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Formation of functionalized nanoclusters by solvent evaporation and their effect on the physicochemical properties of dental composite resins Henry A. Rodríguez a,b , Luis F. Giraldo a , Herley Casanova a,∗ a b

Grupo de Coloides, Instituto de Química, Universidad de Antioquia, Medellín, Colombia New Stetic S.A., Guarne, Colombia

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objective. The aim of this work was to study the effect of silica nanoclusters (SiNC), obtained

Received 14 October 2014

by a solvent evaporation method and functionalized by 3-methacryloxypropyltrimetho-

Received in revised form

xysilane (MPS) and MPS + octyltrimethoxysilane (OTMS) (50/50 wt/wt), on the rheological,

20 January 2015

mechanical and sorption properties of urethane dimethylacrylate (UDMA)/triethylenglycol

Accepted 9 April 2015

dimethacrylate (TEGDMA) (80/20 wt/wt) resins blend.

Available online xxx

Methods. Silica nanoparticles (SiNP) were silanized with MPS or MPS + OTMS (50/50 wt/wt) and incorporated in an UDMA-isopropanol mix to produce functionalized silica nanoclusters

Keywords:

after evaporating the isopropanol. The effect of functionalized SiNC on resins rheologi-

Nanoclusters

cal properties was determined by large and small deformation tests. Mechanical, thermal,

Solvent evaporation method

sorption and solubility properties were evaluated for composite materials.

Functionalized silica nanoparticles

Results. The UDMA/TEGDMA (80/20 wt/wt) resins blend with added SiNC (ca. 350 nm) and

Composites

functionalized with MPS showed a Newtonian flow behavior associated to their spheroidal

Restorative materials

shape, whereas the resins blend with nanoclusters silanized with MPS + OTMS (50/50 wt/wt)

Dental resins

(ca. 400 nm) showed a shear-thinning behavior due to nanoclusters irregular shape. Com-

Silane coupling agents

posite materials prepared with bare silica nanoclusters showed lower compressive strength than functionalized silica nanoclusters. MPS functionalized nanoclusters showed better mechanical properties but higher water sorption than functionalized nanoclusters with both silane coupling agents, MPS and OTMS. Significance. The solvent evaporation method applied to functionalized nanoparticles showed to be an alternative way to the sinterization method for producing nanoclusters, which improved some dental composite mechanical properties and reduced water sorption. The shape of functionalized silica nanoclusters showed to have influence on the rheological properties of SiNC resin suspensions and the mechanical and sorption properties of light cured composites. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

∗ Corresponding author at: Grupo de Coloides, Instituto de Química, Universidad de Antioquia, Calle 70 No. 52-21, Medellín, Colombia. Tel.: +5 74 219 86 50. E-mail address: [email protected] (H. Casanova).

http://dx.doi.org/10.1016/j.dental.2015.04.001 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

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1.

Introduction

Dental composites have displaced dental alloys in restorative dentistry due to their superior esthetic qualities, tooth like appearance, workability and light curing setting. However, composites show some disadvantages such as volumetric contraction, low adhesion to natural dental pieces and less wear resistance [1–3]. A number of reinforcing materials and changes in resins chemical nature have been used to improve resins composites performance. Recently, nanoparticles have been used as reinforcing material due to better mechanical properties of composite materials obtained with them in comparison to bare polymer matrixes or polymers reinforced with microfillers [4,5], and the generation of translucent dental materials, which are not feasible with reinforcing particles sizing more than 200 nm. However, nanoparticles can only be used at a relatively low concentration (500 ◦ C) [15]. The sintered silica nanoclusters can be subsequently functionalized with silane groups anchored to silica particles to improve surface–polymer matrix interaction, or to provide a more hydrophobic material with higher resistant to hydrolysis on a wet mouth environment. Silane coupling agents such as 3-methacryloxypropyltrimethoxysilane (MPS) and octyltrimethoxysilane (OTMS) (Fig. 1) have been used to obtain covalent bonds between the polymeric matrix and the functionalized particles, and to increase nanoparticle surface hydrophobicity, respectively [16–20]. Moreover, the increase in particle surface hydrophobicity could induce changes in the rheology of the resin composite before polymerization. In this way, the viscoelastic properties of the resin composite can be adjusted to improve its handling during application in the dental piece to reproduce its anatomy. The OTMS has been used for this purpose, instead of MPS, because it induces a high

Fig. 1 – Chemical structure of silane coupling agents used in this study, methacryloxypropyltrimethoxysilane (MPS) and octyltrimethoxysilane (OTMS).

increase in the viscosity and hydrophobicity of the system due to its long carbon chain. However, some investigations have shown that OTMS can decrease the mechanical properties and increase the water sorption of dental polymers when used in large amounts [18,19]. The objective of this study was to evaluate the effect of silica nanoclusters, obtained by a solvent evaporation technique and functionalized with MPS and a mixture of MPS + OTMS, on the rheological, mechanical and sorption properties of composite materials.

2.

Materials and methods

2.1.

Materials

Silica nanoparticle dispersion in isopropanol (30 wt%, diameter between 10 and 20 nm, Lot No. P070302), was supplied by Nissan Chemical Corporation (Houston, USA), 3-methacryloxypropyltrimethoxysilane (MPS) (97 wt%, Lot No. 10146396), ethyl-4-dimethylaminobenzoate (4EDMAB) (99 wt%, Lot No. 10059615) were supplied by Alfa Aesar (Ward Hill, Massachusetts, USA), UDMA monomer (90 wt%, Lot No. 715-36) was provided by Esstech Inc. (Essintong, Pensilvania, USA) and TEGDMA monomer (96 wt%, Lot No. 36296HK), photoinitiator camphorquinone (97 wt%, Lot No. 09003AQ) and octyltrimethoxysilane (OTMS) (96 wt%, Lot No. 000181503) were supplied by Sigma Aldrich GmbH (Deisenhofen, Germany). All materials were used as received without further purification. Monomers structures are shown in Fig. 2.

2.2.

Silanization of silica nanoparticles

The isopropanol silica nanoparticle dispersion was heated at 65 ◦ C and stirred during 3 h at atmospheric pressure in reflux. Afterwards, MPS or a MPS + OTMS blend (50/50 wt/wt) was

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Fig. 2 – Chemical structure of monomers used in this study, urethane dimethylacrylate (UDMA) and triethylenglycol dimethacrylate (TEGDMA).

added to the dispersion to obtain a concentration of 12% of silane (s) in relation to silica. This system was kept under stirring during 5 h, holding the temperature at 65 ◦ C under reflux. The isopropanol and remaining volatile byproducts were evaporated from silanized nanoparticles dispersion by drying the system at 80 ◦ C in a vacuum oven for 24 h and analyzed by infrared spectroscopy in a FT-IR Spectrum One (PerkinElmer, USA) using KBr pellet method. The average reported spectra were obtained from 16 scans at a resolution of 4 cm−1 .

2.3.

Dispersion of nanoparticles

UDMA was added to a dispersion of silanized nanoparticles in isopropanol under continuous stirring using a magnetic stirrer. The system was then heated up at 80 ◦ C, holding both temperature and stirring during 8 h. Afterwards, temperature was decreased to 37 ◦ C and kept for 48 h with no agitation to induce full isopropanol evaporation, producing the functionalized silica nanoclusters (SiNC) dispersed in UDMA. Finally, TEGDMA, camphorquinone (0.2 wt%) and 4EDMAB (0.8 wt%) were added. The amount of UDMA and TEGDMA were calculated to keep the 80/20 proportion in all final systems (70 wt% UDMA/TEGDMA, 30 wt% nanoparticles).

2.4.

(DC%) was evaluated using FT-IR spectroscopy by monitoring the symmetrical stretching signal for the aliphatic C C bond (between 1600 and 1650 cm−1 ) and normalizing it against the C H stretching peak (between 2900 and 3000 cm−1 ) according to the formula: DC(%) = 1 −

(1)

where Abs(C C/C H)polymerized is the relation between absorption peaks in the polymerized material and Abs(C C/C H)unpolymerized is the relation between absorption peaks in the non-polymerized material. The infrared spectra were obtained from a FT-IR Spectrum One (PerkinElmer, USA) using a NaCl cell for the non-polymerized materials; KBr pellet method was used to analyze polymerized materials. The average reported spectra were obtained from 16 scans at a resolution of 4 cm−1 .

2.6.

TEM micrographs

Composite material discs were cut using a microtome and sections were observed in a transmission electron microscope JEM-1011 (JEOL, Japan) working at 80 kV.

Viscosity analysis 2.7.

Viscosity curves of silanized silica dispersions in the resins (30 wt% SiNC) were obtained by applying a shear rate sweep between 50 and 800 s−1 in 30 s. A creep test was carried out by applying a stress of 1 Pa during 5 s, and allowing sample relaxation during 30 s. Both rheological tests used a cone and plate geometry (40 mm/2◦ ) and the temperature was kept at 25 ◦ C. A Bohlin Gemini HR Nano rheometer (Malvern Instruments, UK) was used to carry out the tests.

2.5.

Abs(C C/C H)polymerized Abs(C C/C H)unpolymerized

Thermogravimetric analysis

Thermal degradation of SiNP, functionalized SiNP and lightcured composites was studied by thermogravimetric analysis, applying a heat rate of 10 ◦ C/min between 30 and 800 ◦ C in air atmosphere. Weight loss was recorded as a function of time and temperature using a TGA Q500 device (TA Instruments, USA).

2.8.

Mechanical properties

2.8.1.

Flexural strength and flexural modulus

Degree of conversion

Nanocomposites discs (30 mm diameter, 2 mm thick) were prepared by molding at room temperature. Afterwards, the discs were light-cured using a conventional curing device Sunlite 1275 (FEN Dental, USA), irradiating light (450−490 nm) during 60 s in five different zones of the disc to guaranty a complete polymerization of the disc. The total irradiation time was 300 s in each disc side. The degree of conversion

From a disc treated in a similar way to those used for conversion degree analysis, composite material bars (25 mm × 2 mm × 2 mm) were cut by a high speed tool equipped with a diamond disc and used to measure flexural strength and flexural modulus. Five specimen bars were prepared for each composite; these specimens were immersed in distilled water at 37 ◦ C in darkness for 24 h immediately after

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curing. Another set of five specimens were kept for 40 days under the same storage conditions to repeat the mechanical analysis. The specimens were bent in a three-point transverse testing rig with 20 mm between the two supports (3-point bending). The rig was fitted to a mechanical testing machine 4202 (Instron, USA). All tests were carried out with a constant cross-head speed of 0.75 mm/min until fracture occurred. The load and the corresponding deflection values were recorded. The flexural strength () and the flexural modulus (E) were calculated by the following equations: =

3Fl 2bh2

(2)

E=

F1 l3 4bdh3

(3)

where F is the maximum load exerted at the fracture point, F1 represent the load exerted on the specimen, l is the distance between supports, h and b are the height and width of the specimen measured prior to test, and d is the deflection generated by the load F1 .

2.8.2.

Compressive strength

Following the standard specification ISO 9917 [21] for compressive strength, specimens were prepared having 8.0 mm length and 4.0 mm diameter. A two parts cylindrical mold of stainless steel was used to prepare the specimens. Composite resins were placed in the mold in 2 mm layers to fill the mold, and pressure was applied for 1 min at 3000 psi to allow bubble release. Each specimen was light-cured using a conventional curing device Sunlite 1275 (FEN Dental, USA) by overlapping irradiation during 60 s in the two circular plane surfaces and 60 s in the curved surface, for a total irradiation time of 180 s. Five specimens were prepared for each composite. These specimens were immersed in distilled water at 37 ◦ C in darkness for 24 h, immediately after curing. Another set of five specimens were kept for 40 days under the same storing conditions to repeat the mechanical analysis. The compressive strength test was carried out in a universal testing machine 4202 (Instron, USA) applying a crosshead speed of 0.5 cm/min. Compressive strength (CS) was calculated from the formula: CS =

4F d2

(4)

where F is the maximum applied load in Newton and d is the cylindrical specimen diameter in mm.

2.8.3.

2.9.

Sorption and solubility test

Following the methodology proposed by Sideridou and Karabela [18], sorption and solubility tests were determined according to method ANSI/ADA Specification No. 27-1993 for resin based filling materials, which is the same described in ISO 4049-1988. Specimen discs were prepared by filling a Teflon mold (15 mm × 1 mm) with the unpolymerized material. The samples were irradiated for 80 s on each side, using a Sunlite 1275 dental photocuring source (FEN Dental, USA). The unit was used avoiding the light guide to get in contact with the sample. Four specimen discs were prepared for each composite material and used for the sorption experiments. All the specimens were placed in a desiccator and transferred to a pre-conditioning oven at 37 ◦ C during 24 h. Afterwards, specimens were removed, stored in the desiccator for 1 h and weighed to an accuracy of ±0.0001 g using a Mettler Toledo AB 204 balance. This cycle was repeated until a constant mass (mi ) was obtained. The discs were then immersed in water at 37 ± 1 ◦ C, removed at fixed time intervals, blotted dry to remove excess liquid, weighed and returned to the liquid. The uptake of the liquid was recorded for 30 days. The percentage weight increase of specimens, WI (%), was calculated using the following formula: WI(%) =

ms − mi × 100 mi

(5)

where ms represents the weight of the saturated specimen after 30 days of immersion. The WI value corresponds to an apparent value of liquid sorbed, considering that unreacted monomer is simultaneously extracted, resulting in a decrease in specimen weight. For the determination of monomer extracted, the samples were transferred to a drying oven maintained at 37 ◦ C and a similar process to that of sorption was repeated during desorption. The percentage of liquid desorbed of specimens, WD (%), was calculated using the following formula: WD(%) =

ms − md × 100 ms

(6)

where md represents the weight of the specimen after desorption for 30 days. The amount of unreacted monomer extracted by water during immersion for 30 days – known as monomer release (MR) of the composite in these solvents – was calculated from the formula: MR(%) =

mi − md × 100 mi

(7)

Dynamic mechanical analysis

From a disc treated in a similar way to those used for degree of conversion analysis, bar specimens of 18 mm × 8 mm × 2 mm were cut using a high speed tool equipped with a diamond disc. The specimens were analyzed in a Dynamic Mechanical Analysis equipment (DMA) Q800 working in bending deformation mode (T.A. Instruments, USA) using a single cantilever clamp applying an oscillation frequency of 1 Hz and an amplitude of 15 ␮m. The sample was evaluated in the temperature range 30–250 ◦ C.

The percentage amount of water sorbed is then given by the formulae: WS(%) = WI(%) + MR(%)

(8)

Data reported in tables and figures in the present document represent mean values. Significant statistical differences between groups of data were determined using one-way ANOVA and Turkey’s test with a 95% confidence (i.e. p ≤ 0.05).

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Fig. 3 – Fourier-transform infrared spectra of bare silica nanoparticles and functionalized nanoparticles.

3.

Results

3.1. Nanoparticle characterization before polymerization FT-IR spectra of silanized nanoparticles (Fig. 3) shows the characteristic bands associated to nanoparticle functionalization by both silanizing agents, MPS and OTMS. Bands at around 1630 cm−1 and 1700 cm−1 assigned to C C and C O bonds respectively, are due to MPS functionalization, and the band at around 2950 cm−1 assigned to C H bond stretching vibration is characteristic of OTMS and MPS functionalization. The height ratio of the peaks assigned to C H and C C in the IR spectrum, showed a value of 0.4 for the MPS silanized nanoparticles. On the other hand, the MPS + OTMS silanized nanoparticles showed a height ratio of 1.4. Thermogravimetric analysis of MPS and MPS + OTMS functionalized nanoparticles are shown in Fig. 4. The accumulative thermogravimetric data indicates that the amount of silane agent chemically bonded to silica nanoparticle surface was 7.9 wt% for MPS and 7.8 wt% for MPS + OTMS. The viscosity curves of UDMA/TEGDMA (80/20 wt/wt) monomers blend, with 30 wt% silanized nanoparticles added, are illustrated in Fig. 5. The presence of MPS functionalized nanoparticles increased monomers blend viscosity from 0.35 Pa s for the system with no added nanoparticles to 0.40 Pa s, showing a Newtonian flow behavior. Nanoparticles functionalized by MPS + OTMS produced an increase in suspension viscosity, showing a value of 1.8 Pa s at a shear rate of 50 s−1 . By increasing the shear rate applied to the suspension, a decrease in the apparent viscosity was observed, showing a pseudoplastic flow behavior. Fig. 6 presents the creep results for both MPS and MPS + OTMS suspensions, showing a Hookian response by applying a stress of 1 Pa during 5 s. After removing the applied stress, the MPS + OTMS suspension shows a partial recovery of the system structure as detected by a reduction of the compliance value, associated to materials with viscoelastic behavior.

5

Fig. 4 – TGA curves of bare silica nanoparticles and functionalized nanoparticles.

On the contrary, the MPS nanoparticles suspension showed no reduction in the compliance value after removing the applied stress, indicating a liquid-like behavior for the MPS system.

3.2. Characterization of polymerized systems with added functionalized nanoclusters 3.2.1. state

Degree of conversion and nanoparticles aggregation

FT-IR spectra of MPS nanoparticles light cured system [30% nanoparticles, UDMA/TEGDMA (80/20 wt/wt)] is shown in Fig. 7, together with the system before polymerization. In each spectrum is clear the presence of a wide band at 3500 cm−1 assigned to the stretching of N H bond, at 1100 cm−1 appears a wide band corresponding to asymmetric extension of the Si O Si bonds of the silica, in 470 cm−1 there is a band

Fig. 5 – Viscosity curves of UDMA/TEGDMA resins blend with added functionalized nanoparticles (30% nanoparticles).

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Fig. 6 – Creep and recovery test of UDMA/TEGDMA resins blend with added functionalized nanoparticles (30% nanoparticles). Applied stress = 1 Pa.

assigned to flexion of the Si O Si bonds, at 811 cm−1 appears a band due to the flexion of H OH bond of remaining water, the bands at 1636 and 1720 cm−1 are assigned to extension vibration of C C and C O bonds respectively, the peak in 2960 cm−1 can be attributed to the stretching of C H bond, the peak at 1527, 652 and 595 cm−1 correspond to vibrations of N H bond in UDMA monomer. Bands at 1636 and 2960 cm−1 assigned to C C extension and C H bond stretching, respectively, showed a change in their ratio from the unpolymerized sample to the polymerized sample, giving a degree of conversion around 60% for both MPS and MPS + OTMS functionalized nanoparticles. The degree of conversion was also obtained for the UDMA/TEGDMA monomers blend (80/20 wt/wt) without nanoparticles, giving a value of 80%. Thermogravimetric analysis of light cured materials is shown in Fig. 8. The initial 5% weight loss corresponds

Fig. 7 – Fourier-transform infrared spectra of composites before and after polymerization.

Fig. 8 – TGA curves of light cured composite materials.

to sorbed water; the subsequent 52% weight loss, at temperatures between 200 and 350 ◦ C, is due to polymeric matrix degradation, evaporation of silane agents and residual monomers. An additional 8% weight loss, observed between 350 and 450 ◦ C, is associated to silane agent degradation chemically bonded to nanoparticles surface and polymer chains that are highly crosslinked and wrapped by silica agglomerates. The final 8% weight loss between 450 and 600 ◦ C is explained in terms of the silanol groups that did not react with silica nanoparticles surface or the presence of char carbon. Fig. 9 shows the TEM micrograph of MPS and MPS + OTMS nanoclusters. The MPS nanocluster is a spheroidal porous aggregate of functionalized nanoparticles, while the MPS + OTMS nanocluster has a more irregular and compact shape.

3.2.2.

Mechanical properties

Table 1 resume the mechanical properties measured to light cured composites. The flexural strength of composite materials (day 1) made of functionalized nanoparticles (MPS and MPS + OTMS) showed significant lower values than the system made of bare silica nanoparticles (p < 0.005). On the other hand, the compressive strength of these two composite materials (day 1) showed significant higher values than the system made of bare silica nanoparticles (p < 0.005). However, the flexural modulus and compressive strength of these two systems showed no significant differences between them (p < 0.005). The mechanical analysis of specimens kept under water during 40 days (Table 1), showed that the mechanical properties of all tested samples kept relatively unchanged, except the compressive strength of the MPS nanocomposite that decreased significantly. This could be understood in terms of its higher cross linking during the post-polymerization process, therefore, this sample becomes more rigid, increasing its mean flexural modulus, but induces the sample to fracture at a lower pressure as indicated by its low compressive strength value. A similar trend it is observed for the MPS + OTMS system, but the changes in flexural modulus and compressive strength are in a less extent due to the lower amount of MPS present in the sample.

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Table 1 – Mechanical properties of light cured composites. Material

FS () (MPa)

FM (E)(MPa)

CS (MPa)

FS () 40 Days (MPa)

FM (E) 40 Days (MPa)

CS 40 Days (MPa)

UDMA/TEGMA (80/20) UDMA/TEGMA (80/20) with bare nanoparticles (30%) UDMA/TEGMA (80/20) with MPS SiNC (30%) UDMA/TEGMA (80/20) with MPS + OTMS SiNC (30%)

84 (4)a,b 90 (9)b

2004 (248)a 3047 (343)b

220 (24)a 204 (40)a

74 (12)a,b 80 (6)b

1922 (303)a 2589 (223)b

198 (48)a 194 (42)a

76 (6)a

2500 (492)a,b

407 (48)b

82 (10)b

2684 (590)b

333 (25)b

68 (9)c

2267 (401)a

357 (45)b

63 (10)a

2252 (404)a,b

284 (47)b

Standard deviations are shown in parentheses and letters indicate significantly different groups at p ≤ 0.05.

3.2.3.

DMA analysis

Dynamical Mechanical Analysis (DMA) of composite materials is shown in Fig. 10. Here, the storage and loss moduli values for the MPS functionalized nanoclusters are higher than those observed for the bare nanoparticles system (Fig. 10a). On the other hand, the MPS + OTMS functionalized nanoclusters system showed storage and loss moduli values lower than the bare nanoparticles system. The tangent (ı) of the composite materials (Fig. 10b), show values lower than those obtained for both the bare nanoparticles and the MPS + OTMS system at temperatures up to 110 ◦ C. Above this temperature, the MPS composite material showed ı values between the bare nanoparticles and the MPS + OTMS functionalized nanoclusters.

3.2.4.

Sorption and solubility test

Table 2 summarizes the result for water sorption (%), percentage of desorbed compounds (WD%) and percentage of monomer release (MR%) of four systems: the polymeric material with non-added nanoparticles and three composite materials (Table 2). The polymeric matrix and the composite with non silanized nanoparticles produced statistically higher water sorption (p < 0.05) than the systems with silanized nanoclusters. The data of total desorbed compounds correspond to the sum of desorbed water and released monomers, with the MPS + OTMS having the highest value of released monomers (i.e. 1.58%).

4.

Discussion

The addition of MPS functionalized silica nanoparticles to the UDMA/TEGDMA system induced a marginal increase in monomers blend viscosity. This is an unexpected result, considering the high surface area of added nanoparticles and their relatively high concentration in the system (30% wt). A possible explanation for this rheological behavior is that nanoparticles are in an aggregated state, with low interaction between aggregates, diminishing the buildup of any type of microstructure in the system. Favorable interactions between polar methacrylic groups in MPS moieties and polar resin monomers could take places in this low viscosity system [22]. On the other hand, MPS + OTMS functionalized nanoparticle added to the UDMA/TEGDMA blend induced a significant

increase in viscosity, changing the Newtonian type of fluid for the monomers blend to a pseudoplastic behavior. The increase in viscosity at low shear rate for the MPS + OTMS functionalized nanoparticles suspension, could be attributed to interactions between long OTMS moieties present in two different particles aggregates, which induce a weak bridging flocculation among particles. By increasing the shear rate, flocks interconnection is destroyed, inducing a reduction in the apparent viscosity of nanoparticles suspension. Wilson et al. [22] reported a similar effect of OTMS on nanoparticles aggregation state in silica nanoparticles dispersions using a resin composed of 2,2bis[p-(20-hydroxy-30-methacryloxypropoxy)-phenyl] propane (BisGMA) and triethylene glycoldimethacrylate (TEGDMA), based on FE-SEM images of composite surfaces. Here, they found some polymer rich phases associated to poor particles dispersion prior to composite curing. This segregation was explained in terms of the attractive interparticles van der Waals forces between OTMS moieties, which were greater than the interactions between the silanized particles and the matrix. Small-angle neutron scattering studies of OTMS silica functionalized nanoparticles showed a rise in OTMS aggregates size by increasing the amount of OTMS in the system [22]. The higher aggregation state of the MPS + OTMS nanoparticles dispersions induced a more solid like system as corroborated by the creep and recovery test shown in Fig. 6. The degree of conversion of 60% obtained for the UDMA/TEGDMA monomers blend (80/20 wt/wt) with SiNP + MPS and SiNP + MPS + OTMS) is in the range observed for conventional BisGMA/TEGDMA based dental composites, which are typically between 55 and 75% [23]. However, it is significantly lower than that observed for the monomers blend with no added nanoparticles (i.e. 80%). A similar decrease in the degree of conversion was observed by Halvorsona et al. [24] in a BisGMA/TEGDMA resin reinforced with a zirconia/silica fillers mix using MPS as a silane coupling agent. In such work, the reduction in the degree of conversion was attributed to the reduced molecular mobility within boundary regions around the interface of the fillers, with no effect of nanoparticles silanization on the DC. On the other hand, Wilson et al. [19] found that the type of silanizing agent did change the degree of conversion of a BisGMA/TEGDMA resin reinforced with silica filler nanoparticles. They found that the resin composite with OTMS functionalized nanoparticles

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Table 2 – Sorption and solubility values of light cured composites. Water sorbed (%) UDMA/TEGMA UDMA/TEGMA + bare nanoparticles UDMA/TEGMA + MPS SiNC UDMA/TEGMA + MPS + OTMS SiNC

3.44 (0.03)a 3.84 (0.19)b 2.55 (0.06)c 2.07 (0.06)d

Liquid desorbed (%) 3.41 (0.03)a 4.27 (0.21)b 3.27 (0.04)c 3.57 (0.20)a

Monomer release (%) 0.08 (0.03)a 0.59 (0.03)b 0.81 (0.04)c 1.58 (0.15)d

Standard deviations are shown in parentheses and letters following the values denote significantly different groups at p ≤0 .05.

had a DC of 76%, while the MPS functionalized system showed a value of 72%. This decrease in the DC was attributed to the greater concentration of vinyl groups in the MPS nanoparticles that acted as multifunctional monomers, causing gelation at lower conversions and thereby limiting

Fig. 9 – Representative TEM images of light cured composites showing functionalized silica nanoclusters with MPS on the left and with MPS + OTMS on the right.

the advance of the polymerization reaction. In contrast, the OTMS nanoparticles had a plasticizing effect that increased the mobility of dimethacrylate monomers in the growing polymers, enhancing vinyl addition. However, in the present work, the degree of conversion for the MPS nanoparticles resin dispersion was similar to that obtained for MPS + OTMS functionalized nanoparticles (ca. 60%). A complementary explanation to that presented by Halvorsona et al. [24] for the decrease in the degree of conversion in both MPS and MPS + OTMS nanoparticles systems, based on the rheological results obtained here, could be understood in terms of the liquid-like state of the resin matrix at the beginning of the polymerization process, which does not restrict the mobility

Fig. 10 – DMA analysis of light cured composites. On the right the storage modulus G´ı and on the left the tangent value (ı), where maxima correspond to glass transition temperature values.

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of a growing polymer chain. However, once the polymerization has advanced enough to produce a more solid-like material, nanoparticles could act as chain stoppers for the growing polymers due to the presence of the double bonds in the MPS dangling tails for both MPS and MPS + OTMS systems, which reduces the degree of conversion from 80% to 60%; such effect has been reported in similar dental systems elsewhere [23]. The TEM micrographs give experimental evidence that the evaporation method induced the formation of nanoclusters, with no need of a high temperature treatment as required for the sinterization of silica particles [15]. Additionally, the spheroidal shape of the MPS nanocluster is in agreement with the flow properties observed for the resin with MPS nanoparticles added, which showed a Newtonian flow behavior, having non-interacting spheroidal particles. On the other hand, the irregular shape of the MPS + OTMS particles could help to explain the pseudoplastic behavior of this system, considering their difficulties to be aligned with flow layers. The addition of functionalized silica nanoparticles to UDMA/TEGDMA monomers blend did no induce a significant change in the mechanical properties of cured composite materials. This is probably due to the low concentration of inorganic particles used in the present work, 30% instead of 70% or higher added to commercial products. Both composites MPS and MPS + OTMS functionalized nanoclusters, showed a reduction in the flexural strength of the material at values lower than those observed for the bare polymer, showing a viscoelastic flow behavior during the mechanical test. These indicate that the functionalized nanoclusters are not fully integrated to the polymeric matrix, and become a point of failure for the composite material during the flexural test. However, the MPS functionalized nanoclusters showed a slightly higher flexural modulus than the MPS + OTMS system, which suggest that the higher amount of double bonds in the MPS system promotes a better interaction with the polymeric matrix than the MPS + OTMS system [19]. The compressive strength values for the functionalized nanoclusters composites showed a significant improvement in comparison with the bare polymer and the composite material made with non-functionalized nanoparticles. For the MPS system a two fold increase was observed in comparison to the bare polymer, while the MPS + OTMS system had a 1.5 fold increase. This mechanical behavior for the functionalized nanoclusters materials suggests that they are highly effective in dissipating the tensile forces acting on the compressive strength test, despite the fact that they are no fully integrated to the polymer matrix. In this case, nanoclusters made of MPS funtionalized nanoparticles with a spherical like shape are more effective dissipating the tensile forces, due to its more homogeneous structure than the MPS + OTMS clusters that have a more irregular shape. The MPS composite system, after 40 days immerse in water, increased both the flexural strength and the flexural modulus, which could be attributed to a post-polimerization process that reinforce the interaction between the polymeric matrix and the MPS nanocluster. However, the increase in the number of double bonds created during the post-polimerization process induced a slight reduction in the compressive strength of the composite material, probably due to a less effective transmission of the tensile forces from the nanocluster to the

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polymeric matrix. This indicates that there is a compromise between the mechanical properties for the MPS nanoclusters reinforced composite. The significantly higher values of shear modulus for the Dynamical Mechanical Analysis (DMA) of the MPS composite material (Fig. 10), indicates that the double bonds present in the MPS are reacting during the polymerization process. Therefore, MPS functionalized nanoclusters have some degree of integration to the polymeric matrix, inducing the reinforcement of the composite material. On the other hand, the MPS + OTMS composite material showed significantly lower module values than the bare nanoparticles system, indicating that the presence of OTMS as functionalizing agent blocked the interaction with the polymeric matrix and limited the nanoclusters reinforcement effect. It is important to recall that both MPS and MPS + OTMS systems showed a degree of conversion close to 60%, which suggests that the advance in the polymerization, although was the same, it took different routes depending on the functionalizing agent present on the nanoclusters surface. The tangent (ı) value of the composite materials (Fig. 10b) provide additional evidences of the higher covalent interaction for the MPS functionalized nanoclusters, showing values significantly lower than those obtained for both the bare nanoparticles and the MPS + OTMS system at temperatures up to 110 ◦ C. The change in the rheological behavior for the MPS system at around 110 ◦ C, could be due to the presence of two regions in the polymer with incomplete polymerization. One of the regions complete its polymerization at temperatures close to 110 ◦ C, and the second one at temperatures up to 200 ◦ C, considering that the increase in temperature induces chain movements that allow the reaction in the second segment with incomplete polymerization [25]. The presence of non-silanized nanoclusters produced higher water sorption than those systems with silanized nanoclusters. This could be explained in terms of the hydrophilic characteristic of non-silanized nanoparticles, due to the presence of silanol groups on their surface, whereas silanized nanoclusters showed hydrophobic characteristics due to the silane groups on the surface. Therefore, the high water sorption observed in non-silanized nanoparticle material avoids the interaction between the polymeric matrix and the teeth surface, which diminishes composite mechanical properties. Moreover, the MPS + OTMS functionalized nanoclusters showed the lowest water intake, which confirms the benefit of having a long hydrophobic tail (i.e. the OTMS) on the nanoparticles surface to repeal the water from the polymeric matrix. The MPS shows a higher value for the water intake in comparison to the MPS + OTMS system that induces a lower stability for such composite material. The MPS + OTMS have the highest value of released monomers (i.e. 1.58%). This could be due to the low number of chemical bonds between the MPS + OTMS nanoclusters and the polymeric matrix, which gives the monomers a higher level of mobility. Therefore, the high content of free monomers present in the MPS + OTMS nanoclusters system could reduce the mechanical properties of the composite material. As future work, it is necessary to evaluated which of the two factors (water intake or monomer release) is the dominant one in the mechanical stability of the composite material.

Please cite this article in press as: Rodríguez HA, et al. Formation of functionalized nanoclusters by solvent evaporation and their effect on the physicochemical properties of dental composite resins. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.04.001

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Acknowledgments This work was supported by CODI-Estrategia de Sostenibilidad, University of Antioquia, and Colciencias´ı Grant 346-2011.

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Please cite this article in press as: Rodríguez HA, et al. Formation of functionalized nanoclusters by solvent evaporation and their effect on the physicochemical properties of dental composite resins. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.04.001

Formation of functionalized nanoclusters by solvent evaporation and their effect on the physicochemical properties of dental composite resins.

The aim of this work was to study the effect of silica nanoclusters (SiNC), obtained by a solvent evaporation method and functionalized by 3-methacryl...
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