In this study acetyloxypropylene dimethacrylate was synthesized and proposed as an alternative monomer for triethyleneglycol dimethacrylate (TEGDMA) in dental mixtures.
The monomer was prepared by the reaction of glycerol dimethacrylate with acetic anhydride. The exchange reaction was carried out in the presence of catalytic amounts of sulfuric acid. After purification the monomer was mixed with 2,2-dimethoxy-2-phenyloacetophenone and photo-irradiated. Unfilled homopolymer was evaluated for photopolymerization conversion and volumetric curing shrinkage. Water sorption, water solubility, flexural strength and hardness were measured. Also, dynamic mechanical studies were performed. For comparison TEGDMA was homopolymerized and its properties were evaluated.
The proposed dimethacrylate has viscosity comparable to TEGDMA, lower curing shrinkage and lower degree of double bond conversion. After homopolymerization, its water sorption is much lower than that of homopolymerized TEGDMA. Concerning the mechanical properties, the homopolymerized acetyloxypropylene dimethacrylate has higher modulus and hardness than analogically cured TEGDMA.
New dimethacrylate is a promising photocurable dental diluent owing to its low viscosity, good mechanical and water uptake properties.
Photo-curable polymeric composites are routinely used as filling materials for dental restorations. These materials consist of multimethacrylate/acrylate 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 other desirable properties of the dentin compositions although it is not without disadvantages. Bis-GMA is a highly viscous monomer and requires dilution with low viscosity alkoxyalkyl dimethacrylate esters, the most popular of which is triethyleneglycol dimethacrylate (TEGDMA). Adding TEGDMA to a composition enables adequate filling loading and improves handling characteristics. However, dilution also increases water uptake and polymerization shrinkage and as a consequence, worsens marginal adaptation of a composite.
Several approaches to reducing water sorption and curing shrinkage of dental composites have been reported . Partial improvement of the above-mentioned parameters 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 . Also, employing dimethacrylate esters with bulky (substituent or pendant) groups or high molecular weight urethane dimethacrylates could be an alternative .
Recently, the implementation of dental restorative materials based on resins comprised methacrylate–thiol-ene systems has been considered . Because thiol-ene-based resins exhibit a step growth radical polymerization mechanism as opposed to the chain growth polymerization mechanism of dimethacrylates, they have numerous kinetic advantages. Methacrylate–thiol-ene polymerizations result in reduced volume shrinkage and higher functional group conversions. Besides, the risk of oxygen inhibition is minimized in such systems and that makes the polymerization initiation more effective.
The relevant literature shows also examples where highly reactive mono-methacrylate monomers were evaluated as diluents for Bis-GMA-based composites . Such formulations exhibited increased curing rates and superior mechanical properties. Despite all these attempts, TEGDMA is still present in the majority of commercial dental formulations.
In this work the synthesis of acetyloxypropylene dimethacrylate (Acet-GDMA) prepared from glycerol dimethacrylate by a simple one-step exchange reaction with acetic anhydride is reported. Its viscosity was evaluated and compared to that of TEGDMA. The DSC technique was used to measure the double bond conversions and the DMA was performed on the obtained homopolymers. Water sorption and solubility as well as volumetric curing shrinkage were also investigated. Based on the obtained results the comparison of the properties of TEGDMA and Acet-GDMA is presented and the potential of Acet-GDMA as a dental monomer (diluent) is discussed.
Materials and methods
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), methacrylate acid (99%, from Merck KgaA Frankfurt), hydroquinone (analytically pure, from Merck KgaA Frankfurt), 2,2-dimethoxy-2-phenyloacetophenone (Irgacure 651, UV polymerization initiator, from Ciba Specialty Chemicals) benzoil peroxide (BPO, analytically pure, from Sigma–Aldrich Chemie GmbH), sulfuric acid (98%, from POCh Gliwice), acetic anhydride (analytically pure, from POCh Gliwice), diethyl ether (analytically pure, from POCh Gliwice), anhydrous magnesium sulfate (analytically pure, from POCh Gliwice), anhydrous calcium chloride (analytically pure, from POCh Gliwice), sodium hydrogen carbonate (analytically pure, from POCh Gliwice), and zinc chloride (analytically pure, from POCh Gliwice) were used as received.
1 H NMR spectra of purified methacrylate monomers were recorded at 20 °C using an FT-NMR Bruker Avance (Germany) spectrometer operating at the 1 H resonance frequency of 300 MHz. Chemical shifts were referenced to tetramethyl silane serving as an internal standard. Elementary analysis was carried out using the Perkin Elmer CHN 2400 apparatus. Differential scanning calorimetry (DSC) measurements were performed on a NETZSCH 204 apparatus in the temperature range 20–200 °C at 10 °C/min. The samples weighted around 5–6 mg. The calorimeter was calibrated with indium (Δ H fusion 28.6 J/g, melting point 156.6 °C) prior to use. An empty aluminum pan was used as a reference. Viscosities of the methacrylate monomers were measured by means of a rotating spindle rheometer (Brookfield, model DV-III) using appropriate spindles and standard solutions. Viscosity was measured at various spindle speeds (5–150 rpm) and only readings obtained around 50% torque were recorded and expressed as Pa s. Refractive indices were evaluated at 20 °C by means of a Carl Zeiss Jena refractometer according to PN-88/C-89082/06 specification. PH level was measured by a basic titration method (PN-87/C-89082/11 specification).
The effect of temperature on viscoelastic properties of the obtained polymers in the area of linear dependence between stress and strain was determined using a DMA Q800 apparatus produced by TA (USA) calibrated according to the producer’s recommendation. Thermomechanical properties of the cured polymers were evaluated from storage modulus ( E ′), mechanical loss ( E ″) and tan δ curves obtained at constant frequency (10 Hz). Measurements for all samples were made in the temperature range 0–250/300 °C at a constant heating rate of 4 °C/min. Rectangular profiles of the sizes: (4 ± 0.2 mm) × (10 ± 0.2 mm) × (35 mm) were used in the measurements. Flexural properties were determined in a three-point loading configuration using a Zwick Roell Strength Machine (model Z010). Specimen dimensions were: (10 ± 0.2 mm) × (4 ± 0.2 mm) × (75 mm). Measurements were carried out at room temperature with a crosshead speed of 5 mm/min. At least six specimens were tested for every datum point (PN-82/C-89051). Hardness according to Brinell was determined by means of a hardness tester HPK (PN-84/C-89030).
Syntheses of GDMA and Acet-GDMA
Glycerol dimethacrylate (hydroxypropylene dimethacrylate; GDMA) was prepared by the reaction of glycidyl methacrylate with methacrylic acid. The addition reaction of glycidyl methacrylate and the acidic compound was carried out in the presence of tetraethylammonium bromide. The synthesis was conducted in the presence of the inhibitor of free radical polymerization – hydroquinone. The detailed procedure is described elsewhere . The acetylated dimethacrylate (Acet-GDMA) was obtained by reacting acetic anhydride with GDMA.
To a mixture of GDMA (100 g, 0.438 m) and acetic anhydride (45.6 g, 0.446 m), sulfuric acid (5 drops, 0.05 g) was added with stirring at room temperature. The temperature was then kept at 85 °C for 3 h. After cooling the mixture was washed twice with distilled water. Then it was dissolved in diethyl ether and washed repeatedly with 5 wt.% of aqueous NaHCO 3 , and distilled water, and then dried over anhydrous magnesium sulfate. After filtration, the solvent was removed under reduced pressure at 50 °C. A pale yellow liquid was obtained (94 g, 79%).
Molecular structures of dimethacrylates have been confirmed by 1 H NMR spectra and elemental analysis. 1 H NMR: (CDCl 3 ): GDMA: δ 1.9 (6H), 4.3 (4H), 4.4 (1H), 5.6, 6.2 (4H), 3.8 (2H); Acet-GDMA: 1.9 (6H), 5.3 (1H), 4.4 (4H), 5.6, 6.2 (4H), 4.3 (2H), 2.1 (3H);
Elemental analysis of the obtained dimethacrylate esters is as follows.
The theoretical composition of GDMA: %C: 57.88; %H: 7.07; the determined composition: %C: 58.01; %H: 7.19.
The theoretical composition of Acet-GDMA: %C: 57.77; %H: 6.71; the determined composition: %C: 58.27; %H: 6.76.
The obtained dimethacrylates were irradiated by an ultraviolet light (340–360 nm) in the presence of a photoinitiator (Irgacure-651) used in the amount of 1 wt.%. Photopolymerization was carried out in specially designed glass molds and initiated with four TL20W/05 SLV low-pressure mercury lamps at 25 °C. The compositions in the molds were irradiated from both sides for 10 min. The obtained homopolymers were subsequently postcured at 120 °C for 2 h.
Measurement of double bond conversion
The photopolymerization conversion (DC) of dimethacrylates was measured by DSC. For initiating of the polymerization process benzoil peroxide was used in the amount of 1 wt.%. To calculate the DC the following equation was used:
DC ( % ) = Δ H M w H 0 n m × 100 ,
where Δ H is the curing enthalpy of the sample, M w is the molecular weight of a dimethacrylate, H 0 is the theoretical enthalpy of polymerization of one functional group (for the methacrylate double bond it equals 57 kJ/mol ), n is the number of methacrylic groups per a monomer molecule, and m is the weight of the sample.
Measurement of water sorption and solubility
Water sorption and water solubility were measured according to ISO 4049. Five samples, 15 ± 1 mm in diameter and 1.5 ± 0.1 mm thickness were prepared. The photoirradiation conditions were the same as those used in other tests. The samples were dried in air at 37 °C until their weight was constant, and this result was recorded as m 1 . The specimens were then immersed in water and maintained at 37 °C for a week. After this time, the samples were removed, blotted to remove surface water, dried in air for 15 s, and weighed. The result was recorded as m 2 . After this weighing, the specimens were placed in the desiccator that contained anhydrous calcium chloride and dried at 37 °C until a final constant mass was obtained ( m 3 ). The volumes of the specimens ( V ) were also measured.
To calculate the water sorption ( w sp ) and solubility ( w sl ) the following equations were used:
w sp = m 2 − m 1 V , w sl = m 3 − m 1 V .