Preparation and characterization of polyhedral oligomeric silsesquioxane (POSS) using domestic microwave oven

Journal of Non-Crystalline Solids 428 (2015) 82–89

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Preparation and characterization of polyhedral oligomeric silsesquioxane (POSS) using domestic microwave oven Igor Penso a, Eduardo A. Cechinatto a, Giovanna Machado b, Caroline Luvison a, Cesar H. Wanke a, Otávio Bianchi a,⁎, Márcio R.F. Soares a,⁎ a b

Programa de Pós-Graduação em Engenharia e Ciência dos Materiais (PGMAT), Universidade de Caxias do Sul (UCS), Caxias do Sul, Brazil Laboratório de Microscopia e Microanálise, Centro de Tecnologias Estratégicas do Nordeste, Recife, Brazil

a r t i c l e

i n f o

Article history: Received 24 June 2015 Received in revised form 6 August 2015 Accepted 15 August 2015 Available online xxxx Keywords: POSS; Microwave; Nanomaterials; Hydrolysis; Condensation

a b s t r a c t This paper reports the preparation and characterization of polyhedral oligomeric silsesquioxane (POSS) using a domestic microwave oven. The hybrid nanomaterials were synthesized using the hydrolytic condensation of vinyltriethoxysilane (VTES) and a VTES/tetraethyl orthosilicate (TEOS) mixture. The samples were characterized by FTIR, 1H-NMR, 29Si-NMR, GPC, SEM, TEM, WAXD and TGA. The dielectric heating of the VTES and VTES/TEOS systems enabled the production of POSS with increased yield, showing rates 47 and 59 times faster than conventional heating, respectively. The POSS VTES samples prepared by dielectric and conventional heating showed very similar chemical characteristics. However, for the POSS obtained with the addition of TEOS the same behavior was not observed, because of the selective effect occasioned by microwave-assisted synthesis. The morphological shape of the hybrid clusters is dependent on the synthesis method and the precursor type. The simple approach used in this work represents an alternative to obtaining nanomaterials more efficiently. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The synthesis and properties of polyhedral oligomeric silsesquioxane (POSS) nanostructures have recently attracted great interest, in relation to both molecular compounds and inorganic–organic hybrid materials [1], since they possess a synergistic combination of the properties of organic and inorganic materials [1,2]. POSS nanostructures, with the general formula (RSiO3/2)n (n = 6, 8, 10… 14), are nanometer-sized (1–5 nm) hybrid materials that provide a better choice for preparing hybrid materials through copolymerization [10,11], grafting [12,14], or blending [15, 18]. Octafunctional cubic cages, (RSiO3/2)8, called pT8, are the most commonly studied POSS compounds, where eight pendant groups (R) are attached to each of the silicon atoms at the corners of the cage [1]. In most cases, R is an inert group, such as phenyl, cyclopentyl, isobutyl, methyl or vinyl [3]. The chemical groups bonded to the silicon atoms are responsible for the high solubility of the POSS in organic solvents and polymer matrixes [4]. POSS can be found in liquid [2] or solid [5] state at room temperature depending on the type of R group. The POSS nanostructures have a range of applications, for instance, in coatings, optical devices, dielectrics, liquid crystal displays, biomedical applications and polymer nanocomposites [6–9]. The ability to create higher dimensionality through the aggregation or crystallization of POSS has been studied [2]. This self-assembly capability is a key ⁎ Corresponding authors. E-mail addresses: [email protected] (O. Bianchi), [email protected] (M.R.F. Soares).

http://dx.doi.org/10.1016/j.jnoncrysol.2015.08.020 0022-3093/© 2015 Elsevier B.V. All rights reserved.

advantage of POSS nanostructured materials along with their chemical versatility [10]. Octavinyl polyhedral oligomeric silsesquioxane (OVPOSS) is a type of POSS material with eight vinyl double bonds at every corner [11]. OVPOSS has also been used in numerous applications, such as elastomer crosslinking [12–15], preparations of nanomagnetic particles for the selective extraction of fluoroquinolones in milk [16] and pollutantcapture [17], OLEDs [18,19] and polymer nanocomposites [20–22]. This molecule often serves in the preparation of new POSS structures, therefore being of great importance as a starting material for many applications [16–19,23]. Unfortunately, the preparation of these OVPOSS involves timeconsuming synthesis techniques to obtain synthetically useful quantities (around 2 days) [14,15]. An alternative route to preparing these nanomaterials is the use of microwave ovens and microwave reactors, however, the cost of commercial reactors often limits the use of microwave-assisted reactions and this has stimulated researchers to find other approaches, such as the use of an adapted domestic microwave oven [24–27]. The popularity of microwave-assisted chemistry is not surprising considering that these methods often dramatically increase the yield and decrease the reaction times. Microwave radiation acts directly on the molecules of the entire reaction mixture, leading to a rapid rise in the temperature [28,29]. This process is not limited by the thermal conductivity of the reaction vessel and the result is an instantaneous localized superheating of any substance that will respond to either dipole rotation or ionic conductivity [24,28,29]. The acceleration of chemical reactions by microwave exposure results from the

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interactions between the material and electromagnetic field leading to thermal and specific (non-thermal) effects [24,29]. For microwave heating, the substance must possess a dipole moment. The dielectric properties of a non-conductive material govern the way it will heat when exposed to microwave radiation, with a combination of ionic conduction and dipole polarization serving as the predominant heating mechanism [24,28,29]. Chiacchio and coworkers [30] performed the chemical modification of OVPOSS to obtain isoxazole derivatives through the exploitation of the microwave-assisted 1,3 dipolar cycloaddition of nitrone and nitrile oxide. The reaction products were successfully obtained in a short time. Janowski and Pielichowski carried out the microwave-assisted synthesis of cyclopentyltrisilanol and obtained a 16.26% yield with a reaction time of 30 min, while the reaction with conventional heating yielded 14.63% in 180 min [31]. In this study, octavinyl polyhedral oligomeric silsesquioxane (OVPOSS) was synthesized through the hydrolytic condensation of vinyltriethoxysilane using a domestic microwave oven and conventional heating was applied for comparison purposes. The effects of the reaction time and the vinyltriethoxysilane (VTES)/tetraethyl orthosilicate (TEOS) ratio were investigated. The materials were characterized by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), 1H and 29Si nuclear magnetic resonance, gel permeation chromatography (GPC), wide angle X-ray diffraction (WAXD) and scanning (SEM) and transmission (TEM) electron microscopy.

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2.3. Wide angle X-ray diffraction (WAXD) The crystalline structures of POSS were characterized using a Shimadzu XRD-6000 diffractometer in the reflection mode with Cu Kα radiation (λ = 1.5405 Å). The data were collected with a 2θ angle ranging from 1 to 40° at a scanning rate of 0.5° min−1. The WAXD experiments were performed using POSS powders obtained from the purification process. The error associated with this experiment is less than 2%. Rietveld analysis was performed on the POSS–VTES diffraction patterns using the FullProf program [32]. The Le Bail method was applied and all parameters were refined using the least-squares method. A pseudo-Voigt function was used to fit the Bragg peak profiles and the instrumental resolution parameters were obtained from a lanthanum hexaborate standard. The POSS–VTES structure was based on that of Waddon and Coughlin [33] and these data were used as initial parameters for the numeric approximation. 2.3.1. Fourier transform infra-red spectroscopy (FTIR) The Fourier transform infrared spectroscopy was performed on a Perkin-Elmer Spectrum 400 spectrometer in the attenuated total reflection (ATR) mode. The spectra were obtained over the 4000 to 500 cm−1 region employing 62 scans with a resolution of 4 cm− 1. Samples in powder form were used in this experiment. The error associated with this measurement is less than 5%.

2. Experimental 2.1. Materials Vinyltriethoxysilane (98% purity) (VTES) and tetraethoxysilane (98% purity) (TEOS) were purchased from Sigma-Aldrich under codes 235768 and 131903. Spectroscopic-grade acetone, hydrochloric acid and dioxane were purchased from Synth. All chemicals were used as received. 2.2. Methods 2.2.1. Synthesis of oligomeric silsesquioxane For the synthesis with VTES, 102.7 mL of acetone and 13 g of the VTES were placed in a 250 mL flask equipped with a magnetic stirrer. A mixture of 19.7 mL of concentrated hydrochloric acid and 19.7 mL of ultrapure water (Milli-Q) was added dropwise to the reaction mixture, according to a procedure described in the literature [14,15]. In the synthesis of polyhedral oligomeric silsesquioxane with a mixture of precursors (VTES/TEOS 10:1) the same conditions used for the synthesis of VTES were employed. Both reactions were carried out using conventional and microwave-assisted conditions. After the synthesis reaction the samples were washed with ethanol and dried (60 °C). The product of the synthesis was recrystallized from the mixed solvents (dichloromethane and acetone; volume ratio 1:3) and dried (60 °C). 2.2.2. Conventional synthesis The conventional heating reactions were carried out using a reflux system. All chemical reagents were placed in a 250 mL flask and the reaction medium was heated and mechanically stirred using a hot plate with an oil bath, to reduce the convective heat loss, for 48 h. To avoid evaporation of the medium the system was coupled to a condenser. 2.2.3. Microwave-assisted synthesis Microwave-assisted reactions were carried out using an adapted domestic microwave oven with a reflux system. The modification of the microwave oven was performed according to a procedure described in the literature [25–27]. The reactions were carried out using magnetron cycles (on/off) to prevent damage. The reaction time was 2 h under cycles of 5 s with the magnetron on and 5 s with the magnetron off.

2.3.2. Nuclear magnetic resonance (NMR) spectroscopy 1 H NMR spectra for the samples were obtained on a Varian Inova 300 spectrometer. The experimental data were acquired using a chloroform-d solution with a sample concentration of 3 wt.% at 22 °C, operating at 299.95 MHz, using a spectral width of 10 ppm, acquisition time of 2.049 s, relaxation delay of 1 s and 128 scans. The error associated with this experiment is less than 5% Solid state 29Si NMR spectra were obtained on an Agilent Technologies model 500/54 Annual Refill spectrometer. A frequency of 500 Hz was used, with a relaxation time of 5 s, and 12,000–20,000 scans (11.7 T, rotor 4 mm). The error associated with this experiment is less than 10%. 2.3.3. Gel permeation chromatography (GPC) The GPC experiments were performed on a Viscotek TDAmax chromatograph, using a tetrahydrofuran (THF) solution with a sample concentration of 10 mg mL−1, flow rate of 1 mL min− 1 and volume of 150 μL. The samples in solution were filtered through a 0.2 mm-pore size PTFE membrane prior to the analysis. The column (Waters HR 4E, HR 4, HR 3 and HR 2) temperature was set at 45 °C. Polystyrene standards were used to construct the calibration curve. The error associated with this experiment is less than 10%. 2.3.4. Scanning electron microscopy (SEM) The SEM micrographs of the POSS samples were obtained using a Shimadzu SSX-550 microscope. The POSS powder samples were sprinkled onto a carbon tape surface. All samples were sputter-coated with gold before imaging. Samples were also analyzed by energy dispersive X-ray spectroscopy (EDS) to identify the presence of silicon. 2.3.5. Transmission electron microscopy (TEM) The morphology and particle size of the POSS samples were observed by transmission electron microscopy (TEM) using a FEI Tecnai G2 Spirit TWIN microscope operated at an accelerating voltage of 100 kV. TEM samples were prepared by dispersing POSS particles in 2propanol at room temperature. The suspensions were deposited onto holey carbon grids (300 mesh). The excess solution was quickly wiped away with a piece of filter paper and the sample was then dried.

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2.3.6. Thermogravimetric analysis (TGA) The thermo-degradation behavior of the hybrid materials was examined by thermogravimetric analysis (TGA) on a Shimadzu TGA-50 instrument with a sample size of 10 mg, at temperatures of 30 °C to 900 °C, at a heating rate of 10 °C min−1, and under a nitrogen flow of 50 mL min−1. Samples in powder form were used in this experiment. The error associated with this measurement is less than 5%. 3. Results and discussion The sample synthesized with VTES using conventional heating showed a conversion rate of 0.18 ± 0.01 wt.%/h while for the sample prepared with microwave assistance this value was 8.50 ± 0.49 wt.%/ h. The results show that the reaction using dielectric heating is around 47 times more effective when compared to that using the conventional reaction system. For the hybrid materials prepared with VTES/TEOS 90:10 the conversion rates were found to be 0.03 ± 0.01 wt.%/h for conventional heating and 2.23 ± 0.50 wt.%/h for microwave heating, representing a difference of 59 times in relation to conventional synthesis using oil bath. In the literature, values of 18–33 wt.% in 48 h (0.37– 0.68 wt.%/h) have been reported for the conversion reaction using conventional heating (oil bath) [15,34]. The reaction conducted using microwave heating results in a reduction in the reaction time and activation energy [35,36], which is due to the high selective heating rate, caused by the capacity of the reaction medium to absorb microwaves and convert this energy into local heat. Thus, for the reaction using dielectric heating the conversion rate is higher than that for the reaction with conventional heating, which is attributed to a local temperature gradient in the sample. In fact, the application of this heating technique shows that it is possible to obtain POSS in a shorter time compared with conventional heating (convection). This approach has been successfully used to obtain silica nanoparticles in short reaction times [37]. However, longer microwave heating times contribute to the generation of strong convective gradients in the reaction medium, which can result in the breaking of bonds and degradation of synthesis products [24]. The FTIR spectra for the POSS–VTES samples obtained through conventional and microwave-assisted syntheses show the same absorption bands. In Fig. 1, the first peak at ~1604 cm−1 relates to CH_CH vinyl stretching, and the most intense peaks at ~ 1090 and ~ 568 cm−1 are

attributed to the characteristic Si\\O\\Si asymmetrical stretching and O\\Si\\O deformation, respectively, related to silsesquioxane cages. These results are consistent with data reported in the literature [14,15, 34]. In the FTIR spectra for the POSS–VTES samples absorption bands related to the deformation of OH groups were not present. However, the POSS–VTES/TEOS samples prepared by conventional and microwave methods showed the presence of an absorption band at ~3400 cm−1 related to the deformation of the OH groups, besides those mentioned above. This is related to the fact that when the precursors are mixed the formation of a fully condensed structure does not occur. On the FTIR spectra, it is not possible to observe the intensities related to the alkoxide groups, which indicates that the hydrolysis and separation processes were successful. In addition, the absorption due to the \\O\\CH2CH3 group (~2886 cm−1) indicates that the hydrolysis reaction was fully completed. The 1H NMR spectra for all samples are shown in the Supplementary file. For the POSS samples prepared with VTES using the conventional and dielectric heating, signals were observed at 5.88–6.2 ppm, which are related to the vinyl protons of the POSS structure [14,18,20]. At 1.61 and 2.2 ppm methylene proton signals were observed, indicative of degradation of the vinyl bond during synthesis. For samples prepared from mixtures of the precursors (VTES/TEOS) signals related to the vinyl proton at between 5.88 and 6.2 ppm were also observed. All POSS samples synthesized, independent of the method used, showed a signal at 3.5–3.8 ppm, suggesting the presence of silanol without condensation [38]. Moreover, the hybrid materials prepared with TEOS show more evidence of non-condensed structures, as observed in the FTIR results. The 29Si NMR spectra of the samples are shown in Fig. 2, where the presence of T2 (R-Si ≡ (OSi)2(OCH2CH3)), T3 (R-Si ≡ (OSi)3), Q3 (Si(OSi)3(OH)) and Q4 (Si(OSi)4) structural units can be observed [39, 40]. For POSS synthesized from VTES with microwave assistance, the chemical shifts at −78.3 ppm, −69 ppm and −60 ppm can be assigned to T3, T3* and T2 structural units, respectively. T3* is related to the relative shift in the signal for silicon which is linked to vinyl groups that were thermally degraded during the synthesis, as observed in the 1H NMR results (see Supplementary data). The relative proportions of the structure of each silicon species were determined from the area of the corresponding signal. The T3 assignment corresponded to 59.4%, T3* was 12.2% and T2 was 28.4% respectively. For POSS synthesized using conventional heating with VTES the chemical shifts were the same as

Fig. 1. FTIR spectra for (a) POSS–VTES and (b) POSS–VTES/TEOS.

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Fig. 2. Solid-state 29Si NMR spectra of samples.

those for the sample prepared using the microwave-assisted reaction. The relative proportions were 72.2% for T3, 22.4% for T3* and 5.4% for T2. These results indicate that a portion of the hybrid materials was not completely condensed, as suggested by the 1H NMR results. For the hybrid materials prepared using co-condensation and cohydrolysis (VTES/TEOS) applying dielectric heating the chemical shifts observed at − 108.7 ppm, − 99.7 ppm, 78.6 ppm, − 68.5 ppm and − 58.9 ppm can be assigned to Q4, Q3, T3, T3* and T2, respectively [1, 39–43]. The presence of the T3 unit indicates that there is the formation of completely condensed cages, while the presence of T1 and T2 units indicates that partially open cages are also present. However, the use of a tetrafunctional precursor (TEOS) together with VTES results in a hybrid structure with a random characteristic [42,44]. For this sample the relative proportions of silicon structural units were 2.6% for Q4, 1.6% for Q3, 54.4% for T3, 21.3% for T3* and 19.7% for T2 respectively. The POSS with VTES/TEOS synthesized under conditional heating showed chemical shift assignments similar to those of the sample prepared under microwave-assisted conditions. The values found for the relative proportions of structures were 0.8% for Q4, 4.4% for Q3, 36.4% for T3, 19.1% for T3* and 39.3% for T2. The GPC chromatographs for all samples are shown in Fig. 3. For the two samples synthesized with pure VTES signals at the same elution times (38.37–40.28 min) were observed, independent of the heating type. The pattern of the elution profiles obtained for the POSS was the same as that found in the literature [5,45]. For the samples prepared with dielectric heating, from the elution signal at 38.37 min a mass average molar mass (Mw) of 1237 g mol−1 was determined and for the time of 40.28 min the Mw was 558 g mol−1, with a polydispersity index of 1.05. However, for the samples synthesized applying the conventional procedure, the Mw values were 1185 and 561 g mol−1, respectively, with a polydispersity index of 1.03. Thus, it is possible to infer that the largest fraction was comprised of T8 type structures, since the theoretical molecular weight for this structure is around 631 g mol−1. The difference in the experimental and theoretical mass values is due to the hydrodynamic volume of the particles relative to the linear polystyrene standard calibration. The molecular mass found for the elution time of 38.37 min is associated with degradation of the double bond and a subsequent macroradical recombination reaction during synthesis. Thus, one POSS structure is chemically bound to another. The VTES/TEOS hybrid material synthesized by dielectric heating showed two elution peaks, one with signals at 35–39 min, with a

molecular mass of 6580 g mol−1 and polydispersity of 3.11, and the other with an elution time of 40.28 min, an Mw of 571 g mol−1 and a polydispersity index of 1.05. The addition of TEOS resulted in the appearance of a greater number of non-condensed structures with a higher degree of polydispersion. For the VTES/TEOS materials synthesized using conventional heating, peaks were observed at 37.18– 40.28 min with several shoulders, which are due to the formation of structures with different sizes and molecular masses. The Mw values for these samples were in the range of 4451 to 572 g mol−1, indicating that the formation of species of different sizes occurred. The microwaveassisted synthesis of POSS is directly affected by the dipole moment of the reaction medium and the precursors [46]. The precursor molecules react much faster in the conventional synthesis. For this reason, high molecular weight hybrid materials were not obtained with the VTES/ TEOS mixture.

Fig. 3. GPC chromatographs for hybrid samples.

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3.1. Morphological and crystalline structure Fig. 4 shows the SEM micrographs for the POSS samples synthesized. The formation of cubic and spherical structures on the micro and submicron scales can be observed for all samples. This may be related to the sample purification process or the self-assembly of the nanostructures during the synthesis steps. This type of morphology (Fig. 4a and b) has been previously observed for POSS used in polymer nanocomposites [22]. For samples prepared with TEOS, it can be seen that the size of the structures (Fig. 4c and d) is slightly reduced. Possibly the spherical structures are related to the formation of phases with a short-range order. This structure profile has been observed in the synthesis of silica nanoparticles (SiO2) obtained using dielectric heating [37]. The transmission micrographs (Fig. 5) show a similar morphology to that observed by SEM. Fig. 5a and b shows the micrographs of the POSS synthesized with VTES via dielectric heating, where the formation of irregular cubic clusters of submicron order, with an average size of around 0.88 μm, can be observed. This morphology has also been observed in studies related to polymer nanocomposites using VTES as the precursor [34,47]. In Fig. 5a and b, it can be observed that clusters on the micrometer scale were obtained on applying microwave heating. Similar clusters have also been reported in the literature [5,21,47]. Fig. 5c and d shows the transmission micrographs for the POSS/VTES nanoparticles obtained using conventional heating. For the samples obtained by mixing the precursors VTES/TEOS employing both conventional and dielectric heating, it is possible to observe a spherical morphology and regular nanoscale for the structures, as shown in Fig. 6a and b. With the TEOS addition there is the emergence of a greater quantity of non-condensed structures, due to the characteristics of the reaction with two precursors (VTES and TEOS) with different dipole moments [48]. The diffractograms for the conventional and microwave syntheses of POSS VTES and VTES/TEOS, shown in Fig. 7, exhibit six distinct diffraction peaks at 2θ = 9.7°, 13.1°, 19.7°, 21.0°, 22.9° and 23.7°, corresponding to d-spacings of 9.1, 6.5, 4.5, 4.2, 3.9 and 3.7 Å respectively. The first peak corresponding to a d-spacing of 9.1 Å is attributed to the overall size of the POSS molecules (diagonal of the POSS cage), and the subsequent peaks due to a rhombohedral crystal structure with space group

symmetry R3 [11,49,50]. It is also important to note that in the case of the conventional synthesis of POSS VTES/TEOS (Fig. 7d), a diffraction pattern characteristic of an amorphous sample was observed. This diffraction pattern is in agreement with the findings of Cong et al. [12] who reported that the crystalline structure of POSS-aggregates is damaged by HCl solution over a period of 2 days. Le Bail refinement was performed for the POSS–VTES samples using as an initial lattice the parameters based on Waddon and Coughlin [33]. For both samples, the refined structural parameters and levels of agreement obtained were consistent with a rhombohedral unit cell and R3 space group. The results were as follows: a = 13.594 (5) Å, c = 14.289 (8) Å, Rp = 17.3%, Rωp = 23.7%, RB = 1.06% and RF = 1.68% for POSS–VTES samples obtained using the conventional procedure, and a = 13.583 (5) Å, c = 14.275 (9) Å, Rp = 17.9%, Rωp = 21.7%, RB = 0.61% and RF = 1.20% for POSS–VTES samples using the microwave-assisted procedure. The observed (Iobs) and calculated (Icalc) diffractograms and the residual line, as well as indexes for the main reflections for POSS–VTES samples obtained applying the two procedures, are shown in Fig. 8a and b, respectively. The dots represent the experimental data, the black line shows the calculated diffraction pattern profile, the blue line is the difference between the experimental and the calculated profile and the vertical bars indicate the positions of the Bragg reflections. Fig. 9 shows the thermographs for the hybrid materials synthesized with VTES and/or TEOS. The POSS samples synthesized with VTES using both procedures show a single thermal degradation step between 200 and 350 °C. The amount of mass remaining after the thermal degradation process at 800 °C was around 42.4 wt.%. The VTES/TEOS samples synthesized using dielectric heating showed a thermal degradation profile similar to that of the VTES samples, with the remaining mass of 24.8 wt.%. Regarding the VTES/TEOS sample synthesized by conventional heating, the initial weight loss was shifted to 250 °C and the remaining mass was 76.5 wt.%. The thermal degradation of the POSS structures is intrinsically dependent on the organic group linked to the Si\\O cube [51]. In hybrid structures with vinyl groups, the degradation of the vinyl bond occurs with subsequent recombination and formation of a Si\\O\\Cn\\Si structure when carried out in an inert atmosphere. However, when

Fig. 4. SEM micrographs for POSS VTES (a) conventional and (b) microwave heating, and POSS VTES/TEOS (c) conventional and (d) microwave heating.

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Fig. 5. TEM micrographs for POSS VTES (a), (b) microwave heating and (c), (d) conventional heating.

thermal degradation is carried out in an oxidant atmosphere SiO2 is formed [52]. The differences between the structures synthesized with conventional and dielectric heating are associated with the size of the structures, as seen in the GPC results. The use of a tetrafunctional reagent, such as TEOS, when reacted slowly, results in random structures with higher density connections and for this reason there is a change in the thermal degradation profile. The microwave-assisted synthesis provides a quick and reproducible method for the synthesis of POSS hybrid materials with the same chemical, thermal and structural characteristics as samples produced using conventional heating. The materials

Fig. 6. TEM micrographs for POSS VTES/TEOS (a) conventional and (b) microwave heating.

Fig. 7. Diffractograms of POSS obtained for (a) VTES-microwave; (b) VTES-conventional; (c) VTES/TEOS-microwave and (d) VTES/TEOS-conventional synthesis.

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Fig. 8. Le Bail refinement for POSS samples: (a) VTES-microwave-assisted and (b) VTES-conventional synthesis. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)

produced have great potential for application in many fields [5,12–15, 17,18,23]. 4. Conclusions The use of a microwave heating system allows hybrid nanomaterials to be obtained with shorter reaction times. The microwave-assisted synthesis resulted in reaction rates approximately 47 and 59 times greater than those associated with conventional heating, because of specific heating in the reaction medium. All samples showed FTIR bands at ~ 1090 and ~ 568 cm−1 (Si\\O\\Si and O\\Si\\O). The band at ~3400 cm−1 observed only in the case of the VTES/TEOS samples is related to OH groups associated with the formation of non-condensed structures. The 1H and 29Si NMR results show that in all POSS synthesized there is a fraction of the non-condensed hybrid material. The VTES/TEOS structures have a higher degree of polydispersion due to the use of the tetrafunctional alkoxysilanes. The hybrid materials synthesized have clusters of cubic and spherical shaped particles in micro and submicron scales. The use of the

TEOS in the reactions showed a tendency to decrease the size of the agglomerates. The POSS prepared with VTES using the two heating procedures has similar morphology and crystalline structure, regardless of the presence of a fraction of non-condensed POSS. It was noted that the VTES/TEOS hybrid material obtained by conventional synthesis showed an amorphous characteristic, due to the formation of random structures during the synthesis. The structures with similar molecular sizes presented similar thermal degradation profiles. Acknowledgments The authors are grateful to CNPq, CAPES, CETENE and FAPERGS for financial support (473402/2013-0 and ARD 12/1428-3). The authors also acknowledge FINEP (01.13.0359.00) and Laboratório Central de Microscopia Prof. Israel Baumvol for SEM analysis. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.jnoncrysol.2015.08.020. References

Fig. 9. Thermograms of POSS obtained for: (a) VTES/TEOS microwave; (b) VTES conventional; (c) VTES microwave and (d) VTES/TEOS conventional synthesis.

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