This work investigates the graft polymerization of acrylic acid onto nanoclay platelets to be utilized as reinforcing fillers in an experimental dental adhesive. Physical and mechanical properties of the adhesive and its shear bond strength to dentin are studied. The effect of the modification on the stability of the nanoparticle dispersion in the dilute adhesive is also investigated.
Materials and methods
Poly(acrylic acid) (PAA) was grafted onto the pristine Na-MMT nanoclay (Cloisite ® Na + ) through the free radical polymerization of acylic acid in an aqueous media. The resulting PAA-g-nanoclay was characterized using FTIR, TGA and X-ray diffraction (XRD). The modified nanoclays were added to an experimental dental adhesive in different concentrations and the morphology of the nanoclay layers in the photocured adhesive matrix was studied using TEM and XRD. Shear bond strength of the adhesives containing different filler contents was tested on the human premolar teeth. The stability of nanoclay dispersion in the dilute adhesive was also studied using a separation analyzer. The results were then statistically analyzed and compared.
The results confirmed the grafting reaction and revealed a partially exfoliated structure for the PAA-g-nanoclay. Incorporation of 0.2 wt.% of the modified nanoclay into the experimental adhesive provided higher shear bond strength. The dispersion stability of the modified nanoparticles in the dilute adhesive was also enhanced more than 25 times.
Incorporation of the modified particles as reinforcing fillers into the adhesive resulted in higher mechanical properties. The nanofiller containing bonding agent also showed higher shear bond strength due to the probable interaction of the carboxylic acid functional groups on the surface of the modified particles with hydroxyapatite of dentin.
Dental adhesives are applied to bond restorative materials to tooth structure . Adhesion to the enamel is much durable than the dentin due to the dynamic properties of the wet dentin . Dentin is a hydrated biological dynamic composite which consists of about 30% water and organic materials and 70% minerals . A weak bonding between the restorative materials and dentin results in problems such as secondary caries, sensivity after restoration, color change and microleakage . Several mechanisms are involved in the adhesion between dental adhesives and tooth structure including physical adsorption, micromechanical interlocking, chemical and ionic bondings and acid–base interactions. In the developments of dental adhesives attempts have been made to improve the mechanisms or to impart more mechanisms in the adhesion providing a stronger and more reliable bond for adhesive dentistry as well as simplifying the clinical procedures. The attempts have been resulted in the introduction of different generations which are different in chemistry, mechanism, number of bottles, application techniques and clinical effectiveness. In general, new generations of the dental bonding agents contain functional monomers, cross-linking agents, solvents and polymerization initiators. Bifunctional molecules with carboxylic acid or phosphate groups at one end, which are able to bond to the tooth structure through ionic interactions, and (meth)acrylate functions at the other end, which provide covalent bonds with the composite monomers, are the main components of the bonding systems. Water chasing solvents such as ethanol and acetone, however, make the bonding monomers penetrate into the dentin structures which subsequently result in a hybrid layer from polymerized resin and collagen fibrils . Cross-linking takes place via the polymerization of multifunctional (meth)acrylates which are initiated through the activation of the initiator system. A micromechanical retention is then formed between resin and dentin/enamel surfaces along with the physico–chemical interactions . Therefore, any improvement in the mechanical properties of the bonding interface and the enhancement in the physico–chemical interactions between bonding agents and the tooth structure would be beneficial in achieving a strong and durable adhesion and consequently clinical effectiveness.
Nanoparticles, used as reinforcing fillers in composites, have shown remarkable impacts on the nanocomposite properties when incorporated in very small amounts in comparison to the macro/micro fillers .
The application of different nanoparticles in dental adhesives has also been reported to improve the bonding agent properties . The application of modified nanoclay particles, however, is limited to the authors’ previous work ; this might be due to the difficulties in the exfoliation of the nanoclay galleries and the sedimentation of the particles in the dilute adhesive solution. Montmorillonite (Na-MMT) is one of the nanoclay structures which occur in the nature as platelets with about 1 nm thickness and aspect ratio of up to 1000. Because of the high aspect ratio of the platelets, nanoclay is efficiently used for the improvement of mechanical properties of polymeric systems . Grafting is an efficient method to create functional groups onto the surface of nanoparticles. In our previous work, the pristine Na-MMT nanoclay, was modified via graft polymerization of methyl methacrylate, which resulted in significant improvements in the bulk and adhesion properties of an experimental dental adhesive .
In this study, the pristine Na-MMT nanoparticles were modified through graft polymerization of acrylic acid (AA) monomers onto the platelets. AA consists of carboxylic groups in its structure which may interact with Ca 2+ ions of hydroxyapatite providing chemical interactions between the adhesive and dentin structure while the platelet-like nanoclay particles may improve the bulk mechanical properties of the adhesive layer. The poly(acrylic acid) grafted nanoclay platelets (PAA-g-nanoclay) were characterized and their dispersion stability in an experimental dilute dental adhesive was investigated. Microshear bond strength of the adhesive containing different amounts of PAA-g-nanoclay was also studied.
2-Hydroxyethyl methacrylate (HEMA), camphorquinone (CQ), 2-ethyl-2-hydroxymethyl-1,3-propandiol trimethacrylate (TMPTMA), acrylic acid (AA), acetone, ethanol and methanol were purchased from Merck (Germany). N , N -Dimethyl aminoethyl methacrylate (DMAEMA) and tert -dodecyl mercaptan (TDM) were obtained from Fluka (Germany). 2,2-Bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl] propane (Bis-GMA) was kindly supplied by Evonik (Germany). Cloisite ® Na + was obtained from Southern Clays Product, Inc. (USA). 2-Acrylamido-2-methyl-1-propane-sulfonic acid (AMPS), ammonium persulfate and lithium chloride (LiCl) were purchased from Sigma–Aldrich (Germany). Adper™ Single Bond 2, a commercially available nanoparticle containing dentin bonding, were obtained from 3M ESPE (USA). The 37.5% phosphoric acid gel (Kerr Gel Etchant) was obtained from SDS Kerr (USA). Deionized water was used throughout all the experiments.
Graft polymerization of acrylic acid onto the nanoclay platelets
The free radical graft polymerization of acrylic acid onto the surface of nanoclay was carried out in water in presence of AMPS. 0.5 wt.% dispersion of nanoclay in 1 L of deionized water was prepared. The temperature was held at 50 °C for 12 h while stirring the suspension. Aqueous solutions containing 2.5 g AMPS, as reactive surfactant, and 2 g ammonium persulfate, as polymerization initiator were added to the clay dispersions. 50 ml monomer (acrylic acid) and 2 ml TDM as chain transfer agent were then added to the reaction medium. The temperature was increased to 70 °C while stirring. The polymerization reaction was completed in 1 h. The product was then isolated by dropping the reaction mixture into methanol as a non-solvent to AA. The precipitate was vacuum-filtered and washed several times with distilled water. Finally, the filtrate was washed with ethanol to remove the unreacted monomers and vacuum dried at room temperature for 24 h. The product were then Soxhlet-extracted for 72 h using water as solvent to remove the unbonded homopolymers (poly acrylic acid, PAA) produced during the polymerization reaction. The purified PAA grafted nanoclay was vacuum dried and powdered using a ball mill (PM100, Retch, Germany), passed through a 400 mesh (ASTM) sieve and used as filler. The schematic representation of the grafting of acrylic acid onto nanoclay platelets is shown in Fig. 1 .
Characterization of the PAA grafted nanoclay
The pristine Na-MMT and PAA-g-nanoclay were analyzed by FTIR spectroscopy (EQUINOX 55, Bruker, Germany) at a resolution of 4 cm −1 and 32 scans in the range of 4000–400 cm −1 using KBr disc technique. Thermogravimetric analyses of the pristine Na-MMT, PAA-g-nanoclay and pure PAA were performed (TGA-1500, Polymer Laboratories, UK) from room temperature to 600 °C at a heating rate of 10 °C/min and under N 2 atmosphere. X-ray diffraction patterns of pristine Na-MMT and PAA-g-nanoclay were collected in the range of 2 = 1.5–10° and step size of 0.02°, using a Philips X-ray difractometer (Philips, X’pert, Netherlands) with copper target, λ = 1.54056 Å operating at a voltage of 40 kV and a current of 40 mA at the rate of 2°/min. The d 0 0 1 spacings were calculated according to the Bragg’s equation: n λ = 2 d sin.
Reverse ion exchange was used to debond the PAA chains from the clay surface by stirring the purified PAA-g-nanoclay in a 10 g/L LiCl aqueous solution at 50 °C for 5 days . The nanoclay was filtered and dried; and the debonded polymer in the solution was precipitated in acetone, washed, and dried. The molecular weights of the debonded polymer and the unbonded homopolymer which was extracted during Soxhlet-extraction, were determined by gel permeation chromatography (GPC, Agilent 1100 series, USA) using Aquagel column of 7.5 mm × 300 mm (ID × L) with the flow rate of 1 ml/min at 23 °C. Thermogravimetric analyses of the debonded polymer, homopolymer, and the nanoclay were then performed.
Preparation of adhesives
The adhesive was prepared according to the formulation shown in Table 1 . Pristine NA-MMT and PAA-g-nanoclay were added to the adhesive in 0.2, 0.5, 1, 2 and 5 wt.%. The fillers were well dispersed in the adhesive solution by ultra-sonication using a probe sonication apparatus (Sonoplus UW2200, Bandelin, Germany) for 3 min in an ice bath. The stability of the dispersions were investigated by a separation analyzer (LUMiReader ® 416.1, LUM, Germany) working with visible light and intensities of 25% and tilt of 0, for 12 h including 256 intervals.
|Bis[4-(2-hydroxy-3-methacryloyloxyproxy)phenyl] propane (Bis-GMA)||14|
|2-Hydroxy ethyl methacrylate (HEMA)||26|
|2-Ethyl-2-(hydroxymethyl)-1,3-propandiol trimethacrylate (TMPTMA)||8|
|Urethane dimethacrylate (UDMA)||12|
|Camphorquinone (CQ)||0.5 a|
|N , N -Dimethyl aminoethyl methacrylate (DMAEMA)||0.5 a|
Measurement of degree of conversion
A small droplet of the adhesives containing 0.5 wt.% CQ and 0.5 wt.% DMAEMA as photoinitiator system were placed on a polyethylene film. The solvent of the adhesive was gently evaporated for 10 s applying a low-pressure air stream and a second film was placed on it to form a very thin layer. The sandwich was placed into the FTIR spectrometer’s sample holder and the absorbance peaks of the uncured adhesives were collected. The samples were then light cured for 40 s using a dental light source with an irradiance of ca. 600 mW/cm 2 (Optilux 501, SDS Kerr, Germany) and the absorptions were collected for the cured samples. The degree of conversion (DC%) was determined from the ratio of absorbance intensities of aliphatic C–C (peak at 1638 cm −1 ) against internal reference of aromatic C C (peak at 1608 cm −1 ) before and after curing of the specimen as follows:
DC ( % ) = 1 − ( 1638 cm − 1 / 1608 cm − 1 ) peak area after curing ( 1638 cm − 1 / 1608 cm − 1 ) peak area before curing × 100