Structure–property relationship in new photo-cured dimethacrylate-based dental resins
Introduction
The photopolymerization of multifunctional methacrylates and acrylates to form densely crosslinked networks finds wide application in dentistry such as dental composites, dental adhesives, bonding agents and cements, pit and fissure sealants [1], [2], [3], [4], [5], [6].
Typical dental composites consist of dimethacrylate/diacrylate organic systems in which inorganic filling particles are imbedded. As the main component of the organic phase 2,2-bis[4-(2′-hydroxy-3′-methacryloxypropoxy)phenyl]-propane (Bis-GMA) is commonly used. Its rigid aromatic backbone structure provides superior toughness and low polymerization shrinkage of the dentin formulations although it is not without disadvantages. Bis-GMA is a highly viscous monomer and requires dilution. When polymerized in the bulk, it exhibits low degree of double bond conversion due to restriction in segmental motion [2]. Besides, unreacted monomer species are present in the polymer matrix [7]. Incomplete monomer conversion affects the mechanical properties of ultimate restoration [8] whereas the elution of components from the polymer matrix worsens its biocompatibility [7], [9]. Diluting Bis-GMA with low viscosity alkoxyalkyl dimethacrylate esters such as triethyleneglycol dimethacrylate (TEGDMA) enables adequate filling loading and improves handling characteristics [10], [11]. However, it also increases water uptake and polymerization shrinkage and as a consequence deteriorates marginal adaptation of a composite [12], [13].
Several approaches to increasing conversion of double bonds and thus biocompatibility and mechanical strength of dental dimethacrylates have been reported [14], [15], [16], [17], [18], [19]. Partial improvement of performance of dental compositions has been achieved by incorporating other low viscosity dimethacrylate monomers like 1,6-bis(2′-methacryloxyethoxycarbonylamino)-2,4,4-trimethylhexane (UDMA) or hydroxyl free analogs of Bis-GMA [17], [18], [19], [20]. Recently, incorporating thiol-ene-based monomers into compositions with conventional dimethacrylates (and thus changing the chain growth mechanism of polymerization to a step growth one) has been found to increase the overall conversion without compromising other properties like volumetric shrinkage or water sorption [21], [22], [23]. Also, employing dimethacrylates and urethane dimethacrylates with high molecular weights is an alternative [24], [25].
The aim of this work is to investigate the influence of the chemical structure of dimethacrylate esters of different chain lengths on the degree of conversion, polymerization shrinkage and mechanical strength of dental compositions.
Carrying out the experiments, four new hydroxyl-containing monomers of similar chemical origin to Bis-GMA as well as one hydroxyl-free in which the reactive methacrylate groups are relatively far apart were synthesized and identified. An increased flexibility of the monomers in respect to Bis-GMA has been achieved by incorporating a flexible hydrocarbon chain between the aromatic rings in the middle of their structures. The new monomers were photo-copolymerized with TEGDMA under the same conditions in order to obtain comparable results. The properties of new compositions were compared with those of Bis-GMA/TEGDMA analog preparation. Also, acetyloxypropylene dimethacrylate (Acet-GDMA) was evaluated as a dental diluent.
Section snippets
Materials and instruments
Triethyleneglycol dimethacrylate (TEGDMA, 94%, from Merck KgaA Frankfurt), glycidyl methacrylate (GMA, 97%, from Sigma–Aldrich Chemie GmbH), tetraethylammonium bromide (analytically pure, from Merck KgaA Frankfurt), phthalic anhydride (analytically pure, from Merck KgaA Frankfurt), 1,3-propylene, 1,4-butylene, 1,5-penthylene and 1,6-hexylene glycols (99%, from Sigma–Aldrich Chemie GmbH), hydroquinone (analytically pure, from Merck KgaA Frankfurt), 2,2-dimethoxy-2-phenyloacetophenone (Irgacure
Results
The chemical structures of the dimethacrylate monomers are shown in Fig. 1. They have been fully confirmed by FTIR and 1H NMR spectroscopy as well as elemental analysis. Analyzing the spectroscopic data it is evident that the monomers constitute mixtures of linear and branched isomers with primary and secondary hydroxyl and/or acetyl groups. This is denoted by the presence of chemical shifts at 4.4 ppm and 3.8 ppm for hydroxyl-containing dimethacrylates and 4.4 ppm and 5.3 ppm for the acetylated
Discussion
The new dimethacrylate monomers were readily prepared in good yields and their basic physical properties were determined. It has been proved in the present study that, similar to the previous reports, the ring-opening reaction of epoxides leads to a mixture of isomers [26]. Depending upon the reaction temperature and the selectivity of the catalyst, isomers with the primary hydroxyl groups in the amount of 15–25% are produced [26], [27].
The viscosities of the compositions are obviously
Conclusions
A series of homologous dimethacrylates was obtained and evaluated as possible dental resins. The monomer characteristics and the properties of water sorption and solubility were found to be comparable with the control Bis-GMA copolymer. The flexural properties and storage moduli were shown to better than those of Bis-GMA analog preparation. Therefore, the new dimethacrylates show the potential of possible use as dental dimethacrylates. Based on the obtained results, there were shown
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