Influence of nanogel additive hydrophilicity on dental adhesive mechanical performance and dentin bonding

Highlights

  • The hydrophilicity of nanogel may affect the performance of the adhesive.

  • Hydrophobic nanogel had less degradation and the best mechanical properties.

  • An amphiphilic nanogel showed the highest bond strength values.

  • The strongest material did not show the best interaction with the substrate.

Abstract

Objective

To assess the influence of hydrophilicity of reactive nanogels on the mechanical performance of dental adhesives and microtensile bond strength (μTBS) to dentin after 24 h or 3 months of aging.

Methods

A series of three nanogels were synthesized: NG1—IBMA/UDMA; NG2—HEMA/BisGMA; NG3—HEMA/TE-EGDMA. The nanogels were dispersed in solvent, HEMA or BisGMA/HEMA. The degree of conversion (DC) of the materials was measured and the flexural modulus of these polymers was evaluated in dry or wet conditions. For μTBS analysis, a model adhesive was used without nanogel (control) or with the incorporation of nanogels. μTBS was evaluated after storage in distilled water for 24 h or 3 months. The analysis of the fracture was performed after μTBS testing. Data were analyzed using ANOVA and Tukey’s test (α = 0.05).

Results

Water significantly increased the modulus of NG1 and NG2 dispersed in solvent, while significantly decreased the stiffness of NG3. All polymers dispersed in HEMA and BisGMA/HEMA had significantly lower modulus when stored in water. NG2 showed the highest DC in solvent and BisGMA/HEMA. In HEMA, NG1 and NG3 produced the highest DC. After three months, NG2 showed the best μTBS. The μTBS of NG2-containing adhesive resin significantly increased after 3 months, while storage had no effect in the control group, NG1 and NG3.

Significance

The more hydrophobic IBMA/UDMA nanogel showed higher bulk material mechanical property results, but the best dentin bond strength values, and notably strength values that improved upon storage, were obtained with the amphiphilic nanogel based on BisGMA/HEMA.

Introduction

Although high quality adhesive systems are available for applications in restorative dentistry, the great immediate results of bond strength typically decline with storage time. The adhesive interface remains the weakest component of dental restorations based on varied degrees of degradation over relatively short times . This seems to be due to several factors such as substrate characteristics, technical limitations, aging conditions in the oral environment (i.e., temperature, moisture and chewing load) and the composition of adhesives .

To ensure appropriate hybridization of wet collagen matrix, increasing concentrations of hydrophilic and ionic monomers have been added to adhesives . Furthermore, simplified adhesive systems also have a large concentration of solvents and hydrophilic monomers , which interferes in the polymerization and decreases the degree of conversion . The addition of hydrophilic monomers forms a network with low cross-linking density, and increases water sorption/solubility and resin plasticization . These factors are thought to contribute directly to the hydrolytic and potential enzymatic degradation of the polymer resin.

Different strategies have been developed to control and prevent the degradation of the hybrid layer, such as ethanol wet bonding , chlorhexidine , and antioxidants/crosslinking agents . Moreover, good results have also been obtained using nanomaterials in adhesive formulations .

Nanotechnology in the form of reactive nano-scale prepolymeric particles that can be swollen by monomer has attracted substantial interest due to the versatile structures with multiple applications in the drug delivery, tissue engineering and polymer composites . While a large number of biomedical applications envolve nanogels as freely dispersed particles, recent studies have applied nanogels as functional fillers or additives in the preparation of nano-composite polymer networks .

In general, incorporation of nanogels in dental adhesive systems and composites reduced shrinkage and improved mechanical properties such as flexural modulus and flexural strength, both in dry and wet conditions, due to the strengthening of the polymeric network by the presence of the crosslinked particles . Furthermore, water solubility was reduced, and short-term bond strength to dentin was improved significantly with the inclusion of the nanogels, without need to modify the existing application techniques .

Despite the use of nanogels as an interesting option for the modification of dental materials, the effects of comonomer combinations within the nanogels that produce different levels of hydrophilicity have not been studied with regard to adhesive formulations and bonding to a dentin substrate. The interaction between materials and substrates is very important for the establishment of efficient and effective adhesive bonds. It is reasonable to expect that specific types of nanoparticles may be able to influence the properties and durability of polymers. Thus, the aim of this study was to compare three nanogels that systematically differ in terms of hydrophilicity and assess how the incorporation of these nanostructures into dental adhesives will influence the mechanical performance of materials and bond strength to dentin over early storage times. The null hypotheses to be tested were: (1) nanogels with different hydrophilicity would not affect the mechanical performance when exposed to water; (2) nanogels with different hydrophilicity would not interfere with dentin bond strength.

Materials and methods

Nanogel synthesis

Three nanogel (NG) copolymers were synthesized at a 70:30 molar ratio of: isobornyl methacrylate (IBMA; TCI America, Portland, OR, USA) and urethane dimethacrylate (UDMA; Sigma–Aldrich Co., St. Louis, MO, USA) (NG1); 2-hydroxyethyl methacrylate (HEMA; TCI America) and bisphenol A glycerolate dimethacrylate (BisGMA; Esstech, Essington, PA, USA) (NG2); 2-hydroxyethyl methacrylate (HEMA) and tetraethylene glycol dimethacrylate (TE-EGDMA; TCI America) (NG3). 2-Mercaptoethanol (ME; Sigma–Aldrich) was added (10 mol% for NG1, 40 mol% for NG2, and 15 mol% for NG3 relative to monomers) as a chain-transfer agent to avoid macrogelation, control molecular weight/nanogel particle size, and provide sites for post-polymerization refunctionalization with reactive groups. Free radical polymerization was conducted in solution (six-fold excess for NG1, eight-fold excess for NG2, seven-fold excess for NG3 of methyl ethyl ketone (MEK; Fisher Scientific, Waltham, MA, USA) relative to monomer) with 1 wt% 2,2′-azobisisobutyronitrile (AIBN; Sigma–Aldrich) as thermal initiator. A 100 mL round-bottom flask was used as the reactor with monomer batch sizes of approximately 10 g with reaction conditions of 80 °C and a stirring rate of 200 rpm. Methacrylate conversion during nanogel synthesis was calculated from mid-IR spectra (Nicolet 6700, Thermo Scientific, Waltham, MA, USA) before and after polymerization.

Nanogels were purified by precipitation from hexanes (10-fold excess; Fisher Scientific) and filtration. Resulting precipitates were re-suspended in dichloromethane (BDH Chemicals, VWR Analytical, Radnor, PA, USA) for NG1 and NG3, and acetone (Fisher Scientific) for NG2, and reacted at room temperature with a 10 mol% for NG1, 10 mol% for NG2, and 15 mol% for NG3 of 2-isocyanoethyl methacrylate (IEM; TCI America) with a trace amount of dibutyltin dilaurate (Sigma–Aldrich) as catalyst. The polymer precipitation method was repeated to isolate the methacrylate-functionalized reactive nanogel. Residual solvent was removed completely under vacuum until the nanogels were obtained as dry powders.

Nanogel particle characterization

Polymeric nanogels were characterized by triple-detector (refractive index, viscosity, light scattering) gel permeation chromatography—GPC (GPCmax; Viscotek, Malvern Instruments, Malvern, UK) in tetrahydrofuran (EMD Millipore, Billerica, MA, USA). The glass transition temperature (Tg) of nanogel powders was determined by dynamic mechanical analysis (DMA 8000, PerkinElmer, Waltham, MA, USA) by sandwiching 10 mg of nanogel in a thin metallic pocket that was then subjected to single cantilever cyclic displacement of 50 μm at 1 Hz. The nanogel was heated to 150 °C with tan δ data collected as the sample was cooled to 0 °C at 2 °C/min in air.

Mechanical strength characterization

The flexural modulus was evaluated for all nanogels dispersed only with inert solvent or in HEMA or BisGMA/HEMA. For the solvated nanogel groups, NG1 was dispersed in N , N -dimethylformamide (DMF) (Sigma–Aldrich), while NG2 and NG3 were dispersed in ethanol (Fisher Scientific), according to their solubility characteristics. Slight differences in the weight percentages of nanogels were established according to the specific viscosity characteristics of the blends. This was done as needed to normalize the viscosities between formulations and avoid difficulties related to flow in handling and specimen preparation. When in solvent, nanogels were dispersed at 50:50 wt% NG:solvent; when nanogels were dispersed in HEMA, a 50:50 wt% NG:monomer proportion was used for NG2 and NG3, while a 48:52 wt% NG:monomer formulation was used for NG1; and when the nanogels were dispersed in BisGMA/HEMA (40:60 wt%) in a 40:60 wt% NG:resin ratio. Viscosity measurements of the nanogel-modified materials were performed using a cone-plate digital viscometer (CAP2000+; Brookfield, Middleboro, MA, USA) at ambient temperature.

Flexural modulus tests were performed using bar specimens (n = 8) with dimensions of 25 mm × 2 mm × 2 mm, light-cured between glass slides in an elastomer mold by exposure to UV light at 60 mW/cm 2 for 2 min on each side (mercury arc lamp 365 nm—Acticure 4000, EXFO, Richardson, TX, USA). 2,2-Dimethoxy-2-phenylacetophenone (Sigma–Aldrich) was used at 0.2 wt% as an efficient UV photoinitiator to obtain the bulk adhesive resin polymers with high conversion to allow meaningful analysis of the effect of the different nanogels in these materials. The degree of conversion (Nicolet 6700) of all the bar specimens was evaluated following the preparation of the samples, before storage. Methacrylate conversion was calculated by following the area of the methacrylate absorption band in near-IR (6165 cm −1 ) before and after photoactivation. The bars were stored in an oven (Single-Wall Transite Oven; Blue M Electric Company, Blue Island, IL, USA) overnight at 37 °C and then randomly divided into two groups (n = 8): dry storage condition and wet storage condition. After one-week additional storage in dark containers (dry or in distilled water) at room temperature, which provided coordinated testing of dry and wet polymers after sufficient time for near-equilibrium water uptake in the wet storage samples, the flexural modulus was obtained in three-point bending on the MTS testing machine using a span of 20 mm and a cross-head speed of 1 mm/min (MTS Mini Bionix II, MTS, Eden Prairie, MN, USA).

Dentin-bonding study

A dentin-bonding study was conducted with a model ethanol-solvated (30 wt%) BisGMA/HEMA (40:60 wt%) etch-and-rinse 1 bottle adhesive as a control or containing nanogels 1, 2 or 3 at 40 wt% relative to the BisGMA/HEMA monomer content. The 40 wt% nanogel loading level was selected to provide near confluent nanogel packing in the adhesive resin upon evaporation of a majority of the solvent . Camphorquinone (Sigma–Aldrich) and ethyl 4- N , N -dimethylaminobenzoate (Sigma–Aldrich) were added at 0.2 and 0.6 wt%, respectively, as co-initiators.

Forty extracted human molars were obtained under approval of the institutional ethics committee (protocol 022/2014) and stored in distilled water at 4 °C no longer than 6 months. The roots were removed 2 mm beneath the cement-enamel junction (CEJ), while the occlusal enamel was cut 2 mm above the CEJ using a slow-speed water-cooled diamond saw (Isomet, Buehler; Lake Bluff, IL, USA), exposing a flat surface of dentin. The exposed dentin surface was abraded before bonding with a 600-grit SiC paper for 60 s under running water, with the intent to create a standardized smear layer.

Prepared teeth were randomly divided into four groups (n = 10) according to adhesive systems (with or without NG). The samples were subjected to acid etching with 35% phosphoric acid (Scotchbond Universal Etchant, 3M ESPE) for 15 s. All adhesive systems were applied in 2 coats. Light activation was performed for 10 s using a LED Elipar Deepcure-S lamp (1000–1200 mW/cm 2 , 3M ESPE, St. Paul, MN, USA). Composite buildups were constructed using Filtek Z-100 (3M ESPE) in 2 layers (each layer 2 mm thick). Approximately 20–25 sticks (bonding area 1 × 1 mm 2 ) were obtained from each molar by sectioning the bonded teeth. The sticks from the periphery showing remaining enamel were excluded. Half of the sticks were tested after 24 h of restoration and the other half after 3 months of storage in distilled water at 37 °C.

The sticks were fixed to a jig using a cyanoacrylate gel and tested to failure on MTS testing machine at a crosshead speed of 1.0 mm/min. The cross-sectional area of the sticks was measured before failure with a digital caliper. The μTBS of the sticks from the same bonded tooth were averaged and used for the statistical analysis as the statistical unit. Means and standard deviations were calculated and expressed in MPa. A small number of the bonded specimens failed prematurely and were not included in the statistical analysis. This defect exclusion is supported by the very limited numbers involved and that the range of bond strengths achieved was all relatively narrow and well above that associated with spontaneous failure.

The fractured specimens were analyzed using SEM JSM-5600LV (JEOL; Tokyo, Japan). In brief, the specimens were paired and mounted on aluminum stubs and subsequently dehydrated in silica gel. Finally, the specimens were gold-sputter coated and examined using SEM, operated at 15 kV. The fractures were classified as adhesive, cohesive in composite, cohesive in adhesive, cohesive in dentin or mixed.

Sorption and solubility and contact angle

Disc specimens (1 × 5 mm, n = 5) were produced between glass slides with a model ethanol-solvated BisGMA/HEMA (40:60 wt%) etch-and-rinse 1 bottle adhesive as a control or containing nanogels 1, 2 or 3 at 40 wt% relative to the BisGMA/HEMA monomer content. Camphorquinone (Sigma–Aldrich) and ethyl 4- N , N -dimethylaminobenzoate (Sigma–Aldrich) were added at 0.2 and 0.6 wt%, respectively, as co-initiators. Ethanol was included in the adhesive resin at 12 wt% to simulate a adhesive containing residual solvent . The samples were photoactivated for 2 min on each side by exposure to visible light at 100 mW/cm 2 (mercury arc lamp 320–500 nm—Acticure 4000, EXFO).

Water sorption (WS) and solubility (SL) were calculated as: WS = m2 − m3/V and SL = m1 − m3/V, where m1 is the initial adhesive polymer mass before water immersion, m2 is the wet mass of the surfaceblotted water-equilibrated polymer, and m3 is the dry mass of the polymer after 7 days’ storage in a desiccator. The water contact angle of the 12% ethanol-containing pre-immersion discs was measured by means of a goniometer (Ramé-Hart Instruments Co., Netcong, NJ, USA). One drop of distilled water was placed on the surface of the material, then a profile image was recorded and the contact angle (n = 3) measured (DROPimage Advanced; Ramé-Hart).

Statistical analysis

The data were statistically analyzed by equal variance and Kolmogorov–Smirnov normality tests and, after proving the normality of data and homogeneity of variances, the results were analyzed using one-way ANOVA (nanogel, for degree of conversion) and two-way ANOVA (nanogel and condition – dry or wet, for flexural modulus; and nanogel and storage time – 24 h and 3 months, for microtensile test) and Tukey’s test (α = 0.05). For flexural results and degree of conversion, the analyses were performed for each dispersion medium separately (solvent, HEMA and BisGMA/HEMA).

Materials and methods

Nanogel synthesis

Three nanogel (NG) copolymers were synthesized at a 70:30 molar ratio of: isobornyl methacrylate (IBMA; TCI America, Portland, OR, USA) and urethane dimethacrylate (UDMA; Sigma–Aldrich Co., St. Louis, MO, USA) (NG1); 2-hydroxyethyl methacrylate (HEMA; TCI America) and bisphenol A glycerolate dimethacrylate (BisGMA; Esstech, Essington, PA, USA) (NG2); 2-hydroxyethyl methacrylate (HEMA) and tetraethylene glycol dimethacrylate (TE-EGDMA; TCI America) (NG3). 2-Mercaptoethanol (ME; Sigma–Aldrich) was added (10 mol% for NG1, 40 mol% for NG2, and 15 mol% for NG3 relative to monomers) as a chain-transfer agent to avoid macrogelation, control molecular weight/nanogel particle size, and provide sites for post-polymerization refunctionalization with reactive groups. Free radical polymerization was conducted in solution (six-fold excess for NG1, eight-fold excess for NG2, seven-fold excess for NG3 of methyl ethyl ketone (MEK; Fisher Scientific, Waltham, MA, USA) relative to monomer) with 1 wt% 2,2′-azobisisobutyronitrile (AIBN; Sigma–Aldrich) as thermal initiator. A 100 mL round-bottom flask was used as the reactor with monomer batch sizes of approximately 10 g with reaction conditions of 80 °C and a stirring rate of 200 rpm. Methacrylate conversion during nanogel synthesis was calculated from mid-IR spectra (Nicolet 6700, Thermo Scientific, Waltham, MA, USA) before and after polymerization.

Nanogels were purified by precipitation from hexanes (10-fold excess; Fisher Scientific) and filtration. Resulting precipitates were re-suspended in dichloromethane (BDH Chemicals, VWR Analytical, Radnor, PA, USA) for NG1 and NG3, and acetone (Fisher Scientific) for NG2, and reacted at room temperature with a 10 mol% for NG1, 10 mol% for NG2, and 15 mol% for NG3 of 2-isocyanoethyl methacrylate (IEM; TCI America) with a trace amount of dibutyltin dilaurate (Sigma–Aldrich) as catalyst. The polymer precipitation method was repeated to isolate the methacrylate-functionalized reactive nanogel. Residual solvent was removed completely under vacuum until the nanogels were obtained as dry powders.

Nanogel particle characterization

Polymeric nanogels were characterized by triple-detector (refractive index, viscosity, light scattering) gel permeation chromatography—GPC (GPCmax; Viscotek, Malvern Instruments, Malvern, UK) in tetrahydrofuran (EMD Millipore, Billerica, MA, USA). The glass transition temperature (Tg) of nanogel powders was determined by dynamic mechanical analysis (DMA 8000, PerkinElmer, Waltham, MA, USA) by sandwiching 10 mg of nanogel in a thin metallic pocket that was then subjected to single cantilever cyclic displacement of 50 μm at 1 Hz. The nanogel was heated to 150 °C with tan δ data collected as the sample was cooled to 0 °C at 2 °C/min in air.

Mechanical strength characterization

The flexural modulus was evaluated for all nanogels dispersed only with inert solvent or in HEMA or BisGMA/HEMA. For the solvated nanogel groups, NG1 was dispersed in N , N -dimethylformamide (DMF) (Sigma–Aldrich), while NG2 and NG3 were dispersed in ethanol (Fisher Scientific), according to their solubility characteristics. Slight differences in the weight percentages of nanogels were established according to the specific viscosity characteristics of the blends. This was done as needed to normalize the viscosities between formulations and avoid difficulties related to flow in handling and specimen preparation. When in solvent, nanogels were dispersed at 50:50 wt% NG:solvent; when nanogels were dispersed in HEMA, a 50:50 wt% NG:monomer proportion was used for NG2 and NG3, while a 48:52 wt% NG:monomer formulation was used for NG1; and when the nanogels were dispersed in BisGMA/HEMA (40:60 wt%) in a 40:60 wt% NG:resin ratio. Viscosity measurements of the nanogel-modified materials were performed using a cone-plate digital viscometer (CAP2000+; Brookfield, Middleboro, MA, USA) at ambient temperature.

Flexural modulus tests were performed using bar specimens (n = 8) with dimensions of 25 mm × 2 mm × 2 mm, light-cured between glass slides in an elastomer mold by exposure to UV light at 60 mW/cm 2 for 2 min on each side (mercury arc lamp 365 nm—Acticure 4000, EXFO, Richardson, TX, USA). 2,2-Dimethoxy-2-phenylacetophenone (Sigma–Aldrich) was used at 0.2 wt% as an efficient UV photoinitiator to obtain the bulk adhesive resin polymers with high conversion to allow meaningful analysis of the effect of the different nanogels in these materials. The degree of conversion (Nicolet 6700) of all the bar specimens was evaluated following the preparation of the samples, before storage. Methacrylate conversion was calculated by following the area of the methacrylate absorption band in near-IR (6165 cm −1 ) before and after photoactivation. The bars were stored in an oven (Single-Wall Transite Oven; Blue M Electric Company, Blue Island, IL, USA) overnight at 37 °C and then randomly divided into two groups (n = 8): dry storage condition and wet storage condition. After one-week additional storage in dark containers (dry or in distilled water) at room temperature, which provided coordinated testing of dry and wet polymers after sufficient time for near-equilibrium water uptake in the wet storage samples, the flexural modulus was obtained in three-point bending on the MTS testing machine using a span of 20 mm and a cross-head speed of 1 mm/min (MTS Mini Bionix II, MTS, Eden Prairie, MN, USA).

Dentin-bonding study

A dentin-bonding study was conducted with a model ethanol-solvated (30 wt%) BisGMA/HEMA (40:60 wt%) etch-and-rinse 1 bottle adhesive as a control or containing nanogels 1, 2 or 3 at 40 wt% relative to the BisGMA/HEMA monomer content. The 40 wt% nanogel loading level was selected to provide near confluent nanogel packing in the adhesive resin upon evaporation of a majority of the solvent . Camphorquinone (Sigma–Aldrich) and ethyl 4- N , N -dimethylaminobenzoate (Sigma–Aldrich) were added at 0.2 and 0.6 wt%, respectively, as co-initiators.

Forty extracted human molars were obtained under approval of the institutional ethics committee (protocol 022/2014) and stored in distilled water at 4 °C no longer than 6 months. The roots were removed 2 mm beneath the cement-enamel junction (CEJ), while the occlusal enamel was cut 2 mm above the CEJ using a slow-speed water-cooled diamond saw (Isomet, Buehler; Lake Bluff, IL, USA), exposing a flat surface of dentin. The exposed dentin surface was abraded before bonding with a 600-grit SiC paper for 60 s under running water, with the intent to create a standardized smear layer.

Prepared teeth were randomly divided into four groups (n = 10) according to adhesive systems (with or without NG). The samples were subjected to acid etching with 35% phosphoric acid (Scotchbond Universal Etchant, 3M ESPE) for 15 s. All adhesive systems were applied in 2 coats. Light activation was performed for 10 s using a LED Elipar Deepcure-S lamp (1000–1200 mW/cm 2 , 3M ESPE, St. Paul, MN, USA). Composite buildups were constructed using Filtek Z-100 (3M ESPE) in 2 layers (each layer 2 mm thick). Approximately 20–25 sticks (bonding area 1 × 1 mm 2 ) were obtained from each molar by sectioning the bonded teeth. The sticks from the periphery showing remaining enamel were excluded. Half of the sticks were tested after 24 h of restoration and the other half after 3 months of storage in distilled water at 37 °C.

The sticks were fixed to a jig using a cyanoacrylate gel and tested to failure on MTS testing machine at a crosshead speed of 1.0 mm/min. The cross-sectional area of the sticks was measured before failure with a digital caliper. The μTBS of the sticks from the same bonded tooth were averaged and used for the statistical analysis as the statistical unit. Means and standard deviations were calculated and expressed in MPa. A small number of the bonded specimens failed prematurely and were not included in the statistical analysis. This defect exclusion is supported by the very limited numbers involved and that the range of bond strengths achieved was all relatively narrow and well above that associated with spontaneous failure.

The fractured specimens were analyzed using SEM JSM-5600LV (JEOL; Tokyo, Japan). In brief, the specimens were paired and mounted on aluminum stubs and subsequently dehydrated in silica gel. Finally, the specimens were gold-sputter coated and examined using SEM, operated at 15 kV. The fractures were classified as adhesive, cohesive in composite, cohesive in adhesive, cohesive in dentin or mixed.

Sorption and solubility and contact angle

Disc specimens (1 × 5 mm, n = 5) were produced between glass slides with a model ethanol-solvated BisGMA/HEMA (40:60 wt%) etch-and-rinse 1 bottle adhesive as a control or containing nanogels 1, 2 or 3 at 40 wt% relative to the BisGMA/HEMA monomer content. Camphorquinone (Sigma–Aldrich) and ethyl 4- N , N -dimethylaminobenzoate (Sigma–Aldrich) were added at 0.2 and 0.6 wt%, respectively, as co-initiators. Ethanol was included in the adhesive resin at 12 wt% to simulate a adhesive containing residual solvent . The samples were photoactivated for 2 min on each side by exposure to visible light at 100 mW/cm 2 (mercury arc lamp 320–500 nm—Acticure 4000, EXFO).

Water sorption (WS) and solubility (SL) were calculated as: WS = m2 − m3/V and SL = m1 − m3/V, where m1 is the initial adhesive polymer mass before water immersion, m2 is the wet mass of the surfaceblotted water-equilibrated polymer, and m3 is the dry mass of the polymer after 7 days’ storage in a desiccator. The water contact angle of the 12% ethanol-containing pre-immersion discs was measured by means of a goniometer (Ramé-Hart Instruments Co., Netcong, NJ, USA). One drop of distilled water was placed on the surface of the material, then a profile image was recorded and the contact angle (n = 3) measured (DROPimage Advanced; Ramé-Hart).

Statistical analysis

The data were statistically analyzed by equal variance and Kolmogorov–Smirnov normality tests and, after proving the normality of data and homogeneity of variances, the results were analyzed using one-way ANOVA (nanogel, for degree of conversion) and two-way ANOVA (nanogel and condition – dry or wet, for flexural modulus; and nanogel and storage time – 24 h and 3 months, for microtensile test) and Tukey’s test (α = 0.05). For flexural results and degree of conversion, the analyses were performed for each dispersion medium separately (solvent, HEMA and BisGMA/HEMA).

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Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Influence of nanogel additive hydrophilicity on dental adhesive mechanical performance and dentin bonding
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