Abstract
Objective
The objective of this study was to synthesize and characterize novel hyperbranched poly(acrylic acid)s via atom-transfer radical polymerization (ATRP) technique and tether the photo-curable methacrylate onto the poly(acrylic acid), use these polymers to formulate the resin-modified glass-ionomer cements, and evaluate the mechanical strengths of the formed cements.
Materials and methods
The hyperbranched poly(acrylic acid)s were synthesized using a self-condensing vinyl polymerization initiator via ATRP. The effects of the concentrations of both catalyst and initiator on molecular weight (MW) and degree of branching (DB) were studied. Compressive, diametral tensile as well as flexural strengths, fracture toughness, hardness and wear-resistance of the experimental cement were evaluated and compared to those of Fuji II LC cement. The specimens were conditioned in distilled water at 37 °C for 24 h prior to testing.
Results
The concentrations of both catalyst and initiator had significant effects on MW and DB of the synthesized polymers. The concentration of the initiator also significantly affected both CS and DTS values of the cement. The experimental cement showed significantly higher mechanical properties, i.e., 53% in CS, 50% in compressive modulus, 125% in DTS, 95% in FS, 21% in FT and 96% in KHN, higher than Fuji II LC. The experimental cement was only 5.4% of abrasive and 6.4% attritional wear depths of Fuji II LC.
Conclusions
This study developed a novel resin-modified glass-ionomer cement system composed of newly synthesized hyperbranched poly(acrylic acid)s. It appears that this novel experimental cement is a clinically attractive dental restorative and may potentially be used for high-wear and high-stress-bearing site restorations.
1
Introduction
Glass-ionomer cement (GIC) is a widely used dental restorative that exhibits numerous advantages including direct adhesion to tooth and base metals due to capability of crosslinking with calcium ions in tooth or metal ions in base metals, anticariogenic properties due to release of fluoride, thermal compatibility with tooth enamel and dentin due to low thermal expansion coefficients similar to that of tooth, minimized microleakage at the tooth–enamel interface due to low shrinkage, and low cytotoxicity due to no monomer (in most conventional GICs or CGICs) or low content of monomers (in some resin-modified GICs or RMGICs) incorporated .
An acid–base reaction between calcium and/or aluminum cations released from a reactive glass and carboxyl anions pendent on polyacid describes the setting and adhesion mechanism of GICs . The polymer backbones of GICs have been made by poly(acrylic acid) homopolymer, poly(acrylic acid-co-itaconic acid) or/and poly(acrylic acid-co-maleic acid) copolymers . These GICs are called conventional glass-ionomer cements (CGICs) . Despite numerous advantages of CGICs, brittleness, low tensile and flexural strengths have limited the current CGICs for use only at certain low stress-bearing sites such as Class III and Class V cavities . Much effort has been made to improve the mechanical strengths of CGICs and the focus has been mainly on improvement of polymer backbone or matrix . Briefly two main strategies have been applied. One is to incorporate hydrophobic pendent (meth)acrylate moieties onto the polyacid backbone or add methacrylate-containing oligomers such as triethyleneglycol dimethacrylate in CGIC combining with 2-hydroxyethyl methacrylate (HEMA) for enhancing organic/aqueous phase compatibility, to make CGIC become light- or redox-initiated resin-modified GICs or RMGICs and the other is to directly increase molecular weight (MW) of the polyacid . As a result, the former has shown significantly improved tensile and flexural strengths as well as handling properties . The strategy of increasing MW of the polyacid by either introducing amino acid derivatives or N-vinylpyrolidone has also shown enhanced mechanical strengths ; however, the working properties were somehow decreased because strong chain entanglements formed in these high MW linear polyacids resulted in an increased solution viscosity . Using visible light and a photo-initiator system to initiate the polymerization in RMGICs has been well welcomed by dentists and dental community because these RMGICs demonstrate extended working time, low moisture sensitivity, high initial mechanical strengths, and significantly improved diametral tensile and flexural strengths as well as fracture toughness as compared to CGIC . However, current commercially available RMGICs still cannot compete with contemporary dental resin composites especially in Class I and II restoration, because they show lower mechanical strengths and wear resistance than resin composites .
Not long ago, our research group developed a star-shape poly(acrylic acid)-constructed RMGIC system for enhanced mechanical properties . It was found that the synthesized star-shape polymers exhibited a lower solution viscosity as compared to their linear counterparts. It was also found that the formulated GICs showed significantly improved mechanical strengths and wear-resistance as compared to commercial Fuji II LC cement . Although the star-shape polymers demonstrated significant advantages over the liner polymers, the six-arm seems to be the maximum arm number that can be well derivatized to form the star-shape polymers. Further increase in arm number requires using or synthesizing bulky initiators. These bulky initiators may have additional unnecessary groups or fragments attached, which would not make any contributions to either salt-bridge formations or covalent crosslinks and thus the mechanical properties of the resultant GIC might be adversely affected.
It is known that hyperbranched polymers also exhibit low solution or melt viscosity. In 1952, Flory proposed and prepared hyperbranched polymers via a condensation polymerization of AB 2 monomers . In 2005, Frechet et al. developed a polymerization method called self-condensing vinyl polymerization (SCVP) to prepare hyperbranched polymers by using a self-condensing vinyl polymerization initiator or an inimer . The inimer is a vinyl monomer containing a functional group that can be activated to initiate the vinyl polymerization (see Fig. 1 a). Since then several research groups have utilized this method to polymerize styrene and (meth)acrylates via controlled radical polymerization techniques . The resultant hyperbranched polymers exhibited a lower intrinsic viscosity than their linear counterparts. In this research we proposed to use “inimer” as an initiator, which is easier to be prepared than those star-shape initiators, to generate a hyperbranched poly(acrylic acid) system for improved GIC dental restoratives. We also chose to use the newly synthesized hyperbranched polymer to develop a light-curable RMGIC system instead of a self-cured CGIC one because the former is more attractive to dental community and is expected to have significantly improved mechanical properties.
The objective of this study was to synthesize and characterize novel hyperbranched poly(acrylic acid)s via atom-transfer radical polymerization (ATRP) technique and tether the photo-curable methacrylate onto the poly(acrylic acid), use these polymers to formulate the resin-modified glass-ionomer cements, and evaluate the mechanical strengths of the formed cements.
2
Materials and methods
2.1
Materials
2-Hydroxylethylacrylate (HEA), 2-bromopropionic acid (BPA), N,N′-dicyclohexylcarbodiimide (DCC), pyridine, CuBr, N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA), dl-camphoroquinone, 2-(dimethylamino)ethyl methacrylate (DMAEMA), tert-butyl acrylate (t-BA), glycidyl methacrylate (GM), hydrochloric acid (37%), diethyl ether, dioxane, N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) were used as received from VWR International Inc (Bristol, CT) without further purifications. Fuji II LC cement and Fuji II LC glass powders were used as received from GC America Inc (Alsip, IL).
2.2
Synthesis of the initiator for preparing the hyperbranched polymers
The initiator, 2-(2-bromopropionyloxyl) ethyl acrylate (BPEA), for preparing hyperbranched polymers was synthesized as shown below: to a flask containing BPA (10.2 mmol), HEA (9.7 mmol), pyridine (3%, by mole, of HEA) and THF (15 ml) in an ice-water bath, a solution of DCC (10.2 mmol) in THF (25 ml) was added dropwise. The reaction was kept at room temperature for 4 h. After the precipitates were filtered off, the solvent was removed by a rotary evaporator and the product was further purified by distillation under reduced pressure.
2.3
Synthesis of the GM-tethered hyperbranched poly(AA)
The GM-tethered hyperbranched poly(acrylic acid) or poly(AA) was synthesized via three steps: synthesis of hyperbranched poly(t-BA) via ATRP, conversion of poly(t-BA) to poly(AA) and tethering of GM onto poly(AA). For synthesis of poly(t-BA), to a flask containing dioxane, a mixture of BPEA (initiator), PMDETA (ligand) and t-BA was charged with a predetermined ratio. After the above solution was degassed and nitrogen-purged via three freeze–thaw cycles, CuBr (3%, by mole) was incorporated. The solution was then heated to 120 °C to initiate the ATRP . The proton nuclear magnetic resonance ( 1 HNMR) spectrometer was used to monitor the reaction. After the polymerization was complete, the poly(t-BA) polymer was precipitated from water. CuBr and PMDEMA were removed by re-precipitated from dioxane/water. For conversion of poly(t-BA) to poly(AA), the poly(t-BA) polymer was hydrolyzed in a mixed solvent of dioxane and HCl (37%) (dioxane/HCl = 1/3) under refluxed condition for 18 h . The formed poly(AA) was dialyzed against water until the pH became neutral. The purified hyperbranched poly(AA) was obtained through freeze-drying. For GM tethering , to a flask containing the hyperbranched poly(AA), THF and BHT (1%, by weight), a mixture of GM, THF, and pyridine (1% of GM, by weight) was added dropwise. Under a nitrogen blanket, the reaction was initiated and run at 60 °C for 5 h and then kept at room temperature overnight. The polymer tethered with GM was recovered by precipitation from diethyl ether, followed by drying in a vacuum oven at room temperature. The overall synthesis scheme is shown in Fig. 2 .
2.4
Characterization
The synthesized initiator and polymers were characterized by 1 HNMR spectroscopy using a 500 MHz Bruker NMR spectrometer (Bruker Avance II, Bruker BioSpin Corporation, Billerica, MA). The deuterated methyl sulfoxide ( d -DMSO) and chloroform (CDCl 3 ) were used as solvents. The molecular weight (MW) and molecular weight distribution (MWD) or polydispersity index (PDI) of the synthesized poly(t-BA)s were determined in THF via a Waters GPC unit (2410 reflective index detector, 717 autosampler and 510 pump, Waters Corp., Milford, MA), with standard GPC techniques, using a polystyrene standard.
2.5
Sample preparation
The experimental cements were formulated with a two-component system (liquid and powder) . The liquid was formulated with the GM-tethered polymer, water (polymer/water ( P / W ) ratio = 70/30, by weight), CQ (photo-initiator, 0.9%, by weight) and DMAEDA (activator, 1.8%). Fuji II LC glass powder was used to formulate the experimental cements with a powder/liquid ratio of 2.7 unless specified. Fuji II LC cement was used as control and prepared per manufacturer’s instruction where the P/L ratio = 3.2.
Specimens were fabricated at room temperature according to the published protocols . Briefly, the specimens were prepared for different tests following the geometries below: (1) cylindrical specimens (4 mm in diameter × 8 mm in length) for compressive strength (CS); (2) disk specimens (4 mm in diameter × 2 mm in thickness) for diametral tensile strength (DTS); (3) rectangular specimens (3 mm in width × 3 mm in thickness × 25 mm in length) for flexural strength (FS); (4) rectangular specimens (4 mm in width × 2 mm in thickness × 20 mm in length), fitted with a sharp blade for generating 2-mm-long notch, for fracture toughness (FT) ; (6) disk specimens (4 mm in diameter × 2 mm in height), where the smooth surface at the diametral side was generated by pressing the cement against a microscopic slide before setting, for Knoop hardness; and (7) rectangular specimens (4 mm in width × 2 mm in thickness × 10 mm in length) for wear tests. All the specimens were exposed to blue light (EXAKT 520 Blue Light Polymerization Unit, GmbH, Germany) for 2 min, followed by conditioning at 37 °C in 100% humidity for 15 min and then in distilled water for 24 h prior to testing.
2.6
Evaluation
CS, DTS, FS and FT tests were performed on a screw-driven mechanical tester (QTest QT/10, MTS Systems Corp., Eden Prairie, MN), with a crosshead speed of 1 mm/min. The FS and FT tests were performed in three-point bending, with a span of 20 mm and 16 mm, respectively, between supports. The sample sizes were n = 6–8 for each test. CS was calculated using an equation of CS = P /π r 2 , where P = the load at fracture and r = the radius of the cylinder. DTS was determined from the relationship DTS = 2 P /π dt , where P = the load at fracture, d = the diameter of the cylinder and t = the thickness of the cylinder. FS was obtained using the expression FS = 3 P l/2 bd 2 , where P = the load at fracture, l = the distance between the two supports, b = the breadth of the specimen, and d = the depth of the specimen. The FT was calculated from the equation K IC = P S / B Wf ( a / W ), where K IC = the index for FT, P = the load at fracture, S = the distance between supports, a = the length of notch, B = the thickness, and W = the width of specimen. The f is a function of ( a / W ), as shown below :
f ( x ) = 3 x 0.5 [ 1.99 − x ( 1 − x ) ( 2.15 − 3.93 x + 2.7 x 2 ) ] 2 ( 1 + 2 x ) ( 1 − x ) 1.5
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