introduced as a dental resin in 1974 by Foster and Walker [8] as an alternative monomer to Bis-GMA. It is the reaction product of 2,4,4-trimethylhexamethylene diisocyanate and 2-hydroxyethyl methacrylate (HEMA). Advantages of the UDMA monomer when compared to Bis-GMA include a reduced viscosity, increased filler loading and greater toughness due to the flexibility of the urethane linkages [9], [10]. Studies have shown that when evaluating only the monomer systems UDMA based resin composites have improved mechanical properties compared to composites prepared from Bis-GMA [4], [11]. Polymers made with the UDMA monomer exhibit similar or slightly less water sorption than those prepared from Bis-GMA [12], [13]. However, UDMA polymers show significantly more water uptake than polymers based on non-hydroxylated Bis-GMA analogues such as Bis-EMA [7], [12]. Excess water sorption may lead to hydrolytic degradation of the polymer matrix and a reduction in mechanical properties [14]. Chemical erosion may also result in the release of unreacted monomers and degradation by-products into the oral environment [14], [15]. In order to improve the mechanical properties of UDMA based polymers, a modified UDMA polymer was prepared which incorporated carboxylic acid side groups [16]. Improved mechanical properties were obtained through increased intermolecular non-covalent crosslinking, however, the addition of polar carboxylic side groups would tend to make the polymer more hydrophilic. Other studies have prepared high molecular weight polyurethane methacrylates in order to improve toughness [17] and reduce polymerization shrinkage [18]. In one of these studies the polyurethane polymer showed a slight decrease in water sorption but a general reduction in mechanical properties when compared to the Bis-GMA based polymer [17].
This study seeks to develop and evaluate novel urethane dimethacrylate monomers that contain various side groups in order to improve the water resistance of the resultant polymers. Therefore, the objective of this investigation was to evaluate the flexural strength, flexural modulus, water contact angle, water sorption and solubility of six experimental urethane dimethacrylate monomers as well as the commercially available urethane dimethacrylate, UDMA. An additional objective was to calculate Hoy's solubility parameters of the urethane copolymers to determine their relative hydrophilicity.
Section snippets
Materials and instrumentation
1,6-Hexamethylene diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate (2,4,4-trimethyl diisocyantohexane), 1,3-bis(isocyanatomethyl) cyclohexane, m-tetramethylxylene diisocyanate [1,3-bis(isocyanato-1-methylethyl) benzene], 1,6-hexanediol methacrylate (HDDM), camphorquinone (CQ), 2-(dimethylamino)ethyl methacrylate; butylated hydroxyquinone (BHT), 4-(dimethylamino)pyridine (DMAP), dibutyltin dilaurate, sodium sulfate, 4-(4-nitrobenzyl)pyridine, tetraethylene pentamine, hydroxybutyl
Results
The results of the flexural strength, flexural modulus, water sorption and solubility tests are presented in Table 1. ANOVA (p < 0.001) and REGW Multiple Range Test (p < 0.05) indicated significant differences between the urethane composite resins for all four testing parameters. Generally lower mean flexural strengths were noted with resins which contained pendant phenyl methoxy (UHP and UTP) or ethyl (UXB) substituents as compared to resins without pendant moieties extending off the polymer
Discussion
Various factors relating to the chemistry and structure of the polymeric network control the amount of water that a material absorbs when placed in an aqueous environment [14], [26], [27]. Polymerized networks which contain polar groups such as hydroxy, carbamate (urethane), amide, ester and/or ether linkages are hydrophilic in nature and tend to be more susceptible to moisture absorption [10], [14], [28]. This is primarily due to the ability of these groups to form hydrogen bonds with water [7]
Conclusions
The incorporation of pendant hydrophobic substituents within the monomer backbone may be an effective method in reducing the water sorption and water solubility of urethane based dimethacrylate systems. Composite polymers made with the aromatic UXY monomer showed similar physical properties as polymers made with UDMA. These similarities, along with a higher modulus value, give support to making the UXY modified urethane resin a viable alternative to the more commonly used UDMA resin, especially
Acknowledgement
The authors wish to thank Dr. Sunny Skaria for his assistance in evaluating 1H NMR spectrograms.
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