Effects of 5 Nanosilica Concentrations and Humid Environments at 6 Different pH Levels on Fracture Toughness and Moisture Absorption of Dental Polymethyl Methacrylate Resin‎ Reinforced With Silica Nanoparticles: An Explorative Experimental Scanning Electron Microscopy Study
Materials and Methods This was an explorative in vitro experimental study. As it did not involve any humans, animals, or life forms, the need for any consent was waived by the Research Committee of the University. The study protocol and its ethics were approved by the Research Committee of the Islamic Azad University (Thesis Code: 352648000617703105008162424173). 2.1 Materials and Equipment in Use In this experimental laboratory study, PMMA was used as a denture base material (Taiwan Chemical Company, Taiwan) with a specific gravity of 1.19 g/cm 3 . The process of polymerization and production of PMMA were carried out at the Razi Metallurgy Research Institute in Iran and the production process was carried out under the petrochemical license of Taiwan. Silica NPs with a size of 20 nm and a purity of 99.5% (Acrosun, Spain) were used as base polymer reinforcements. To reduce the viscosity, TEGDMA thinning resin (Evonic, Germany) was applied. Molding of the samples was performed using a two‐piece aluminum mold with dimensions of 6 × 4 × 50 mm. To prevent the samples from sticking inside the mold, an acrylic separating fluid (Major, Italy) was used. Also, to modify the surface of silica NPs, the acrylate silane modifying agent, γ‐MPS, was used (Sigma Aldrich, Germany) as it is the most widely used silane coupling agent in dental resins (Karkanis et al. ; Kundie, Azhari, and Ahmad ; Rossi Canuto de Menezes et al. ). To measure the weight of the samples, a laboratory scale at 0.001 g accuracy (model M1003i, Bel, Italy) was used. An STM250 tensile device (Centam, England) was used to measure the fracture toughness of the samples. A mechanical stirrer (blender type) with a maximum power of 2000 revolutions per minute was applied to combine silica NPs with the base polymer. To measure the pH of the environment, the pH meter model PH‐80, manufactured by HM Digital Company, was used. 2.2 Sample Production 2.2.1 Nanosilica In order to achieve a uniform composition of silica NPs and PMMA, a vacuum mixer was used. In order to reduce the formation of agglomerates and achieve a better distribution of the filler particles inside the matrix, the surface of silica NPs was modified using an acrylate silane agent, ϒ‐MPS, and the hydrolysis–condensation method. For this purpose, a solution of water and alcohol with a weight ratio of 70 to 30 was used for the hydrolysis of ϒ‐MPS for 30 min. The pH was adjusted to around 3−4 by acetic acid. Then, NPs were added to this solution and kept at room temperature for 1 week. The NPs were washed with ethanol and then in room air, the remaining alcohol was evaporated, and the NPs were dried. 2.2.2 Incorporation of Nanosilica Into PMMA To produce each desired combination of nanosilica‐incorporated PMMA resin specimens, first, modified silica NPs with specific weight percentages (0% [control], 2%, 5%, 7%, and 10%) were mixed with polymer methyl methacrylate (PMMA) powder in a mixer. Grinding was performed for 12 min at a frequency of 20 Hz and a speed of 100 rpm. To produce nanomaterial‐enhanced resin specimens, the mixture of methyl methacrylate powder and milled NPs was mixed with a diluent liquid (TEGDMA) according to the manufacturer's instructions. This process was carried out mechanically at room temperature using a small stirrer. 2.2.3 Preparation of Each Specimen To prevent the produced paste from sticking to the mold, the mold surface was covered with a layer of acrylic separating liquid. Once the surface of the mold was dried (after about 3 h), the paste‐like liquid was poured into each part of the mold; then, the two parts of the mold were secured together. Next, the mold was immersed in hot water at a constant temperature of 80°C for 50 min. After that, the mold was removed and cooled down to room temperature, and then the polymerized sample was removed from the mold. Afterward, the samples were polished first with a sandpaper with 600 grit and then with a sandpaper with 1000 grit. 2.3 Grouping 2.3.1 Sample Size There were a total of 30 subgroups in this study: five groups of nanosilica percentages, each divided into six subgroups of pH levels. The sample size was determined as three specimens per subgroup (as per the ASTM D5045 standard) totaling 90 specimens in general. 2.3.2 Grouping Based on Nanosilica Percentage Initially, for each of the five weight percentages of nanosilica (i.e., 0%, 2%, 5%, 7%, 10%), 50 samples were produced. They were inspected carefully, and for each of the five groups, the best 15 samples with the least number of defects, the smoothest surfaces, and the fewest bubbles were selected. The group with zero percent of nanosilica was the control. The groups were named A (0% nanosilica, control), B (2%), C (5%), D (7%), and E (10%). 2.3.3 Grouping Based on pH There were six groups of pH. Considering that the pH range of a healthy adult human mouth is 5−9, there were five groups of pH, with pH = 5, 6, 7, 8, and 9. A “dry” control group was included as well, in which there was no moisture in the environment. The 18 specimens in each of the above five nanosilica groups were randomly divided into six subgroups of pH ( n = 5 nanosilica groups × 6 pH groups). One of these subgroups (in each of the five groups of nanosilica, n = 5 × 3) was the control, in which the samples were not placed in a humid environment. The rest of the samples were divided randomly into the remaining five pH groups as mentioned above ( n of each = 3 samples in each subgroup, 25 subgroups, overall n = 75). The samples were named as shown in Table . 2.4 Moisture Absorption Test All the specimens, except the ones within the dry environment (subgroups A0 to E0 in Table ), were placed in closed containers with a pH of 5−9 for 1 week. The moisture absorption test was performed under the ASTM D570 standard. As the pH range of a healthy and adult human is between 5 and 9, for this purpose, we performed moisture absorption tests in this range (at pH values of 5, 6, 7, 8, and 9). Sealed containers were used to increase the contact surface of the samples with the liquid and also to exclude any potential effects of the environment humidity or temperature on the pH of the tested liquid. Thus, each sample was placed in a coded container. The duration of the test for the moisture absorption test was 7 days. The samples were immersed in the solution for a week. For pH = 7, we used the same distilled water, the pH of which was measured with a device. ‎The liquid with different pH was also purchased from Pishgaman Shimi Company. These were distilled water that had been acidified using phosphoric acid or alkalinized using sodium bicarbonate. The pH of the solution was measured and calibrated in the laboratory of the Dezful Standard Department using a P811 pH meter with an accuracy of 0.1 (Yoko Company). During the tests, the liquid bath was in temperature equilibrium with the environment at a temperature of 20 ± 2°C. A digital scale with an accuracy of 0.001 g (M1003i, Bel, Italy‎) was used to weigh the samples. The amount of moisture absorption was determined by measuring the weight of the samples before and after placing them in the liquid bath according to the following ASTM D570 standard‎ formula (Equation ): (1) Δ w W = M 2 − M 1 M 1 where M1 is the primary mass of the sample and M2 is the secondary mass of the sample. 2.5 Fracture Resistance Test The fracture resistance test of nanosilica‐enhanced resin was performed based on the ASTM D5045 standard and using the three‐point bending method. In this test, a universal testing machine (AL‐7000‐M, Gotech, Taiwan) with a speed of 1.0 mm/min and a load cell of 20 KN was used. The method of loading the resin is shown in Figure , in which a rectangular piece with an edge crack of length is subjected to a three‐point bending load. Before conducting the fracture toughness test, according to the standard, prefabricated rectangular pieces were cut using a thin saw blade with a thickness of 250 mm to create an initial crack in the samples. The notches created by the thin blade were sharpened by pressing a razor. In this way, the grooves created in the samples can be assumed to have acceptable accuracy as a sharp crack to perform the fracture toughness test. Finally, the length of the edge crack created in the middle of each sample is equal to 3 mm. As a result, the crack length ratio to width of the sample in all experiments of this research is a constant value of 0.5. The fracture toughness test was performed on all of the 90 specimens described in Table . Of these, 75 specimens were placed in closed environments with different pH values for 1 week. In both groups of samples, before performing the bending test, the samples were removed from the containers and kept for 24 h in the standard conditions of the laboratory environment (temperature: 23 ± 2°C, humidity: 50 ± 5%). Then, the cracked samples were placed inside the supports of the three‐point bending fixture (with a support distance of 40 mm) in the universal testing machine and were monotonically loaded at a constant rate of 1 mm/min (Figure ). Loading continued until the moment of crack growth from the initial crack tip and complete failure of the parts, and finally, the load–displacement curve of each sample was obtained. Using the critical load corresponding to the fracture moment of each sample ( F S ), the value of fracture toughness ( K IC ) of dental resin specimens was calculated using Equation . (2) K IC = f a w F S t w × MPa m 1 2 where f ( a / w ) is the geometric coefficient, and it depends on the type of part and applied loading. This coefficient was calculated using finite element modeling of the tested part for the conditions of s / w = 3.3 and a / w = 0.5. Also, t is the thickness of the sample (in millimeters) and w is the width of the sample (in millimeters) (Equation ). (3) f a w = 2 + a w ( 0.886 + 4.64 a w − 13.32 a 2 w 2 + 14.72 a 3 w 3 − 5.6 a 4 w 4 ) ( 1 − a w ) 1 2 2.6 SEM After performing the fracture toughness test, we evaluated the fracture section of the samples ‎using an electron microscope (VEGA/TESCAN‐XMU model, the Czech Republic) at different magnifications. The ‎samples were selected based on two factors: the highest amount of moisture absorption and ‎the highest amount of fracture toughness. All samples were placed at pH = 7 and all samples had 5% ‎filler.‎ To perform the SEM test, the samples were first placed in a dehumidifier and completely dried. The samples were coated with gold according to the laboratory standard RMRC‐WI‐550‐103‐01. Afterward, electron micrographs were taken from the samples at magnifications of ×200, ×1000, and ×3000. The details of SEM images, including the voltage, field of view, depth, and magnification, are specified below each image. 2.7 Statistical Analysis Descriptive statistics and 95% confidence intervals (CIs) ‎were calculated. Two‐way analysis of variance (ANOVA) and the Bonferroni post hoc test of SPSS 25 (IBM, Armonk, NY, USA) were used to evaluate the effects of nanosilica concentrations and environment pH on moisture absorption and fracture toughness. The Pearson's correlation coefficient was used to evaluate the correlation between moisture absorption and fracture toughness. The level of significance was predetermined as 0.05. 

Materials and Equipment in Use In this experimental laboratory study, PMMA was used as a denture base material (Taiwan Chemical Company, Taiwan) with a specific gravity of 1.19 g/cm 3 . The process of polymerization and production of PMMA were carried out at the Razi Metallurgy Research Institute in Iran and the production process was carried out under the petrochemical license of Taiwan. Silica NPs with a size of 20 nm and a purity of 99.5% (Acrosun, Spain) were used as base polymer reinforcements. To reduce the viscosity, TEGDMA thinning resin (Evonic, Germany) was applied. Molding of the samples was performed using a two‐piece aluminum mold with dimensions of 6 × 4 × 50 mm. To prevent the samples from sticking inside the mold, an acrylic separating fluid (Major, Italy) was used. Also, to modify the surface of silica NPs, the acrylate silane modifying agent, γ‐MPS, was used (Sigma Aldrich, Germany) as it is the most widely used silane coupling agent in dental resins (Karkanis et al. ; Kundie, Azhari, and Ahmad ; Rossi Canuto de Menezes et al. ). To measure the weight of the samples, a laboratory scale at 0.001 g accuracy (model M1003i, Bel, Italy) was used. An STM250 tensile device (Centam, England) was used to measure the fracture toughness of the samples. A mechanical stirrer (blender type) with a maximum power of 2000 revolutions per minute was applied to combine silica NPs with the base polymer. To measure the pH of the environment, the pH meter model PH‐80, manufactured by HM Digital Company, was used. 

Sample Production 2.2.1 Nanosilica In order to achieve a uniform composition of silica NPs and PMMA, a vacuum mixer was used. In order to reduce the formation of agglomerates and achieve a better distribution of the filler particles inside the matrix, the surface of silica NPs was modified using an acrylate silane agent, ϒ‐MPS, and the hydrolysis–condensation method. For this purpose, a solution of water and alcohol with a weight ratio of 70 to 30 was used for the hydrolysis of ϒ‐MPS for 30 min. The pH was adjusted to around 3−4 by acetic acid. Then, NPs were added to this solution and kept at room temperature for 1 week. The NPs were washed with ethanol and then in room air, the remaining alcohol was evaporated, and the NPs were dried. 2.2.2 Incorporation of Nanosilica Into PMMA To produce each desired combination of nanosilica‐incorporated PMMA resin specimens, first, modified silica NPs with specific weight percentages (0% [control], 2%, 5%, 7%, and 10%) were mixed with polymer methyl methacrylate (PMMA) powder in a mixer. Grinding was performed for 12 min at a frequency of 20 Hz and a speed of 100 rpm. To produce nanomaterial‐enhanced resin specimens, the mixture of methyl methacrylate powder and milled NPs was mixed with a diluent liquid (TEGDMA) according to the manufacturer's instructions. This process was carried out mechanically at room temperature using a small stirrer. 2.2.3 Preparation of Each Specimen To prevent the produced paste from sticking to the mold, the mold surface was covered with a layer of acrylic separating liquid. Once the surface of the mold was dried (after about 3 h), the paste‐like liquid was poured into each part of the mold; then, the two parts of the mold were secured together. Next, the mold was immersed in hot water at a constant temperature of 80°C for 50 min. After that, the mold was removed and cooled down to room temperature, and then the polymerized sample was removed from the mold. Afterward, the samples were polished first with a sandpaper with 600 grit and then with a sandpaper with 1000 grit. 

Nanosilica In order to achieve a uniform composition of silica NPs and PMMA, a vacuum mixer was used. In order to reduce the formation of agglomerates and achieve a better distribution of the filler particles inside the matrix, the surface of silica NPs was modified using an acrylate silane agent, ϒ‐MPS, and the hydrolysis–condensation method. For this purpose, a solution of water and alcohol with a weight ratio of 70 to 30 was used for the hydrolysis of ϒ‐MPS for 30 min. The pH was adjusted to around 3−4 by acetic acid. Then, NPs were added to this solution and kept at room temperature for 1 week. The NPs were washed with ethanol and then in room air, the remaining alcohol was evaporated, and the NPs were dried. 

Incorporation of Nanosilica Into PMMA To produce each desired combination of nanosilica‐incorporated PMMA resin specimens, first, modified silica NPs with specific weight percentages (0% [control], 2%, 5%, 7%, and 10%) were mixed with polymer methyl methacrylate (PMMA) powder in a mixer. Grinding was performed for 12 min at a frequency of 20 Hz and a speed of 100 rpm. To produce nanomaterial‐enhanced resin specimens, the mixture of methyl methacrylate powder and milled NPs was mixed with a diluent liquid (TEGDMA) according to the manufacturer's instructions. This process was carried out mechanically at room temperature using a small stirrer. 

Preparation of Each Specimen To prevent the produced paste from sticking to the mold, the mold surface was covered with a layer of acrylic separating liquid. Once the surface of the mold was dried (after about 3 h), the paste‐like liquid was poured into each part of the mold; then, the two parts of the mold were secured together. Next, the mold was immersed in hot water at a constant temperature of 80°C for 50 min. After that, the mold was removed and cooled down to room temperature, and then the polymerized sample was removed from the mold. Afterward, the samples were polished first with a sandpaper with 600 grit and then with a sandpaper with 1000 grit. 

Grouping 2.3.1 Sample Size There were a total of 30 subgroups in this study: five groups of nanosilica percentages, each divided into six subgroups of pH levels. The sample size was determined as three specimens per subgroup (as per the ASTM D5045 standard) totaling 90 specimens in general. 2.3.2 Grouping Based on Nanosilica Percentage Initially, for each of the five weight percentages of nanosilica (i.e., 0%, 2%, 5%, 7%, 10%), 50 samples were produced. They were inspected carefully, and for each of the five groups, the best 15 samples with the least number of defects, the smoothest surfaces, and the fewest bubbles were selected. The group with zero percent of nanosilica was the control. The groups were named A (0% nanosilica, control), B (2%), C (5%), D (7%), and E (10%). 2.3.3 Grouping Based on pH There were six groups of pH. Considering that the pH range of a healthy adult human mouth is 5−9, there were five groups of pH, with pH = 5, 6, 7, 8, and 9. A “dry” control group was included as well, in which there was no moisture in the environment. The 18 specimens in each of the above five nanosilica groups were randomly divided into six subgroups of pH ( n = 5 nanosilica groups × 6 pH groups). One of these subgroups (in each of the five groups of nanosilica, n = 5 × 3) was the control, in which the samples were not placed in a humid environment. The rest of the samples were divided randomly into the remaining five pH groups as mentioned above ( n of each = 3 samples in each subgroup, 25 subgroups, overall n = 75). The samples were named as shown in Table . 

Sample Size There were a total of 30 subgroups in this study: five groups of nanosilica percentages, each divided into six subgroups of pH levels. The sample size was determined as three specimens per subgroup (as per the ASTM D5045 standard) totaling 90 specimens in general. 

Grouping Based on Nanosilica Percentage Initially, for each of the five weight percentages of nanosilica (i.e., 0%, 2%, 5%, 7%, 10%), 50 samples were produced. They were inspected carefully, and for each of the five groups, the best 15 samples with the least number of defects, the smoothest surfaces, and the fewest bubbles were selected. The group with zero percent of nanosilica was the control. The groups were named A (0% nanosilica, control), B (2%), C (5%), D (7%), and E (10%). 

Grouping Based on pH There were six groups of pH. Considering that the pH range of a healthy adult human mouth is 5−9, there were five groups of pH, with pH = 5, 6, 7, 8, and 9. A “dry” control group was included as well, in which there was no moisture in the environment. The 18 specimens in each of the above five nanosilica groups were randomly divided into six subgroups of pH ( n = 5 nanosilica groups × 6 pH groups). One of these subgroups (in each of the five groups of nanosilica, n = 5 × 3) was the control, in which the samples were not placed in a humid environment. The rest of the samples were divided randomly into the remaining five pH groups as mentioned above ( n of each = 3 samples in each subgroup, 25 subgroups, overall n = 75). The samples were named as shown in Table . 

Moisture Absorption Test All the specimens, except the ones within the dry environment (subgroups A0 to E0 in Table ), were placed in closed containers with a pH of 5−9 for 1 week. The moisture absorption test was performed under the ASTM D570 standard. As the pH range of a healthy and adult human is between 5 and 9, for this purpose, we performed moisture absorption tests in this range (at pH values of 5, 6, 7, 8, and 9). Sealed containers were used to increase the contact surface of the samples with the liquid and also to exclude any potential effects of the environment humidity or temperature on the pH of the tested liquid. Thus, each sample was placed in a coded container. The duration of the test for the moisture absorption test was 7 days. The samples were immersed in the solution for a week. For pH = 7, we used the same distilled water, the pH of which was measured with a device. ‎The liquid with different pH was also purchased from Pishgaman Shimi Company. These were distilled water that had been acidified using phosphoric acid or alkalinized using sodium bicarbonate. The pH of the solution was measured and calibrated in the laboratory of the Dezful Standard Department using a P811 pH meter with an accuracy of 0.1 (Yoko Company). During the tests, the liquid bath was in temperature equilibrium with the environment at a temperature of 20 ± 2°C. A digital scale with an accuracy of 0.001 g (M1003i, Bel, Italy‎) was used to weigh the samples. The amount of moisture absorption was determined by measuring the weight of the samples before and after placing them in the liquid bath according to the following ASTM D570 standard‎ formula (Equation ): (1) Δ w W = M 2 − M 1 M 1 where M1 is the primary mass of the sample and M2 is the secondary mass of the sample. 

Fracture Resistance Test The fracture resistance test of nanosilica‐enhanced resin was performed based on the ASTM D5045 standard and using the three‐point bending method. In this test, a universal testing machine (AL‐7000‐M, Gotech, Taiwan) with a speed of 1.0 mm/min and a load cell of 20 KN was used. The method of loading the resin is shown in Figure , in which a rectangular piece with an edge crack of length is subjected to a three‐point bending load. Before conducting the fracture toughness test, according to the standard, prefabricated rectangular pieces were cut using a thin saw blade with a thickness of 250 mm to create an initial crack in the samples. The notches created by the thin blade were sharpened by pressing a razor. In this way, the grooves created in the samples can be assumed to have acceptable accuracy as a sharp crack to perform the fracture toughness test. Finally, the length of the edge crack created in the middle of each sample is equal to 3 mm. As a result, the crack length ratio to width of the sample in all experiments of this research is a constant value of 0.5. The fracture toughness test was performed on all of the 90 specimens described in Table . Of these, 75 specimens were placed in closed environments with different pH values for 1 week. In both groups of samples, before performing the bending test, the samples were removed from the containers and kept for 24 h in the standard conditions of the laboratory environment (temperature: 23 ± 2°C, humidity: 50 ± 5%). Then, the cracked samples were placed inside the supports of the three‐point bending fixture (with a support distance of 40 mm) in the universal testing machine and were monotonically loaded at a constant rate of 1 mm/min (Figure ). Loading continued until the moment of crack growth from the initial crack tip and complete failure of the parts, and finally, the load–displacement curve of each sample was obtained. Using the critical load corresponding to the fracture moment of each sample ( F S ), the value of fracture toughness ( K IC ) of dental resin specimens was calculated using Equation . (2) K IC = f a w F S t w × MPa m 1 2 where f ( a / w ) is the geometric coefficient, and it depends on the type of part and applied loading. This coefficient was calculated using finite element modeling of the tested part for the conditions of s / w = 3.3 and a / w = 0.5. Also, t is the thickness of the sample (in millimeters) and w is the width of the sample (in millimeters) (Equation ). (3) f a w = 2 + a w ( 0.886 + 4.64 a w − 13.32 a 2 w 2 + 14.72 a 3 w 3 − 5.6 a 4 w 4 ) ( 1 − a w ) 1 2 

SEM After performing the fracture toughness test, we evaluated the fracture section of the samples ‎using an electron microscope (VEGA/TESCAN‐XMU model, the Czech Republic) at different magnifications. The ‎samples were selected based on two factors: the highest amount of moisture absorption and ‎the highest amount of fracture toughness. All samples were placed at pH = 7 and all samples had 5% ‎filler.‎ To perform the SEM test, the samples were first placed in a dehumidifier and completely dried. The samples were coated with gold according to the laboratory standard RMRC‐WI‐550‐103‐01. Afterward, electron micrographs were taken from the samples at magnifications of ×200, ×1000, and ×3000. The details of SEM images, including the voltage, field of view, depth, and magnification, are specified below each image. 

Statistical Analysis Descriptive statistics and 95% confidence intervals (CIs) ‎were calculated. Two‐way analysis of variance (ANOVA) and the Bonferroni post hoc test of SPSS 25 (IBM, Armonk, NY, USA) were used to evaluate the effects of nanosilica concentrations and environment pH on moisture absorption and fracture toughness. The Pearson's correlation coefficient was used to evaluate the correlation between moisture absorption and fracture toughness. The level of significance was predetermined as 0.05. 

Results The raw results pertaining to moisture absorption of all 75 specimens are presented in Table . Overall, the mean and standard deviation (SD) for moisture absorption in all 75 samples were ‎0.0149880 ± ‎0.01249969‎ (standard error: ‎0.00144334, 95% CI: ‎0.0121121 to ‎0.0178639, minimum: ‎0.00140‎, maximum: ‎0.06460), respectively. ‎Descriptive statistics and 95% CIs for moisture absorption in different groups and subgroups are presented in Tables and . Raw results of fracture resistance are presented in Table . Overall, the mean and SD for moisture absorption in all 90 samples were ‎25.3506 ± 22.47070‎ (standard error: 2.40911, 95% CI: ‎20.5614 to ‎30.1397, minimum: ‎9.54‎, maximum: ‎107.40), respectively. ‎Descriptive statistics and 95% CIs for fracture resistance in different groups and subgroups are presented in Tables and . 3.1 Fracture Resistance The two‐way ANOVA showed a significant difference between fracture resistances of all nanosilica groups ( p < 0.00001) and between fracture resistances of all pH groups ( p < 0.00001). The interaction of nanosilica and pH was ‎significant ( p < 0.00001). ‎ The Bonferroni test showed significant pairwise comparisons between fracture resistances in different pH groups (including the dry control, Table ) and different nanosilica groups (Table , Figure ). 3.2 Moisture Absorption Two‐way ANOVA identified both the variables nanosilica ( p < 0.00001) and pH ( p < 0.00001). The interaction of nanosilica and pH was significant as well ( p < 0.00001). The Bonferroni test showed significant pairwise comparisons between moisture absorptions in different pH groups (Table ) and different nanosilica groups (Table , Figure ). 3.3 Correlation The Pearson's coefficient showed a significant negative correlation between fracture toughness and moisture absorption ( R = −0.382, p = 0.0009). 3.4 SEM SEM images taken from the surface of samples show that moisture absorption causes cavities and swelling in the base polymer, and the ‎samples break as a result of the creation and expansion of cracks that are created around these ‎cavities (Figures , , , , , ). SEM images showed that up to 7 wt% of the filler, the dispersion of silicon particles was good, but for the case where 10 wt% of the filler was used, the dispersion of the particles was not uniform. 

Fracture Resistance The two‐way ANOVA showed a significant difference between fracture resistances of all nanosilica groups ( p < 0.00001) and between fracture resistances of all pH groups ( p < 0.00001). The interaction of nanosilica and pH was ‎significant ( p < 0.00001). ‎ The Bonferroni test showed significant pairwise comparisons between fracture resistances in different pH groups (including the dry control, Table ) and different nanosilica groups (Table , Figure ). 

Moisture Absorption Two‐way ANOVA identified both the variables nanosilica ( p < 0.00001) and pH ( p < 0.00001). The interaction of nanosilica and pH was significant as well ( p < 0.00001). The Bonferroni test showed significant pairwise comparisons between moisture absorptions in different pH groups (Table ) and different nanosilica groups (Table , Figure ). 

Correlation The Pearson's coefficient showed a significant negative correlation between fracture toughness and moisture absorption ( R = −0.382, p = 0.0009). 

SEM SEM images taken from the surface of samples show that moisture absorption causes cavities and swelling in the base polymer, and the ‎samples break as a result of the creation and expansion of cracks that are created around these ‎cavities (Figures , , , , , ). SEM images showed that up to 7 wt% of the filler, the dispersion of silicon particles was good, but for the case where 10 wt% of the filler was used, the dispersion of the particles was not uniform. 

Discussion As there was no study similar to our explorative research, we are limited to discussion of more general aspects of the study. The literature does not contain adequate studies in this regard, and the existing ones are about scattered and different aspects and properties of this field, not to mention that they are controversial at points. Zhen et al. ‎  and Balos et al. ‎  found that by increasing silica NPs from 0% to 5%, the tensile strength and elongation at the breaking point of the resins improved. Also, the bending strength and elastic modulus increased significantly. Fatalla, Tukmachi, and Jani  observed a slight increase in impact strength, transverse strength, and hardness with a negligible amount of 5% wt of nanosilica. Siot et al.  asserted that although silica NPs increase the static modulus and yield strength of resins, the final properties of the resin are strongly affected by the dispersion properties of the NPs, such that a better performance of the silica filler is related to its regular dispersion. Evaluating the effect of mixing modified and unmodified silica NPs, Mussatto et al. ‎  concluded that by adding silica NPs, the bending strength of the resin in both modified and unmodified states decreased by 20%−27%; this reduction was not dependent on the amount of filler and the method of combining NPs. Moreover, they found that the surface modification of nanosilica did not cause any major improvement in the mechanical behavior of nano‐resins, but it seemed to improve the dispersion because the surface roughness was smaller. Salman, Jani, and Fatalla  observed very significant improvements in impact strength, transverse strength, and stiffness by adding nanosilica to PMMA at 3%, 5%, and 7% by weight. They concluded that the addition of NPs in the optimal state could yield better physical and mechanical properties. Topouzi et al.  observed a significant increase in fracture toughness by adding modified and unmodified silica NPs to PMMA; nevertheless, they concluded that increasing the concentration of the filler decreased the fracture toughness. Also, they did not report any significant difference between modified and unmodified fillers. Kundie, Azhari, Muchtar, et al. ‎  concluded that the effect of NPs on the mechanical properties of dental resins strongly relied on factors such as the type and mechanical properties of mineral nanofillers, the uniform dispersion of nanofillers in the polymer matrix, the volume of the filler particles, and the type of silane used. In the present study, polymeric resins filled with different percentages of hard mineral particles in nano dimensions showed different behaviors with increasing percentage of particles. Polymer water absorption is the result of molecular polarity, unsaturated bonds of molecules, and unbalanced intermolecular forces in polymers. According to the ISO 1567 standard, water absorption for thermal and self‐baking materials should not exceed 32 mg/mm 3 (Karabela and Sideridou ). According to our results, with increasing percentage of nanosilica, the amount of moisture absorption decreases. The reason for this could be related to the amorphous structure of the background polymer. The distance between PMMA molecules in the absence of silica filler particles was greater than the matrix to which NPs were added. Silica NPs fill the space between molecules, so the more the amount of nanosilica in the base polymer, the smaller the distance between molecules, and more regular the structure of the polymer, and hence a decrease in the amount of moisture absorption (Zirak et al. ). The results of the experiments also showed the lowest amount of moisture absorption at 10 wt% of nanosilica (Zirak et al. ). Furthermore, by comparing the findings obtained at different pH levels, it was observed that the more neutral the pH of the liquid bath, the higher the absorption rate. The highest amount of moisture absorption was observed at pH = 7. The chemical structure of PMMA shows that the background polymer atoms do not tend to react with acidic and basic solvents, and as a result, the amount of absorption in these environments decreases. Also, silica NPs that were in the base polymer did not tend to react with acids and bases (Ilie et al. ). Therefore, it was expected that as the liquid became acidic or basic, the amount of absorption decreased. Studies show that the main problem with using silica NPs in dental resins is the accumulation of silica NPs next to each other when combined with PMMA, which leads to the formation of micrometer particles, and this reduces the mechanical properties of resins enriched with NPs (Siot et al. ; Kundie, Azhari, Muchtar, et al. ). Therefore, surface modification of the filler particles is needed to reduce the formation of agglomerates and achieve a better distribution of the filler particles inside the matrix (Mussatto et al. ). Our findings indicated that increasing the amount of silica filler increased the fracture toughness of the samples. However, when the amount of filler was more than 5 wt%, not only did the fracture toughness not improve but also it began to decrease. Thus, in the case of 10 wt% nanosilica, the fracture resistance of these samples was lower than that of samples without silica filler (Balos et al. ). As polymers have an amorphous and irregular structure, increasing the filler particles reduces the distance between the particles, which reduces the crack propagation rate. Increasing the length of the crack propagation path increases the fracture toughness of dental resin. Smaller sizes of the filler particles increase the surface energy created at the junction of the resin and the filler, and this improves flexural strength. The addition of NPs also improved the hardness because solid mineral particles usually have higher hardness compared to polymeric matrices. Researchers believe that the reason for the increase in the fracture toughness of resins by addition of filler is due to the interaction of the crack tip with the filler phase. Moreover, increasing the amount of filler increases the accumulation of particles and causes the formation of agglomerates. The formation of these points accelerates crack growth and decreases fracture toughness (Kundie, Azhari, Muchtar, et al. ). Although in this research, an attempt was made to create a more uniform distribution of silica NPs on the surface of the base polymer by modifying the filler particles, in high weight percentages, the accumulation of NPs created agglomerates. Therefore, the fracture toughness of resins is a complex process, and its increase or decrease cannot be attributed only ‎to the concentration of fillers. The study on the mechanism of failure in resins shows that the type of filler, the process of failure, the protrusion and thickness of the crack tip, the plastic yield limit of the matrix, the structure of the resin, the separation of the interface between the particles and the matrix, and especially the accumulation of filler particles are among other factors that can affect the test results of the fracture toughness of resins. Also, by examining the fracture toughness of samples that were in a humid environment, it was found that moisture absorption caused a significant decrease in the fracture toughness of dental samples. As the amount of moisture absorption increased, the fracture toughness decreased. Hence, at pH = 7, which had the highest moisture absorption, the greatest decrease in failure toughness was observed. SEM images showed that moisture absorption caused the formation of microcracks in the base polymer, and this led to crack growth and, as a result, reduced fracture toughness of the dental samples. Furthermore, PMMA swelled slightly by absorbing water, which is different from the polymer composition of the particle components, and this weakened the mechanical properties, including the fracture toughness of the dental samples (Karabela and Sideridou ; Zirak et al. ). 

Conclusion It can be concluded that 1. Increasing the amount of silica filler improves the fracture toughness, but there is an optimal limit (5 wt% nanosilica filler). 2. Increasing the percentage of nanosilica filler leads to the accumulation of nanosilica particles and the formation of agglomerates. 3. Increasing the nanosilica filler concentration reduces moisture absorption. 4. The tested samples react to acidic and alkaline environments. Accordingly, the more the acidity or alkalinity of the liquid becomes neutral, the more the absorption of moisture in the dental samples increases. 5. Also, in samples with less than 10% filler weight, the amount of moisture absorption in acidic environments is higher than that in alkaline environments, but at 10 wt% of the filler, the amount of moisture absorption in an alkaline environment is higher compared to an acidic environment. 6. With an increase of moisture absorption, the fracture toughness of dental samples decreases significantly. 

All the authors meet the ICMJE criteria for authorship. Nima Refahati and Mohammad Ali Golshokouh made substantial contributions to the conception or design, acquisition, analysis, and interpretation of data. Mohammad Ali Golshokouh, Nima Refahati, and Pouyan Roodgar Saffari drafted and critically reviewed the work for important intellectual content, and approved the final version. All authors agreed to be accountable for all aspects of the work, ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. 

The authors declare no conflicts of interest. 

