Abstract
Objectives
Developing a novel dentin bonding system containing poly(methacrylic acid)-grafted-nanoclay (PMAA-g-nanoclay) as reinforcing filler, with high stability of nanoparticle dispersion and improved bond strength and mechanical properties were the main objectives of this study.
Materials and methods
Poly(methacrylic acid) (PMAA) was grafted onto the pristine sodium montmorrillonite (Na-MMT) nanoclay surface and characterized using FTIR, TGA, and X-ray diffraction (XRD). The PMAA-g-nanoclay was incorporated into an experimental dentin bonding system as filler in different concentrations and stability of nanoclay dispersion in the dilute adhesive, morphology of nanoclay layers in the photocured adhesive matrix, shear bond strength to caries-free extracted human premolar teeth, and mode of failure were studied. The mechanical properties including diametral tensile strength (DTS), flexural strength (FS), and flexural modulus (FM) were also investigated. The measured FM was also compared to theoretical prediction models.
Results
The grafting of PMAA onto the nanoclay surface was confirmed and the results revealed a partially exfoliated structure for PMAA-g-nanoclay. The dispersion stability of the modified nanoparticles in the dilute adhesive increased more than 45 times in comparison with the pristine nanoclay. The incorporation of 0.5 wt.% PMAA-g-nanoclay to the adhesive resulted in a significant increase in microshear bond strength, DTS, and FS. Higher PMAA-g-nanoclay contents resulted in increased flexural modulus. The experimental flexural modulus was in good agreement with the Halpin–Tsai theoretical model.
Significance
Incorporation of PMAA-g-nanoclay particles as novel functional fillers into dental adhesive could result in the development of bonding systems with improved physical, mechanical, and adhesion properties.
1
Introduction
The main function of dental adhesives is to bond dental restorative materials to the tooth structure . The adhesion is necessary to avoid secondary carries, sensitivity after restoration, color change, and microleakage . Dentin is a hydrated biological composite which consists of water, organic matrix and mineral materials with dentinal tubules located in the radius of dentin in which a fluid flows from inside to outside direction . Due to the wet and dynamic structure, adhesion to dentin is more complicated than enamel . Penetration of bonding monomers into the dentin structures, which subsequently results in a hybrid layer of polymerized resin and collagen fibrils and strengthening of adhesion to dentin, is increased by using water chasing solvents like ethanol . Formation of the hybrid layer along with the resin tags provide a micromechanical retention between resin and dentin . The elastic modulus of the adhesive layer formed at the resin–dentin interface is the lowest comparing to dentin and restorative materials . The low elastic modulus causes the failure of the restoration when the occlusal loading is more than the strength of the layer . It has been shown that the incorporation of small amounts of nanofillers into polymeric matrices improves mechanical and fracture properties of the composites . Na-MMT is a nanoclay which occurs in the nature as platelets which are 1 nm thick and up to 1 μm in diameter. Because of the high aspect ratio, nanoclay is efficiently used to improve mechanical properties of polymeric systems . Incorporation of clay nanoparticles into the dilute systems such as dentin bondings, however, results in the fast sedimentation of the particles due to their higher densities. Surface modification of nanoclay platelets with a polymer, has been shown, to solve the problem due to the decrease in the density of the particles and providing interactions between the newly attached functional groups on the surface of particles with the solvent. Grafting is an efficient method to introduce functional groups onto the surface of nanoparticles . In our previous works, Na-MMT pristine nanoclay surface, was modified via graft polymerization of methyl methacrylate and acrylic acid to be used as reinforcing fillers for dental adhesives .
In this study, the Na-MMT pristine nanoclay surface was modified with methacrylic acid monomers, which consist of carboxylic groups. The carboxylic acid functional groups could interact with Ca 2+ ions of hydroxyapatite promoting the adhesion between the restorative materials and dentin . Having grafted PMAA chains onto the surface of nanoclay, the grafted nanoclay platelets were characterized and their dispersion stability in an experimental dental adhesive was investigated. Mechanical properties and microshear bond strength of the dentin bonding system containing PMAA-g-nanoclay were investigated. The elastic properties of the adhesives were also modeled using theoretical prediction models.
2
Experimental
2.1
Materials
2-Hydroxyethyl methacrylate (HEMA), 2,3-butanedione (BD), 2-ethyl-2-hydroxymethyl-1,3-propandiol trimethacrylate (TMPTMA), methacrylic acid (MAA), acetone, ethanol and methanol was purchased from Merck (Germany). tert -Dodecyl mercaptan (TDM) were obtained from Fluka (Germany). 2,2-Bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl] propane (Bis-GMA) was kindly donated by Evonik (Germany). Na-MMT (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, was 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.
2.2
Methods
2.2.1
Graft polymerization of methacrylic acid onto the surface of nanoclay platelets
The graft polymerization of methacrylic acid onto the surface of nanoclay was carried out according to the previous works . Briefly, MAA was grafted onto the clay platelets in an aqueous solution in the presence of AMPS as reactive surfactant and TDM as a chain transfer agent to control the molecular weight of the grafted polymer.
2.2.2
Characterization of the PMAA grafted nanoclay
The pristine Na-MMT and PMAA-grafted 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, PMAA-g-nanoclay and pure PMAA were performed (TGA-1500, Polymer Laboratories, UK) from room temperature to 600 °C at a heating rate of 10 °C min −1 and under N 2 atmosphere. X-ray diffraction patterns of pristine Na-MMT and PMAA-g-nanoclay were collected in the range of 2 Θ = 2–10° and step size of 0.02°, using a Philips X-ray diffractometer (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 −1 . The d 0 0 1 spacings were calculated according to the Bragg’s equation: nλ = 2 d sin Θ . The PMAA-g-nanoclay was then stirred at 50 °C for 5 days in a 10 g L −1 LiCl solution to debond the PMAA chains from the clay surface . The debonded nanoclay was filtered and dried, and the polymer in the solution was precipitated in acetone, washed and dried. Thermogravimetric analyses of the debonded polymer and nanoclay were performed. The molecular weight of the debonded polymer and the molecular weight of the unbonded homopolymer which was extracted during Soxhelet-extraction were determined by GPC (GPC, Agilent 1100 series, USA) using Aquagel column of 7.5 mm × 300 mm (ID × L) with the flow rate of 1 mL min −1 at 23 °C.
2.2.3
Preparation of adhesives
The adhesive was prepared according to the formulation shown in Table 1 . Pristine NA-MMT and PMAA-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 was investigated by a separation analyzer (LUMiReader ® 416.1, LUM, Germany) working with visible light and intensities of 25% and tilt of 0, for 20 h (PMAA-g-nanoclay) and 30 min (pristine nanoclay), including 256 intervals.
| Materials | %Weight |
|---|---|
| Bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl] propane (Bis-GMA) | 14 |
| 2-Hydroxyethyl methacrylate (HEMA) | 26 |
| 2-Ethyl-2-hydroxymethyl-1,3-propandiol trimethacrylate (TMPTMA) | 8 |
| Urethane dimethacrylate (UDMA) | 12 |
| Ethanol | 39 |
| 2,3-Butanedione (BD) | 1 a |
a The photoinitiator was added after sonication to prevent un-wanted polymerization before applying the adhesive.
2.2.4
Measurement of degree of conversion
A droplet of the adhesives containing 1% BD as photoinitiator was 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 then placed on it to form a very thin layer. The sandwich was placed into the FTIR spectrometer’s sample holder and the absorbance peaks obtained by transmission mode of FTIR before and after 40 s photocuring using a dental light source with an irradiance of ca. 600 mW cm −2 (Optilux 501, SDS Kerr, Germany). 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 .
2.2.5
Transmission and scanning electron microscopies (TEM and SEM)
Adhesive specimen containing 1 wt.% PMAA-g-nanoclay was solvent evaporated and light cured in a mold. An ultra-microtome (DMU3, Reichert, Austria) was used to prepare approximately 70 nm thick TEM specimen. A Philips TEM (CM200, FEG, Netherlands) was used for TEM observations. The composite–dentin interface was observed using SEM (TESCAN, VEGAII, XMU, Czech Republic) to study the mode of failure in the microshear bond strength test. EDX elemental composition analyzer was used to map the distribution of nanoclay in the photocured dentin bonding system. A probe current of 2.0 × 10 −9 A, an accelerating voltage of 30 kV, and spot size of 500 nm with a collection time of 100 s, were used during the mapping. The elemental compositions of the products were quantitatively identified by energy dispersive X-ray analysis (EDXA, Model: QX2, RONTEC Co.) which was coupled with the SEM.
2.2.6
Mechanical tests
2.2.6.1
Diametral tensile strength
Diametral tensile strength (DTS) test was performed adopting the procedure of ANSI/ADA specification no. 27 for light cured resins using a universal testing machine (STM-20, Santam, Iran) at a cross-head speed of 10 mm min −1 with a 5 KN load cell.
2.2.6.2
Flexural strength
Flexural strength of the solvent-free adhesives was conducted according to the 3-point bending method suggested in ISO 4049 using the same universal testing machine at a cross-head speed of 1 mm min −1 with a 200 N load cell.
A number of theories and models have been developed to describe the elastic properties of polymeric composites based on the volume fraction, shape, size, aspect ratio, and distribution of the reinforcing particles, the stiffness of the matrix and filler phase, and filler–matrix interactions. Among them Halpin–Tsai equation has successfully been used to predict the elastic properties of nano-particle containing polymeric composites . The experimental data were fitted to the model equations and compared.
2.2.6.3
Microshear bond test
The microshear bond test was performed according to previous works . A commercially available dentin bonding (Adper™ SingleBond ® 2, 3M ESPE, USA), as control group, was also applied following the manufacturer’s instruction and resin composite (Premise, Kerr, USA) was used as the filling material .
2.2.7
Statistical analyses
The results were analyzed and compared using one-way ANOVA and Tukey test at the significance level of 0.05. The reported values are the average of 10 measurements for microshear bond strength, 5 repeats for DTS, FS, and FM and 3 measurements for degree of conversion.
2
Experimental
2.1
Materials
2-Hydroxyethyl methacrylate (HEMA), 2,3-butanedione (BD), 2-ethyl-2-hydroxymethyl-1,3-propandiol trimethacrylate (TMPTMA), methacrylic acid (MAA), acetone, ethanol and methanol was purchased from Merck (Germany). tert -Dodecyl mercaptan (TDM) were obtained from Fluka (Germany). 2,2-Bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl] propane (Bis-GMA) was kindly donated by Evonik (Germany). Na-MMT (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, was 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.
2.2
Methods
2.2.1
Graft polymerization of methacrylic acid onto the surface of nanoclay platelets
The graft polymerization of methacrylic acid onto the surface of nanoclay was carried out according to the previous works . Briefly, MAA was grafted onto the clay platelets in an aqueous solution in the presence of AMPS as reactive surfactant and TDM as a chain transfer agent to control the molecular weight of the grafted polymer.
2.2.2
Characterization of the PMAA grafted nanoclay
The pristine Na-MMT and PMAA-grafted 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, PMAA-g-nanoclay and pure PMAA were performed (TGA-1500, Polymer Laboratories, UK) from room temperature to 600 °C at a heating rate of 10 °C min −1 and under N 2 atmosphere. X-ray diffraction patterns of pristine Na-MMT and PMAA-g-nanoclay were collected in the range of 2 Θ = 2–10° and step size of 0.02°, using a Philips X-ray diffractometer (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 −1 . The d 0 0 1 spacings were calculated according to the Bragg’s equation: nλ = 2 d sin Θ . The PMAA-g-nanoclay was then stirred at 50 °C for 5 days in a 10 g L −1 LiCl solution to debond the PMAA chains from the clay surface . The debonded nanoclay was filtered and dried, and the polymer in the solution was precipitated in acetone, washed and dried. Thermogravimetric analyses of the debonded polymer and nanoclay were performed. The molecular weight of the debonded polymer and the molecular weight of the unbonded homopolymer which was extracted during Soxhelet-extraction were determined by GPC (GPC, Agilent 1100 series, USA) using Aquagel column of 7.5 mm × 300 mm (ID × L) with the flow rate of 1 mL min −1 at 23 °C.
2.2.3
Preparation of adhesives
The adhesive was prepared according to the formulation shown in Table 1 . Pristine NA-MMT and PMAA-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 was investigated by a separation analyzer (LUMiReader ® 416.1, LUM, Germany) working with visible light and intensities of 25% and tilt of 0, for 20 h (PMAA-g-nanoclay) and 30 min (pristine nanoclay), including 256 intervals.
| Materials | %Weight |
|---|---|
| Bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl] propane (Bis-GMA) | 14 |
| 2-Hydroxyethyl methacrylate (HEMA) | 26 |
| 2-Ethyl-2-hydroxymethyl-1,3-propandiol trimethacrylate (TMPTMA) | 8 |
| Urethane dimethacrylate (UDMA) | 12 |
| Ethanol | 39 |
| 2,3-Butanedione (BD) | 1 a |
a The photoinitiator was added after sonication to prevent un-wanted polymerization before applying the adhesive.
2.2.4
Measurement of degree of conversion
A droplet of the adhesives containing 1% BD as photoinitiator was 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 then placed on it to form a very thin layer. The sandwich was placed into the FTIR spectrometer’s sample holder and the absorbance peaks obtained by transmission mode of FTIR before and after 40 s photocuring using a dental light source with an irradiance of ca. 600 mW cm −2 (Optilux 501, SDS Kerr, Germany). 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 .
2.2.5
Transmission and scanning electron microscopies (TEM and SEM)
Adhesive specimen containing 1 wt.% PMAA-g-nanoclay was solvent evaporated and light cured in a mold. An ultra-microtome (DMU3, Reichert, Austria) was used to prepare approximately 70 nm thick TEM specimen. A Philips TEM (CM200, FEG, Netherlands) was used for TEM observations. The composite–dentin interface was observed using SEM (TESCAN, VEGAII, XMU, Czech Republic) to study the mode of failure in the microshear bond strength test. EDX elemental composition analyzer was used to map the distribution of nanoclay in the photocured dentin bonding system. A probe current of 2.0 × 10 −9 A, an accelerating voltage of 30 kV, and spot size of 500 nm with a collection time of 100 s, were used during the mapping. The elemental compositions of the products were quantitatively identified by energy dispersive X-ray analysis (EDXA, Model: QX2, RONTEC Co.) which was coupled with the SEM.
2.2.6
Mechanical tests
2.2.6.1
Diametral tensile strength
Diametral tensile strength (DTS) test was performed adopting the procedure of ANSI/ADA specification no. 27 for light cured resins using a universal testing machine (STM-20, Santam, Iran) at a cross-head speed of 10 mm min −1 with a 5 KN load cell.
2.2.6.2
Flexural strength
Flexural strength of the solvent-free adhesives was conducted according to the 3-point bending method suggested in ISO 4049 using the same universal testing machine at a cross-head speed of 1 mm min −1 with a 200 N load cell.
A number of theories and models have been developed to describe the elastic properties of polymeric composites based on the volume fraction, shape, size, aspect ratio, and distribution of the reinforcing particles, the stiffness of the matrix and filler phase, and filler–matrix interactions. Among them Halpin–Tsai equation has successfully been used to predict the elastic properties of nano-particle containing polymeric composites . The experimental data were fitted to the model equations and compared.
2.2.6.3
Microshear bond test
The microshear bond test was performed according to previous works . A commercially available dentin bonding (Adper™ SingleBond ® 2, 3M ESPE, USA), as control group, was also applied following the manufacturer’s instruction and resin composite (Premise, Kerr, USA) was used as the filling material .
2.2.7
Statistical analyses
The results were analyzed and compared using one-way ANOVA and Tukey test at the significance level of 0.05. The reported values are the average of 10 measurements for microshear bond strength, 5 repeats for DTS, FS, and FM and 3 measurements for degree of conversion.
3
Results
FTIR spectra of the pristine Na-MMT and PMAA-g-nanoclay are shown in Fig. 1 . Determined by GPC, the Soxhelet-extracted PMAA homopolymer shows a weight average molecular weight ( M w ) of 7500 (PDI = 1.8), while the chromatogram of the reverse ion exchange debonded PMAA, reveals a M w of about 2300 (PDI = 1.4). The TGA curves of the pristine Na-MMT, PMAA-g-nanoclay, nanoclay after ion exchange process and neat PMAA are represented in Fig. 2 . The PMAA was synthesized in the same conditions as the graft polymerization. Considering the ash content of neat PMAA in the thermogravimetric analysis (≈5 wt.% at 550 °C), the grafting PMAA percentage was calculated about 43 wt.%. Fig. 3 illustrates the XRD patterns of pristine Na-MMT, PMAA-g-nanoclay and cured adhesive containing 1 wt.% PMAA-g-nanoclay. Comparison of the XRD patterns shows that the peak position from 2 Θ = 9.41°, corresponding to an interlayer spacing of d 0 0 1 = 9.39 Å for pristine Na-MMT, is shifted to 2 Θ = 6.15°, representing an interlayer spacing of d 0 0 1 = 14.36 Å for PMAA-g-nanoclay; this peak is disappeared for the adhesive containing 1 wt.% PMAA-g-nanoclay. TEM image of light cured adhesive containing 1 wt.% PMAA-g-nanoclay is shown in Fig. 4 , which indicates intercalation/exfoliation of PMAA-g-nanoclay particles in the adhesive system. Fig. 5 is the separation analysis results which shows the sedimentation behavior of the nanoparticles in the adhesives for the specimens containing 1 wt.% of pristine Na-MMT and treated nanoclay. The transmission of the adhesive containing PMAA-g-nanoclay did not reach 100% during 20 h. The transmission of 80% (at the middle point of testing tube) was obtained after ca. 7 min for the pristine nanoclay containing adhesive whereas the same transmission was reached after ca. 315 min for the adhesive containing PMAA-g-nanoclay, which shows about 45 times increase in the stability of the modified particles in the adhesive solution. To investigate the dispersion of modified particles in the adhesives, the cross section of cured adhesives was examined by EDXA. The Si map of the cured adhesive containing 0.5 wt.% PMAA-g-nanoclay was collected ( Fig. 6 ). Figs. 7 and 8 show DTS and FS of the adhesive samples containing different concentrations of PMAA-g-nanoclay. DTS and FS show a maximum corresponding to the adhesive containing 0.5 wt.% PMAA-g-nanoclay significantly higher than those of the unfilled resin ( p = 0.000). Fig. 9 presents the FM of the adhesives containing PMAA-g-nanoclay. An increasing trend is observed in the FM of the adhesive composites upon the increase in filler content. The model predictions for the FM of adhesives containing different amounts of PMAA-g-clay are also calculated and presented in Fig. 9 . DC% values of the adhesives were in the range of 78–85% and there were no significant difference between the DC% of the adhesives with different filler contents and SingleBond ® ( p > 0.05).
Fig. 10 shows the microshear bond strength of the adhesives containing different percentages of PMAA-g-nanoclay. It reveals a significant increase ( p = 0.025) in the microshear bond strength of the adhesive containing 0.5 wt.% PMAA-g-nanoclay. In Fig. 11 the penetration of the adhesive into the dentinal tubules is clearly observed in low filler concentrations (0.5 wt.%) while most of the tubules are left unfilled in high concentrations (5 wt.%). Agglomeration of the particles in higher concentrations of the fillers results in the decrease of the adhesive penetration into the dentin tubules. The failure mode in the shear bond test was mostly adhesive from the adhesive–dentin interface which is representatively illustrated in Fig. 12 .
4
Discussion
In the graft polymerization, the pristine Na-MMT is dispersed in water in the presence of a reactive surfactant, AMPS, which contains amido and sulfonic acid groups in its structure. It has been suggested that the interaction of AMPS with clay occurs by proton transfer from sulfonic acid to nitrogen, to form a protonated amido group and ion exchange with sodium ions on the silicate layers . AMPS can also be adsorbed onto the surface of the clay galleries by formation of hydrogen bonds between its amido groups and water molecules surrounding the exchangeable cations, and ion–dipole interactions between its sulfonate groups and the interlayer exchangeable cations . These interactions lead to an increase in the basal spacing of pristine Na-MMT. The adsorption of AMPS onto clay by formation of hydrogen bonding between its amido group and the hydroxyl groups on the edges of the clay platelets, however, has no effect on the basal spacing. Polymerization of methacrylic acid monomers in the presence of nanoclay platelets, AMPS, and a water soluble free radical initiator, ammonium persulfate, leads to both graft polymerization onto clay surface involving the vinyl groups of AMPS, and the homopolymerization of MAA in aqueous phase. Therefore, the reaction product is a mixture of PMAA-g-nanoclay and PMAA homopolymers. Having Soxhelet-extracted with water for 72 h to remove PMAA homopolymers completely, the remained PMAA-g-nanoclay, was characterized using different analytical techniques. The grafting of PMAA onto the pristine Na-MMT was confirmed by FTIR spectroscopy ( Fig. 1 ). The appearance of two PMAA characteristic peaks at 1736 cm −1 and 2955 cm −1 which are assigned to the stretching vibrations of carbonyl and C H groups in poly(methacrylic acid) structure, respectively, is the indication of the presence of grafted PMAA.
TDM, which is a chain transfer agent, results in the formation of low molecular weight grafted PMAA and the homopolymer chains. Steric hindrance adjacent to the surface of nanoclay platelets is probably responsible for the low molecular weight of the grafted chains in comparison to the PMAA homopolymers. According to the thermogravimetric results in Fig. 2 , the percentage of grafted PMAA is about 43 wt.%. As the PMAA-g-nanoclay was Soxhelet-extracted for 72 h with water, which is a good solvent for PMAA, the weight loss in the grafted clay is due to the thermal degradation of bonded PMAA polymer chains. There is no sharp weight loss for degradation of PMAA in the grafted clay, which is probably due to the thermal stabilization effect of nanoclay on the polymer . TGA curve for neat PMAA, indicates two stages of breakdown starting at ca. 240 °C and 460 °C, respectively. An initial weight loss below 100 °C is due to the release of absorbed water. The first stage of break down (up to ca. 300 °C) accounts for about 25% weight loss. According to the degradation mechanism, anhydropoly(methacrylic acid) is formed at the first stage. The dehydration reaction occurs by intramolecular cyclisation of adjacent monomer units to lose water and give six-membered anhydride ring structures. At higher temperatures, the second stage, the polymer undergoes massive degradation evolving both water and carbon dioxide and the anhydride structures are decomposing . In Fig. 3 the characteristic XRD peak of nanoclay in the PMAA-g-nanoclay has been broadened and also shifted toward lower angles, which is an indication of the lower regularity and larger basal spacing as a result of the intercalation of PMAA chains into the clay galleries. The grafted PMAA chains prevent the clay sheets to stack up after removing from the aqueous reaction medium. The solvating power of ethanol of the adhesive causes the grafted PMAA chains to swell and allow the diffusion of multifunctional monomers of the adhesive into the clay layers. Removing the solvent and curing the adhesive leaves additional organic components between the layers and results in increased basal spacing and disappearance of XRD peak in the cured adhesive containing 1 wt.% PMAA-g-nanoclay confirming an exfoliated structure which is further supported by the TEM observations.
Dispersion stability analysis of modified nanoclay containing adhesives showed a significant increase in the sedimentation time of the particles through the dilute bonding system ( Fig. 5 ). Ethanol is a good solvent for PMAA (solubility parameters of ethanol and PMAA are 11.7 and 12.48 cal 1/2 cm −3/2 , respectively) , therefore polymer chains, grafted onto the nanoclay surface, swell in the adhesive and force the nanoclay platelets to separate from each other which results in decreasing the overall density of the filler particles and increasing the sedimentation time. A uniform distribution of the particles in the adhesive resin matrix is also observed in the Si-mapping of the cured adhesive ( Fig. 6 ).
Mechanical properties of the resin-based bonding systems are strongly depend upon the degree of conversion (DC%). Therefore, they should preferably be compared in the same degree of conversion. DC% measurements showed that there is no significant difference between the DC% of the test specimens providing a reliable comparison and analysis ( p > 0.05).
DTS test is valid for the brittle materials in which plastic deformation is negligible . The force–displacement curves in this test revealed a brittle behavior validating the DTS results. The increased DTS and FS of adhesive containing 0.5% PMAA-g-nanoclay comparing to the unfilled resin ( Figs. 7 and 8 ) are probably due to high strength of fillers, crack lengthening mechanism and good interaction of polymer grafted filler platelets with the polymeric matrix. In specimens with higher filler content, however, some weak points are formed due to agglomeration of PMAA-g-nanoclay which can result in crack initiation and reduced DTS and FS, subsequently.
The elastic modulus of a composite depends on the elastic properties of its components. Since the inorganic fillers generally have higher stiffness, the modulus of the polymeric matrix of dental adhesives is improved by incorporation of the rigid fillers. The higher elastic modulus observed in the filled adhesives ( Fig. 9 ) is due to the higher elastic modulus of the nanoclay particles in comparison to the polymeric resin matrix.
Theoretical or semi-empirical models have been developed to describe the effect of reinforcing phase on the elastic properties of the composites. In this study, different models have been applied to describe the elastic modulus of the nanoparticle filled dental adhesives.
To apply the models on the experimental data, the following parameters were measured and/or calculated:
The aspect ratio of ultrasound dispersed montmorillonite Cloisite ® Na + , considered as 200 , montmorillonite density as 2.86 g cm −3 , montmorillonite modulus as 170 GPa ( E clay ) , density of PMAA-g-nanoclay as 1.82 g cm −3 (experimentally measured), unfilled resin density as 1.09 g cm −3 (calculated by rule of mixtures taking into account the density and volume fraction of the monomers and solvent in the adhesive), unfilled resin modulus as 1.11 GPa ( Fig. 9 ), PMAA density as 1.23 g cm −3 (experimentally measured), and PMAA modulus as 0.79 GPa ( E PMAA ) . The modulus of the modified nanoclay particles was approximated as 62 GPa using the law of mixtures:
E PMAA-g-clay = E clay V clay + E PMAA V PMAA
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