Microhardness and wear behavior of nanodiamond-reinforced nanocomposites for dental applications

RESEARCH ARTICLE Microhardness and wear behavior of nanodiamond-reinforced nanocomposites for dental applications Rocío Moriche 1,2 | Joaquín Artigas-Arnaudas 1 | Bhanu Chetwani 1 | María S anchez 1,3 |M  onica Campo 1 | Margarita G. Prolongo 4 | Joaquín Rams 1,3 | Silvia G. Prolongo 1,3 | Alejandro Ureña 1,3 1 Materials Science and Engineering Area, Escuela Superior de Ciencias Experimentales y Tecnología, Universidad Rey Juan Carlos, M ostoles, Madrid, Spain 2 Departamento de Física de la Materia Condensada, ICMS, CSIC-Universidad de Sevilla, Sevilla, Spain 3 Instituto de Investigaci on de Tecnologías para la Sostenibilidad, Universidad Rey Juan Carlos, Madrid, Spain 4 Department of Aerospace Materials and Production, E.T.S.I. Aeron autica y del Espacio, Universidad Politécnica de Madrid, Madrid, Spain Correspondence Joaquín Artigas-Arnaudas, Materials Science and Engineering Area, Escuela Superior de Ciencias Experimentales y Tecnología, Universidad Rey Juan Carlos, Calle Tulip an s/n, 28933 M ostoles (Madrid), Spain. Email: [email protected] Funding information Agencia Estatal de Investigaci on Abstract In polymer-based dental composites, wear is a three-body wear system mainly abrasive, because of the food particles and wear products suspended in the oral cavity, which are transferred to the microcavities of the surface of the replacements. Due to this fact, the incorporation of nanodiamond as reinforcement in these polymer–matrix composites, which promotes the creation of a solid lubricant tribofilm surface could be advantageous. With the reinforcement of nanodiamonds, BisGMA/TEGDMA-based composites increase their microhardness by 95%–420%. A maximum hardness exceeding 65 HV is achieved with a reinforcement of 3.2 wt%. The specific wear rate of neat BisGMA/TEGDMA is near 10 4 mm 3 /Nm and the Archard's coefficient is 2.6 10 5 . The incorporation of a content of 1.6 wt% ND is enough to cause a diminution of 78% in the friction coefficient and a reduction of the specific wear rate and Archard's coefficient of 50%. Nevertheless, the addition of relatively high contents reduces the effectiveness of photoinitiation and photocuring, which is related to the scattering and absorption of light radiation by ND. This causes a significant decline in elastic properties starting at 50 μm from the surface. Highlights •Photocuring polymer resin was successfully reinforced with nanodiamonds. •Microhardness increases from 95% up to 420%, close to commercial composites. •Friction coefficient and wear rate are reduced with 1.6 wt% nanodiamonds. •High levels of reinforcement reduce the effectiveness of photocuring. KEYWORDS hardness, nanodiamond, photocuring, polymer–matrix composite, wear Received: 29 July 2024 Revised: 20 September 2024 Accepted: 20 September 2024 DOI: 10.1002/pc.29104 This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. © 2024 The Author(s). Polymer Composites published by Wiley Periodicals LLC on behalf of Society of Plastics Engineers. Polymer Composites. 2024;1–12. wileyonlinelibrary.com/journal/pc 1 1|INTRODUCTION Nowadays, resin-based composites (RBCs) are widely used in dentistry for applications such as filling materials, restorations, endodontic posts and cores, and adhesives. 1 The most common matrices used for these applications are bisphenol A-glycidyl methacrylate (BisGMA), urethane dimethacrylate (UDMA), and triethylene glycol dimethacrylate (TEGDMA). 2–4 Regarding the reinforcements utilized to provide mechanical properties, the use of inorganic particles stands out. 5,6 The most common reinforcements are based on the use of silica glasses, which have high hardness and can be easily obtained in a wide variety of sizes and shapes. 7 In addition, their optical properties are important to improve light scattering. Other ceramic materials, such as silica (SiO 2 ), zirconia (ZrO 2 ), and alumina (Al 2 O 3 ) are also commonly employed to improve mechanical properties, especially wear resistance. 8 Furthermore, reinforcements based on hydroxyapatite (HA) or calcium phosphate (Ca 3 (PO 4 ) 2 ) are employed to enhance biocompatibility. 9 Although they have been used for 50 years, enhancement of mechanical properties and reduction of shrinkage need to be further investigated. 10 For this reason, dental polymer–matrix composites with high wear resistance are a current challenge, which is also one of the key properties in long-term restorations. 11 In addition, the restoration component must prevent the antagonist tooth from wearing down, as this can affect the functionality of the natural tooth. 12 The use of certain ceramic nanoparticles, 13 such as TiO 2 ,ZrO 2 ,Al 2 O 3 , and SiC, and graphene nanoplatelets, 14 as fillers have been demonstrated to enhance tribological properties due to the increase of hardness, diminution of friction coefficient, or the formation of a transfer film between the material and the counterpart. These nanocomposite materials are also potential candidates to prevent contact between erosive agents and the dental surface, that is, to prevent dental erosion, by creating a physical barrier. 15,16 Different authors, such as Santos et al., 17 have also corroborated that commercial dental composites incorporating reinforcements with smaller size distributions show lower friction coefficients, what leads to lower contact forces and, as a consequence, lower wear rates. Additionally, the use of nanoreinforcements instead of microfillers, due to their higher specific surface area, contributes to an enhancement of mechanical properties and hardness because of the increase in the contact between the polymeric matrix and the nanoparticles. 18 The curing of the polymer–matrix is also significantly affected by the particle size of the reinforcement. Nanometer-sized particles allow reduced light scattering compared to micrometer-sized particles. 6 In the case of SiO 2 fillers, the combination of micro- and nano-sized particles results in an improvement of the mechanical behavior of dental polymeric composites, due to the properties that each particle size can provide. 19 Particularly, nanodiamond (ND) particles have been previously studied to develop new formulations of lubricant colloids, 20,21 showing promising results related to the wear behavior of sliding parts. 22 Golchin et al. 23 have also obtained wear resistance improvements in ultra-high molecular weight polyethylene (UHMWPE) reinforced with ND. Regarding the effect on mechanical properties, Haleem et al. 24 corroborated that the addition of ND (0.5–2 wt%) as nanoreinforcement of epoxy matrixes causes an increase in fracture toughness and fracture energy. An increase in hardness and Young's modulus has also been demonstrated in ND-reinforced polymers compared to other carbon nanoparticles. 25 This combination of properties makes ND reinforcement relevant for use with polymer resins in dental applications where mechanical requirements are very important. The wear behavior of epoxy resins can be influenced by various parameters, with the inclusion of particles being one option to reduce the wear rate. 26–28 Among the reinforcements used, natural-origin particles can also be found. 29,30 Dental resins, such as BisGMA/TEGDMA have been reinforced with carbon nanoparticles, demonstrating their potential to be fabricated using techniques such as 3D printing. 31 However, a study is needed with reinforcements that provide the necessary properties for its application in dental reconstructions. This study proposes the use of ND as nanoreinforcement in dental polymer composites to take advantage of its contribution to mechanical and tribological properties. In order to facilitate comparison of the benefits, SiO₂ microparticles, a common component of dental composites, were also employed in the study. A study of the microhardness and wear behavior is carried out as a function of ND content (0.6, 1.2, and 3.2 wt%). Additionally, the influence of the addition of ND on photocuring is also studied to establish the limited contents to ensure functionality. 2|EXPERIMENTAL 2.1 |Materials The polymeric matrix used in nanocomposites was a Bisphenol A bis(2-hydroxyl-3-methacryloxypropyl) ether (BisGMA) and triethylene glycol dimethacrylate (TEGDMA) (Esschem Europe, Durham, UK) in a 50:50 ratio. As an initiator agent, camphorquinona (CQ) (Esschem Europe, Durham, UK) 2MORICHE ET AL. 15480569, 0, Downloaded from https://4spepublications.onlinelibrary.wiley.com/doi/10.1002/pc.29104 by Readcube (Labtiva Inc.), Wiley Online Library on [12/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License was added in a content of 0.7 wt%, which is activated at a light wavelength range of 400–500 nm (maximum peak at 468 nm). With the aim of promoting polymerization, dimethylaminoethylmethacrylate (DMAEM) (Esschem Europe, Durham, UK) was used in a 0.32 wt%. Diamond nanopowder (ND) with a particle size lower than 10 nm and carbon purity ≥97% (Sigma-Aldrich, Germany) was used as a nanofiller. Micrometric amorphous silica (SiO 2 ) powder with a specific surface area (BET) of 200 ± 25 m 2 /g and 99.8% purity (Scharlab, Barcelona, Spain) was also used as reinforcement for comparison. The processed materials were compared with commercially available dental composites: 3 M ESPE Filtek, Tetric Evoflow, and Tetric Restorative, all purchased from authorized distributors. 2.2 |Methods 2.2.1 | Composites manufacturing Dispersion of the reinforcement was carried out by probe sonication (UP400S Ultrasonicator, Hielscher Ultrasonics, Teltow, Germany). Nanocomposites with ND contents of 0.8, 1.6, and 3.2 wt% and silica composites with SiO 2 contents of 1.6, 3.2, 10, and 25 wt% were respectively prepared. Contents were selected based on the processability of nanocomposites to ensure their quality. Sonication was applied for 30 min with a power amplitude of 50% and a cycle of 0.5 seconds. Once dispersed, the mixture was poured into a 50 30 3mm 3 transparent open mold and cured with a photocuring lamp equipped with a full arch attachment, which consists of six aligned LED units (Radii Plus LED light, SDI Ltd., Bayswater, Victoria, Australia). The lamp head is oriented toward the open top face of the mold, covering the entire length, and cures with continuous light exposure for 8 min. This lamp operates in the 440–480 nm wavelength range, is suitable for CQ activation, and has a light intensity peak of 1500 mW/cm 2 . Commercial composites were prepared in accordance with the manufacturer's instructions. 2.2.2 | Microhardness and wear characterization In order to characterize the prepared composites with all the techniques, samples of 20 20 3mm 3 were cut from the bulk material previously prepared. The top face, where the characterization is performed, has been prepared by creating a flat surface by grinding with SiC papers up to a grit size of P1200. Vickers microhardness tests were performed following the ASTM E 92–82 in a microhardness tester (HMV-2, Shimadzu, Shimadzu Corporation, Kyoto, Japan). The applied load was 980.7 mN and the maximum load was maintained for 30 s. At least, five measurements were carried out in one sample for each condition, and the mean value and the standard deviation were calculated. The density of materials was measured by Archimedes' method using three samples from each condition. The calculations were conducted in accordance with the standard ASTM D792, and the mean value and the standard deviation were calculated. Tribological tests were carried out in a tribometer (MT400, MicroTest S.A., Madrid, Spain) using a pinon-disk configuration, in dry conditions and with the same environmental conditions, following the requirements of ASTM G99. An alumina ball with a diameter of 6 mm was used as a counterpart, the applied load was 10 N and the track diameter was 10 mm. Sliding was performed until completing a total distance of 500 m with a speed of 200 rpm. Five wear tests were made for each material in samples of 20 20 mm, and the average value and standard deviation were determined. The tribological behavior was evaluated by measuring the volume loss, friction coefficient (μ), and wear rate (Q) of the composites. The wear testing machine continuously recorded the friction coefficient. Specific wear rate (Q) was calculated from the mass loss (m), the applied load (W), density (ρ) and total length (L) as in Equation (1): Q¼m WρLð1Þ Additionally, the Archard's coefficient (K) was obtained from the empirical Archard's law (Equation 2): Q¼KW Hð2Þ where His the hardness of the tested material. The sample's surface preparation was performed prior to the wear tests. They were ground with different SiC abrasive papers (grit size P1200) to obtain a similar surface roughness that would not influence the wear properties. The average roughness (Ra) of the samples, determined by a profilometer (SJ-301 Surftest, Mitutoyo, Japan) was 0.37 ± 0.05 μm. Three-dimensional profiles of the wear tracks were analyzed by using optical profilometry (Zeta-20, Zeta Instruments, KLA-Tencor, Milpitas, CA, USA). The profiles were generated from 1500 measurements taken with a step size of 0.146 μm. Scanning electron microscopy (SEM) (S-2400 N, Hitachi, Tokyo, Japan) was also used to MORICHE ET AL.3 15480569, 0, Downloaded from https://4spepublications.onlinelibrary.wiley.com/doi/10.1002/pc.29104 by Readcube (Labtiva Inc.), Wiley Online Library on [12/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License evaluate the morphology and the wear mechanisms of composites. SEM images were obtained at high vacuum with the secondary electron detector operating at 15 kV. 2.2.3 | Photocuring characterization Based on the obtained results of the evaluation of microhardness and wear behavior of ND-based nanocomposites, a nanoindentation profile was conducted in 3.2 wt% reinforced ones in order to evaluate photocuring. Tests were performed using a nanoindenter (MTS XP Nanoindenter, MTS Systems Co., Nano Instruments, Oak Ridge, TN, USA). The profile was generated through the polished cross-section, which reached a depth of 375 μm. The distance between adjacent nanoindentations was 25 μm and the maximum applied load was 100 mN with a holding time of 10 s. Glass transition temperature was measured by differential scanning calorimetry (DSC) (DSC822e, Mettler Toledo, Greifensee, Switzerland). The samples used have the same thickness as the bulk material prepared during light curing. Thermal scans were conducted in the temperature range of 10–200C at a heating rate of 20C/min under a nitrogen atmosphere, in accordance with ASTM D3418-21, which specifies that a 10 mg specimen mass is satisfactory. One sample was tested for each condition. Fourier-transform infrared spectroscopy (FTIR) was used to analyze differences in photocuring through thickness. Measurements were carried out in an infrared microscope (Nicolet iN10 MX, Thermo Scientific, Thermo Fisher Scientific Inc, Waltham, MA, USA) with a Ge attenuated total reflection (ATR). Five equidistant FTIR spectra between 700 and 4000 cm 1 wavelengths were recorded along 1500 μm from the surface of the nanocomposite where the light was irradiated. 3|RESULTS AND DISCUSSION 3.1 |Microhardness of ND- nanocomposites and SiO 2 -reinforced composites The addition of ND to the polymeric matrix is thought to contribute to mechanical properties in order to replace current micrometric reinforcement with nanometric ones to enhance polishing and mechanical and tribological behavior. Figure 1shows Vickers microhardness (HV0.1) of ND-based nanocomposites as well as SiO 2 -reinforced composites, for comparison, as a function of reinforcement content. The values of microhardness of commercially available composite materials, measured using the same method, have been also added for comparison. Reinforcement contents in these commercial composite materials, with particle sizes from the nanometer scale up to 50 μm in some cases, ranges from 60 up to 80 wt%. The incorporation of 0.8 wt% of ND causes an increase in microhardness of 95%, while the quantity needed to reach similar microhardness for micrometric SiO 2 reinforcement is above 3.2 wt%. Moreover, microhardness increases by 420% in the case of using ND with a content of 3.2 wt%; this value has not been reached with SiO 2 even with concentrations up to 25 wt %. Additionally, it is important to point out that the microhardness of composites reinforced with 10 wt% of micrometric SiO 2 , which is 23.7 ± 0.4, is still lower than the one of nanocomposites filled with 1.6 wt% of ND, being 27.7 ± 3.4. If these values are compared with the microhardness of commercial dental composites, nanocomposites reinforced with contents as low as 0.8 wt% present values above some of the mentioned commercially available composites. Taking into account that other reinforcements used to achieve multifunctionality in dental composites are included in the commercial ones and the fact that the levels of filler ratios in the polymeric matrix are considerably higher, the obtained results evidence the potential application of ND as nanoreinforcement to be used in dental composite materials. The low contents of ND needed are also advantageous to new manufacturing methods, such as 3D printing, where low reinforcement contents are required to keep the composites flowable and avoid different curing degrees. 11 The study of the mechanical behavior of composite materials is highly complex due to the multitude of parameters that can influence it. In the case of dental FIGURE 1 Vickers microhardness of ND-reinforced nanocomposites and SiO 2 -reinforced composites; and experimental microhardness values for commercial composites Tetric Evoflow, 3 M ESPE Filtek, and Tetric Restorative. 4MORICHE ET AL. 15480569, 0, Downloaded from https://4spepublications.onlinelibrary.wiley.com/doi/10.1002/pc.29104 by Readcube (Labtiva Inc.), Wiley Online Library on [12/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License nanocomposites, this is particularly relevant since they typically contain a high content of different types of reinforcements. The hardness increments obtained may be related to better interfacial adhesion of the nanoparticles with the matrix. The increase in adhesion is attributed to the mechanical interlocking at the interface. The ND nanoparticles exhibit a larger contact surface with the matrix, thereby achieving a larger area of mechanical interlocking in the interface. The matrix-reinforcement interface is a key element in the mechanical toughness of dental polymeric composites. 32 3.2 |Wear behavior of ND- nanocomposites and SiO 2 -reinforced composites Figure 2shows the evolution of the friction coefficient (μ) during the test (Figure 2A) and the average μ, calculated as the average value in the stabilized region (Figure 2B). Results show that stabilization occurs at longer times as the NDs content increases, except in the case of 1.6 wt%. The friction coefficient diminishes by 24% with a content of 0.8 wt% and is considerably reduced by 78% when the ND content is duplicated, that is, 1.6%. When using an ND content of 3.2 wt%, an increase in friction coefficient is observed but it still is lower than the BisGMA/TEGDMA one (0.57 ± 0.06). The reasons are related to the increase in microhardness and wear mechanisms that take place, which will be discussed later. A previously published work reported by Koumoulos et al. 33 shows that the friction coefficient, calculated from nanoscratch tests, of epoxy-matrix nanocomposites incorporating ND as reinforcement diminishes as the ND contents increase from 0.4 up to 5.0 wt%. The friction coefficient (μ) value reduced from 0.45 down to 0.40, that is, 11%; which elucidates the lubricant effect created by detached ND. The volume loss of ND-reinforced nanocomposites and SiO 2 -reinforced composites depending on the filler content is plotted in Figure 3. Although the volume loss of nanocomposites reinforced with 0.8 wt% of ND experienced an increase, when the content is augmented up to 1.6 wt%, the volume loss decreased by nearly 50%. This fact is attributed to the effect that a certain level of concentration of ND induces autolubricity in nanocomposites. The mechanism taking place is that ND nanoparticles favor sliding between the three bodies, that is, nanocomposite, ND, and alumina ball. When ND nanoparticles detach due to wear, they act as bearing, promoting, consequently, a lubricity effect 34 and giving rise to the formation of a film, acting as a lubricating layer (tribofilm). 35 This proposed mechanism has been previously published by Tortora et al. 21 and Ivanov et al. 20 Additionally, the increase in microhardness also contributes to the enhancement observed in wear behavior. 23 In contrast to the reduction observed for ND- reinforced nanocomposites, when micrometric SiO 2 is FIGURE 2 Friction coefficient (μ)of ND-reinforced nanocomposites (A) time dependence and (B) friction coefficient values. FIGURE 3 Volume loss after wear tests for ND-reinforced nanocomposites and SiO 2 -reinforced composites. MORICHE ET AL.5 15480569, 0, Downloaded from https://4spepublications.onlinelibrary.wiley.com/doi/10.1002/pc.29104 by Readcube (Labtiva Inc.), Wiley Online Library on [12/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License added as filler, the volume loss is considerably higher, beingtwicetheoneoftheBisGMA/TEGDMApolymer for a content of 3.2 wt% and 4 times higher in the case of adding a concentration of 25 wt%. In this case, detached SiO 2 microparticles act as an abrasive body between the counterpart and the surface of the composite causing higher wear and, therefore, higher volume loss. 36 The increase in wear resistance by incorporation of inorganic reinforcements has been attributed to the enhancement in hardness, 37 but once these microparticles are detached, they can get involved in a three-body wear system. Figure 4shows specific wear rates of the composites for the tested condition. The specific wear rate of BisGMA/TEGDMA is near 10 4 mm 3 /Nm and the Archard's coefficient is 2.6 10 5 .Chaddaetal. 38 obtained a specific wear rate for non-reinforced BisGMA/TEGMA at a load of 25 N of the same order of magnitude. With the addition of 0.8 wt%, the specific wear rate as well as the Archard's coefficient increases, which is in accordance with the results discussed above. In contrast, a content of 1.6 wt%, due to the formation of the solid lubricant tribofilm, induces a reduction in both specific wear rate and Archard's coefficient of 50%. The contrary happens when micrometric SiO 2 is used as reinforcement, an increase in specific wear rate and Archard's coefficient occurs, due to the abrasion of the detached SiO 2 microparticles. In order to analyze the mechanisms taking place, Figure 5shows representative SEM micrographs of the surfaces of the wear tracks produced by wear. The surface of the BisGMA/TEGDMA wear track (Figure 5A,E) shows plowing marks in the direction of the sliding movement of the counterpart, which is significant for micro-cracks propagation along that direction. Additionally, there is accumulation of polymeric material, that is, pile-up, in the track contour. These two phenomena are representative of a combined abrasive and adhesive wear mechanism. The addition of low contents of ND does not induce appreciable modifications (Figure 5B,F), although a slight change in the morphology of the surface of wear track and pile-up is less significant. Initially, detachment of ND/BisGMA/TEGDMA occurs forming the debris. During the ball movement, debris act as a third body 39 as the ND content is not enough to create an effective solid lubricant tribofilm. Additionally, as it will be discussed later (Section 3.3), the addition of ND influences the photocuring of nanocomposites, as ND acts as the light scattering center. If debris detached due to wear has no enough ND content to create the mentioned tribofilm, material is torn out, 40 creating voids, as the degree of curing slightly diminishes along the thickness. In contrast, a content of 1.6 wt%, as shown in the previous discussion, considerably changes tribology. Wear is significantly reduced, and wear track is near imperceptible (Figure 5C,G). This is due to the effective autolubricant properties of the nanocomposite material already mentioned above, together with the increase in microhardness. The wear track surface of SiO 2 -based composites shows differences, Figure 5D,H show SEM micrographs of composites reinforced with 10 wt%, which are representative to define mechanisms. Due to the presence of SiO 2 , the accumulation of material in the contour of the wear track is negligible, which is indicative of a significant reduction of adhesive wear and more dominancy of abrasive wear. In this case, detached SiO 2 particles act as abrasive in the three-body wear system, not only in micrometric but also in nanometric scale size, as has been previously reported by Abenojar et al., 36 Zhang et al., 14 and Zhao et al. 41 in other polymeric matrix composites. Generally, in dental composites, abrasive wear is the most common mechanism, which originates under plastic conditions and because of the contact of hard particles with a softer surface. 42 In polymer-based dental composites, the usual wear is mainly abrasive, because of the food particles and wear products suspended in the oral cavity, which are transferred to the microcavities of the surface of the replacements. 43 Due to this fact, the creation of the lubricant tribofilm surface could be advantageous. All the results mentioned above are also in accordance with the morphology and size of wear tracks. Figures 6and 7show representative profilometries of the wear tracks after completing the wear tests, as well as geometrical parameters. In these profilometries, the effect of the addition of 1.6 wt% of ND nanoparticles FIGURE 4 Specific wear rates (Q) and Archard's coefficient (K) for ND-reinforced nanocomposites and SiO 2 -reinforced composites. 6MORICHE ET AL. 15480569, 0, Downloaded from https://4spepublications.onlinelibrary.wiley.com/doi/10.1002/pc.29104 by Readcube (Labtiva Inc.), Wiley Online Library on [12/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License can be clearly appreciated as the wear is nearly nonappreciable (Figure 6C) and the depth is nearly negligible (Figure 7). It is also important to point out the wider and deeper geometry of wear tracks induced in SiO 2 -based composites, significantly higher than that of the ND-based composites with similar Vickers microhardness. FIGURE 5 SEM micrographs of wear tracks of (A, E) BisGMA/TEGDMA; (B, F) 0.8 wt% and (C, G) 1.6 wt% ND- reinforced nanocomposites; and (D, H) 10 wt% SiO 2 -reinforced composites. MORICHE ET AL.7 15480569, 0, Downloaded from https://4spepublications.onlinelibrary.wiley.com/doi/10.1002/pc.29104 by Readcube (Labtiva Inc.), Wiley Online Library on [12/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 3.3 |Photocuring analysis of ND-based nanocomposites Due to differences found in the wear behavior of 3.2 wt% ND-reinforced nanocomposites, a deep analysis of photocuring of this composition was carried out with the aim of elucidating the influence of the addition of ND in relatively high contents. Results of additional nanoindentation tests along the cross-section, as well as FTIR and DSC analysis, are shown in Figure 8. The evolution of the elastic modulus and hardness calculated from nanoindentation profile curves (Figure 8A) shows a progressive diminution of mechanical properties from the surface of the nanocomposite with depth. The observed decrease is attributed to differences in photocuring along the cross-section of the sample. To corroborate FIGURE 6 Profilometry of wear tracks of (A) BisGMA/TEGDMA; (B) 0.8 wt% and (C) 1.6 wt% ND-reinforced nanocomposites; and (D) 3.2 wt%, (E) 10 wt%, and (F) 25 wt% SiO 2 -reinforced composites. FIGURE 7 Wear track width and depth of ND-reinforced nanocomposites and SiO 2 -reinforced composites. 8MORICHE ET AL. 15480569, 0, Downloaded from https://4spepublications.onlinelibrary.wiley.com/doi/10.1002/pc.29104 by Readcube (Labtiva Inc.), Wiley Online Library on [12/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License this statement, FTIR spectra were recorded depending on the distance to the surface. Figure 8B shows the correspondent spectra. If the peak associated with C Cofmethacrylate groups that have not reacted is analyzed, which is located at 1635 cm 1 , 44 it increases as the FTIR spectrum recorded corresponds to a deeper section of the nanocomposites. Thus, the concentration of non-reacted methacrylate groups is higher with increasing depth. This fact is related to the scattering and absorption of light radiation by ND already observed in nanodiamond hydrosols by Vu et al., 45 and has been also detected with other reinforcements for dental applications, as reported by Rodrigues de Menezes et al., 46 what reduces the effectiveness of photoinitiation and the creation of the radical species. To solve these heterogeneities, glass transition temperature (T g ) has been examined across a range of photocuring times (8, 16, 24, and 32 min), although they may be not effective enough to be applicable in cases of in situ polymerization. Figure 8C shows T g of BisGMA/ TEGDMA and ND-reinforced nanocomposites for photocuring times of 8, 16, 24, and 32 min. The discrepancy observed between the neat resin with the reinforced resin in an exposition time of 8 min implies that the full cure of the nanocomposite may not be achieved in the presence of reinforcement. As the sample represents the entire thickness of the plate exposed to light, the observed decline in T g within the sample reinforced with ND suggests the presence of regions within the material that have not achieved a homogeneous degree of curing. The exposure to light for longer times makes possible the increment of T g values up to the same value of the neat BisGMA/TEGDMA matrix. Although ND should act as steric blockers of polymeric chain movement resulting in an increase of T g , because of the FIGURE 8 Analysis of photocuring of 3.2 wt% ND-reinforced nanocomposites: (A) elastic modulus and hardness profiles, and (B) FTIR as a function of the distance to the surface; and (C) glass transition temperature (T g ). MORICHE ET AL.9 15480569, 0, Downloaded from https://4spepublications.onlinelibrary.wiley.com/doi/10.1002/pc.29104 by Readcube (Labtiva Inc.), Wiley Online Library on [12/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License