Cleavage of Organosiloxanes with Dimethyl Carbonate: A Mild

Jul 25, 2018 - Chuiko Institute of Surface Chemistry of National Academy of Sciences of Ukraine, Kiev 03164 , Ukraine. ⊥ Key Laboratory of Microbial...
3 downloads 0 Views 5MB Size
Download PDF
Article Cite This: Langmuir 2018, 34, 9719−9730

pubs.acs.org/Langmuir

Cleavage of Organosiloxanes with Dimethyl Carbonate: A Mild Approach To Graft-to-Surface Modification Iryna Protsak,†,‡,⊥ Ian M. Henderson,§ Valentyn Tertykh,∥ Wen Dong,*,†,⊥ and Zi-Chun Le‡ College of Environment and ‡College of Science, Zhejiang University of Technology, Hangzhou 310023, China § Omphalos Bioscience, LLC, Albuquerque, New Mexico 87110, United States ∥ Chuiko Institute of Surface Chemistry of National Academy of Sciences of Ukraine, Kiev 03164, Ukraine ⊥ Key Laboratory of Microbial Technology for Industrial Pollution Control of Zhejiang Province, Hangzhou 310014, China Downloaded via KAOHSIUNG MEDICAL UNIV on October 8, 2018 at 20:23:09 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information *

ABSTRACT: In this work, we explore the depolymerization of poly(dimethylsiloxane) (PDMS-100) and poly(methylphenylsiloxane) (PMPS) using dimethyl carbonate (DMC) and develop a surface functionalization method by utilizing the DMC-imparted active methoxy end groups of the partially depolymerized polysiloxanes. The efficiency of dimethyl carbonate as a reagent for organosiloxane cleavage was confirmed by means of 1H NMR spectroscopy, sizeexclusion chromatography, and viscosity measurements. The reaction of fumed silica with organosiloxanes (PMPS, PDMS50) in the presence of DMC was investigated using the ζpotential, 29Si and 13C solid-state NMR spectroscopy, IR spectroscopy, CHN analysis, contact angle goniometry, thermogravimetric analysis, scanning and transmission electron microscopy (TEM), and rheology. It was found that the interaction of PMPS/DMC with an SiO2 surface produced hydrophobic and thermally stable moieties (up to 550 °C) with a densely packed (average 2.2 groups/nm2) alkylsiloxane network for SiO2/PMPS + DMC in comparison with SiO2/PMPS (average 1.4 groups/nm2). Surface functionalization was successfully attained at a relatively moderate temperature of 200 °C. Scanning electron microscopy data show that the average size of aggregates of PMPS/DMC-modified silica nanoparticles is smaller than that of the initial silica and silica modified with neat PMPS. TEM images reveal uniform distribution of the PMPS/ DMC mixture across the SiO2 surface. Rheology studies show thixotropic behavior in industrial oil (I-40A), a fully reversible nanostructure and shorter structure recovery time for fumed silica modified in the presence of DMC.

■

INTRODUCTION Chemical functionalization of silica nanoparticles is a powerful approach to the design of materials with the desired level of solid−liquid interaction.1−6 This is of great interest in sorption and separation, polymer composites, fillers, and other applications.1−6 The development of reproducible synthetic approaches for the selective surface modification of various silica surfaces is therefore required. Organophilization of silica surfaces can be performed using various traditional types of modifying agents such as alkoxy-, halo-, aminosilanes, and organosilazanes.7−13 However, due to the high reactivity and moisture sensitivity of the modifying agents given above, purification is often critical for these hydrolyzable precursors. Poly(organosiloxanes) with methyl-terminated groups provide a viable and environmentally benign alternative to the chemical functionalization of oxides, as they are characterized by high carbon content, hydrophobic properties, thermal stability, chemical inertness, are noncorrosive reagents, and generate only water as a byproduct.13−25 Moreover, according to the literature, fumed silica treated with silicone oil shows a greater © 2018 American Chemical Society

thickening effect than either silica treated with octyltriethoxysilane or hydrophilic fumed silica.6 Methyl-terminated poly(dimethylsiloxanes) (PDMSs) are, however, typically considered to be inert and not suitable for surface functionalization reactions because of the absence of readily hydrolyzable groups.4 One of the peculiarities of PDMS molecules is that they can form a helix structure due to the corresponding rotations around the Si−O bonds. This limits the number of PDMS segments which are capable of interacting with active silica sites.21,22,26 One of the probable ways of increasing the reactivity of an organosilicon polymer is the partial depolymerization of high molecular poly(dimethylsiloxanes), followed by grafting of formed oligomers (with terminated alkoxy groups) on the silica surface. The cleavage reaction of the Si−O bond by various depolymerization reagents is a widely studied reaction. Complete Received: May 13, 2018 Revised: July 20, 2018 Published: July 25, 2018 9719

DOI: 10.1021/acs.langmuir.8b01580 Langmuir 2018, 34, 9719−9730

Article

Langmuir

an internal capillary diameter of 2.10 mm (see S1). Poly(dimethylsiloxane) depolymerization was investigated for four different systems. In each of them, 4.97 mol (290 mL) of ethanol was added to 0.018 mol of PDMS-100 (50 mL). However, different catalytically active depolymerization agents were additionally injected. In the first system, an alkali (NaOH) was used as the most effective catalyst for polysiloxane depolymerization. Thus, 0.29 mol (11.6 g) of sodium hydroxide was injected into the solution. In the second system, 0.99 mol (53 mL) of dimethyl carbonate and 0.0034 mol (0.2 g) sodium chloride (NaCl) were added into the solution. In the third system, 0.99 mol (0.083 mL) of dimethyl carbonate was added to the alcohol solution. In the fourth system, only 0.0034 mol (0.2 g) of NaCl was added to the polymer solution. All the solutions were stirred at room temperature for 1 h. Each polymer fraction was centrifuged for 15−20 min. Separation of the newly formed fractions was carried out by using a separatory funnel. Modification of Fumed Silica Nanoparticles and Characterization. The modification of fumed silica surface with PMPS was performed at three different temperatures: 200, 250, and 300 °C for 2 h with or without the addition of dimethyl carbonate (DMC). The amount of modifier agent was determined to be 17 wt % of silica weight. In the first series, the fumed silica was modified with neat PMPS. In the second series, the surface treatment was carried out with a mixture of PMPS/DMC, where the amount of DMC added was 0.6 mmol (per 1 g of silica). Modification of the surface was carried out using the PMPS and DMC mixture, using 1.8 mmol (per 1 g of silica) in the third series. As organosiloxanes are not sensitive to absorbed water present inside a silica skeleton, temperature surface treatment was not applied. The modification process was performed in a glass reactor with a stirrer with a rotational speed of 20−300 rpm (see Figure S2 of the Supporting Information). After the reactor was loaded with fumed silica, the air volume in the reactor was replaced with nitrogen and the reactor was heated up to the desired temperature. Next, the nitrogen feed to the reactor was stopped and the modifying agent was added by means of aerosol-nozzle spray. The modification of fumed silica with poly(dimethylsiloxane) (PDMS-50) and its mixing with DMC was described in our previous work.46,47 Removal of the physically adsorbed reactants was carried out in a Soxhlet apparatus with n-hexane as a solvent at 68 °C for 1 h. The washed samples were then dried at 80 °C for 2 h. The drying process was performed in air using the muffle furnace (ThermoLab SNOL 7,2/1100, Kyiv, Ukraine). The samples were subsequently cooled to room temperature. 1 H NMR Spectroscopy. 1H NMR spectra were recorded at 90 MHz with an Anasazi Eft-90 spectrometer (Anasazi Instruments), in deuterated chloroform (CDCl3). A 50 mL round-bottom flask was charged with ethanol (14.5 mL), dimethyl carbonate (2.5 mL), and organosiloxane (PDMS-100) (2.5 mL). The reaction mixture was stirred at room temperature for 2 h. Depolymerization of PMPS was carried out without ethanol at 200 °C for 2 h. After the reaction was complete, the volatile unreacted products were removed via a rotary evaporator. The resulting depolymerized polymer was dissolved in CDCl3 and the solution was analyzed by 1H NMR spectroscopy. Size-Exclusion Chromatography (SEC) Measurements. The SEC-UV analysis was performed at 237 nm based on spectra measurements made directly in the chromatographic detector of the device. Chromatographic measurements were performed using an high-performance liquid chromatograph (HPLC) consisting of a Shimadzu LC-10AT instrument (Shimadzu, Canada), a DGU-14A degasser, a SPD-10 UV−vis detector, and an SCL-10 control unit. The mobile phase used was tetrahydrofuran HPLC grade purchased from Merck (Germany). The system was calibrated using polystyrene standards purchased from Sigma-Aldrich in the range of 2.75 MDa to 92 Da. The reaction mixtures of poly(methylphenylsiloxane) and dimethyl carbonate were prepared for chromatographic analysis with volumes in the ratio of 1:1. Reagents were mixed in a shaking incubator (Benchtop Incubator Shaker Innova 40, Eppendorf) at 50 and 150 °C for 30 min. Due to the microprocessor thermocontroller inside the chamber of the incubator, the temperature remained constant during

depolymerization (to monomers) of poly(dimethylsiloxanes) can be achieved by treating siloxanes with toxic agents such as various amines.27,28 Amines are not environmentally benign reagents: they are toxic, which is unfavorable for recycling. An excellent review of the literature can be found in a paper by Voronkov et al.,26 which reports that the cleavage reaction readily occurs when organosiloxanes are treated in alcohol solution in the presence of acids or bases. Depolymerization can also be realized by thermal degradation (300−400 °C), treatment with sulfuric acids, thionyl chloride, and mixtures of alkali (NaOH, KOH), or with alcohols (methanol, ethanol).29−34 It should be noted, however, that the use of these catalysts is technologically complicated because of the potential electrolyte presence in the resulting products. This is not desirable, especially when using modified silicas as fillers in silicone cable rubbers and thickeners in insulating electrical greases. Okamoto et al. found that the mixture of dimethyl carbonate (DMC) and methanol using different catalysts can completely depolymerize the Si−O bond in PDMS to dimethoxydimethylsilane and methoxytrimethylsilane monomers.35−37 Eventually, modification of silica with monomers will lead to poor density grafting on the silica surface. Our purpose was to find the conditions that produce partially depolymerized organosiloxane (see reaction 1), with the aim of using the resulting end-functionalized depolymerization products for surface modification of fumed silica. (H3C)3 Si − [O − Si(CH3)2 ]x − [O − Si(CH3)2 ]y − OSi(CH3)3 + (H3CO)2 CO → (H3C)3 Si − [O − Si(CH3)2 ]x − OCH3 + H3C−[O − Si(CH3)2 ]y−OSi(CH3)3 + CO2

(1)

Dimethyl carbonate is a mild reagent that meets the main requirements of “green chemistry”.38−41 It is therefore widely used as an alkylating and carboxylating reagent in many chemical processes, where it can successfully replace more aggressive reagents, such as phosgene and methyl halides.42−45 Therefore, in the present work we have explored DMC as a mild reagent in the backbone cleavage of organosiloxanes and the use of the resulting methoxy-functionalized oligomers in the surface functionalization of silica.

■

EXPERIMENTAL METHODS

Chemicals. Commercial poly(dimethylsiloxane) (code name: PDMS-100, linear, −CH3 terminated, viscosity 95−105 mm2/s, degree of polymerization n = 35−68, molecular weight Mw ∼ 5000− 6000 g/mol), poly(dimethylsiloxane) (code name: PDMS-50, linear, −CH3 terminated, viscosity 45−55 mm2/s, degree of polymerization n = 23−28, molecular weight Mw ∼ 3500−3780 g/mol), and poly(methylphenylsiloxane) (code name: PMPS−4, linear, −CH3 terminated, viscosity 600−1000 mm2/s, degree of polymerization n = 300−600) were purchased from Kremnepolimer, Zaporizhzhya, Ukraine. Fumed silica (A-300) (purity 99.87%, S = 260 m2/g) was supplied by the Pilot Plant of the Chuiko Institute of Surface Chemistry, Kalush, Ukraine. Sodium hydroxide containing 96.0 wt % NaOH, sodium chloride containing ≥99.0 wt % NaCl, tetrahydrofuran containing ≥99.9 wt % C4H8O, ethanol containing ≥99.0 wt % C2H5OH, n-hexane containing ≥99.0 wt % of C6H6, dimethyl carbonate containing ≥99.0 wt % (CH3O)2CO, and deuterated chloroform (CDCl3) containing ≥99.0 wt % were purchased from Sigma-Aldrich. Viscosity Measurements. The polymer solution efflux time was measured using a stopwatch by means of an Ostwald viscometer with 9720

DOI: 10.1021/acs.langmuir.8b01580 Langmuir 2018, 34, 9719−9730

Article

Langmuir mixing. Before application, all the samples were diluted 200 times in tetrahydrofuran using an eVol XR by DSG (U.K.) for volumetric measurements. Calibration was done with polystyrene standards. ζ-Potential. A Zetasizer Nano ZS instrument (Malvern, UK) utilizing a 632.8 nm HeNe laser was used to study the average ζ-potential. Measurements were performed using a Malvern dip cell kit in 50/50 THF/water. All of the measurements were performed at 25 °C. Three runs of each sample were taken. Infrared Spectroscopy. IR spectra were recorded using a Specord M-80 spectrophotometer (Carl Zeiss, Germany) in the range of wavenumbers 4000−1100 cm−1. The silica samples were pressed into pellets on 28 × 8 mm2 plates of 25 mg weight. The transmittance spectra were re-plotted into absorbance spectra for quantitative analysis. The modification degree (θ) was calculated as is described in eq S3. 29 Si and 13C CP/MAS NMR Measurements. Solid-state 29Si CP/ MAS NMR spectra were recorded on a Bruker Avance 400 III HD spectrometer (Bruker, with a magic field strength of 9.3947 T) at a resonance frequency of 79.49 MHz for 29Si through cross polarization (CP), magic-angle spinning (MAS), and a high-power 1H decoupling. The powder samples were placed in a pencil-type zirconia rotor of 3.2 mm o.d. The spectra were measured on a 3.2 mm MAS probe at a spin speed of 3 kHz, with a 2.4 μs 1H π/2 pulse, a 6 ms CP pulse, and a recycle delay of 5.5 s. The Si signal of tetramethyl silane at 0 ppm was used as a reference of 29Si chemical shift. Solid-state 13C CP/MAS NMR spectra were recorded on a Bruker Avance 400 III HD spectrometer (Bruker, with a magic field strength of 9.3947 T) at a resonance frequency of 100.61 MHz for 13C through cross polarization (CP), magic-angle spinning (MAS), and a highpower 1H decoupling. The powder samples were placed in a penciltype zirconia rotor of 3.2 mm o.d. The spectra were measured on a 3.2 mm MAS probe at a spin speed of 15 kHz, with a 2.4 μs 1H π/2 pulse, a 2 ms CP pulse, and a recycle delay of 5 s. The methylene signal of adamantane at 38.5 ppm was used as the reference of the 13C chemical shift. The solid-state NMR measurements performed are only of a qualitative nature because no calibration between the intensities of NMR peaks and the concentrations of the surface species was made. Elemental Analysis. The content of grafted organic groups in the synthesized samples was measured by a PerkinElmer 2400 CHNanalyzer (Perkin Elmer). The anchored layer was oxidized to produce H2O and CO2 during heating of the samples in the oxygen flow at 750 °C. The bonding density of attached layers was calculated as described in eq S4. Contact Angles. Contact angle measurements were performed using a commercial Contact Angle Meter (GBX Scientific Instruments, France) equipped with a temperature- and humiditycontrolled measuring chamber and a digital camera (20 °C, relative humidity is 50 %). First, hydrophobic powders were pressed into thin pellets (180 bars for 15 min). A drop of deionized water was then placed onto the surface and the plate was checked to determine the distribution of water on the surface. From measurement of the width and height of the droplet, the contact angle was calculated using a computer program. To obtain the averaged values, measurements were performed for six water droplets placed on each sample. Thermal Analysis. The thermal behavior of the grafted organic layer in the synthesized samples was studied by means of thermogravimetric analysis (TGA, STA 449 Jupiter F1, Netzsch, Germany) in air at temperature intervals of 30−950 °C at a heating rate of 10 °C/min. Rheological Measurements. Modified silica suspensions were prepared at concentrations of the solid phase 5 wt % in industrial oil (I-40A) at room temperature (25 °C). The rheological properties of the silica suspensions were studied with a rotational viscosimeter Reotest RV2.1 (Mettingen, Germany) equipped with a cylindrical system at shear rates (γ) from 9 to 1312.2 s−1. Industrial oil was employed as a dispersion media, as hydrophobic silica has a wide range of applications as thickeners for nonpolar fluids. Transmission and Scanning Electron Microscopy. Transmission electron microscopy (TEM) was performed using a TECNAI

G2 F30 microscope (FEI-Philips, Holland) at an operating voltage of 300 kV. The powder samples were added to acetone (chromatographic grade) and sonicated. A drop of the suspension was then deposited on a copper grid with a thin carbon film. After acetone evaporation, sample particles that remained on the film were studied with TEM. Scanning electron microscopy (SEM) was performed on a FE-SEM (Hitachi S-4700, Japan) at an operating voltage of 15 kV at magnification ranges of 5000−100 000. Dried powders of modified silica particles were investigated.

■

RESULTS AND DISCUSSION H NMR and Viscosimetric Study of the Reaction of PDMS-100 and PMPS with DMC. In this work, we studied the partial depolymerization of PDMS-100 using four catalytic systems (Figure 1) and explored the effect: of alkali 1

Figure 1. Viscosity change of PMDS-100 in alcohol solutions of sodium hydroxide (1), dimethyl carbonate and sodium chloride (2), neat dimethyl carbonate (3) and neat sodium chloride (4).

(NaOH) alone (curve 1), DMC (curve 3), and NaCl (curve 4) on the degree of viscosity change of PDMS-100, and the effect of NaCl + DMC together (curve 2) on the viscosity change of PDMS-100, with the intention of determining if there is a synergistic effect between these two components. The efficiency of each system was examined by means of change of poly(dimethylsiloxane) viscosity under the direct influence of each system. We fitted the change in viscosity with a standard single exponential fit: μ = A e−kt + B

According to the results obtained (Figure 1, curve 4), treatment with the NaCl/alcohol solution results in a slight viscosity change approached at a slow rate (the actual coefficient (k4) of rate of viscosity change is 0.00273 s−1). In contrast, as can be seen from Figure 1, curve 3, the viscosity of PDMS-100 changed much more upon reaction with DMC alone (k3 = 0.18 s−1). To elucidate whether the combination of DMC with NaCl is synergistic in relation to the rate of viscosity change of PDMS-100, both reactants were employed in the reaction system simultaneously. It was found that NaCl has almost no influence on viscosity change, as this system (Figure 1, curve 2, k2 = 0.22 s−1) behaves the same as with DMC alone (Figure 1, curve 3, k3 = 0.18 s−1). When using a system containing only alkali (Figure 1, curve 1) with a 9721

DOI: 10.1021/acs.langmuir.8b01580 Langmuir 2018, 34, 9719−9730

Article

Langmuir

with the methoxy end groups. Peaks at 7.26 and 2.16 ppm represent the residual solvent shifts of CDCl3 and acetone, respectively. The appearance of the methoxy end group confirms that PDMS-100 was depolymerized by DMC and lower-weight polymers were formed. To calculate the degree of polymerization of the resulting products, the 1H NMR spectrum was integrated. As can be seen, the peak of the methoxy group (δ = 3.47 ppm) is normalized with a peak area of 1.00, the shift of the ethoxy group (3.85 ppm) is normalized with a peak area of 1.43, and the shift of the methyl group (from 1.82 to 1.65 ppm) is normalized with a peak area of 653.24. On the basis of end-group analysis (see eq S6), the amount of repeat units of the resulting depolymerized PDMS100 was calculated and was found to be 52.09, down from an estimated average degree of polymerization of 67.6 for PDMS100 (assuming an Mn of 5000 g/mol). As the silica modification (discussed later) was performed with PMPS and DMC at 200 °C, depolymerization of PMPS by DMC was also carried out at 200 °C. After the depolymerization process was complete, the resulting product was analyzed by 1H NMR spectroscopy. The liquid state 1H NMR spectrum (Figure S7) of the depolymerized PMPS shows the presence of methoxy end groups (OCH3, δ = 3.47 ppm), which confirms that the PMPS was partially depolymerized by the DMC. The mechanism by which DMC has its marked effect on polyorganosiloxane depolymerization is still, however, unclear, and further research is needed. To better understand this mechanism, we have explored some theoretical aspects of the interaction of DMC with the Si−O bonds of the polymer backbone, as discussed below. As is known,33,34 strong nucleophilic reagents in the form of OH− and F− anions demonstrate the highest activity in the cleavage of the siloxane bonds, whereas other, sometimes stronger, nucleophiles, such as Cl−, Br−, I−, SO42−, and PO43− do not show similar activity. It is the authors’ opinion33,34 that it is possible that small atomic radii (such as those of OH- and F-anions) are crucial to nucleophilic substitution on the silicon atom, as smaller radii are essential for effective entry into the coordination sphere of the silicon atom and the formation of transitional complexes. As a result of effective entry of the appropriate nucleophile or electrophile into the coordination sphere of the atoms of the siloxane bond, redistribution of electron density on the Si−O bond atoms takes place: the nucleophilicity of the oxygen atom and the electrophilicity of the silicon atom increases, resulting in the heterolytic cleavage of the siloxane bond.32 Dimethyl carbonate is an ambident electrophilic reagent which contains a relatively hard electrophile (carbon atom of the CO bond) and a soft electrophile (methoxy carbon atoms).42 According to the principle of “hard” and “soft” acids and bases (the HSAB principle), soft nucleophiles preferentially react with soft electrophiles and vice versa. The siloxane bond (Si−O) has two active centers: the silicon atom which is a soft electrophile and the oxygen atom which is a soft nucleophile (Figure 4). According to this, the attack by DMC on the PDMS molecule can be carried out through the formation of a six-membered activated complex involving the carbon atom in the methyl group (CH3) and the oxygen atom in the methoxy group (O−CH3), which can attack the atoms of Si and O in the Si−O bond, respectively (Figure 4). In our previous study using simulations,48 we hypothesized that the attack of DMC on the Si−O bond is most likely to

concentration threefold lower than that of DMC, PDMS-100 depolymerization occurs at a slightly faster rate. Figure 2 reveals the influence of different concentrations of DMC on the dept h of depo ly mer i za tio n po ly -

Figure 2. Viscosity change of PDMS-100 at different molar ratios of poly(dimethylsiloxane) and dimethyl carbonate.

(dimethylsiloxane) at different ratios of DMC and PDMS100. The results show that increasing the amount of DMC in the reaction mixture has a slight effect on the depth of depolymerization for PDMS: the depolymerization occurs deeper and at a faster rate (k1 = 0.083, k2 = 0.104, k3 = 0.148). The viscosity data show that the viscosity of PDMS-100 reduced under the influence of DMC over time at ambient temperature. The liquid state 1H NMR spectrum (Figure S5) of neat PDMS-100 reveals signals of protons of the methyl groups with satellites from 1.82 to −1.65 ppm. The 1H NMR spectrum of depolymerized PDMS-100 by DMC (Figure 3) in

Figure 3. 90 MHz 1H NMR spectrum of depolymerized PDMS-100; the inset shows the alkoxy group shifts of depolymerized PDMS-100.

alcohol solution shows signals of protons of the methyl groups with satellites from 1.82 to −1.45 ppm; the end groups of linear products of depolymerized PDMS-100 were methoxy (OCH3, δ = 3.47 ppm) and ethoxy (O−CH2−CH3, δ = 3.85 ppm) and the CH2 peak was covered by the satellite at 1.24 ppm. The ethoxy end groups are the result of ethanol exchange 9722

DOI: 10.1021/acs.langmuir.8b01580 Langmuir 2018, 34, 9719−9730

Article

Langmuir

Figure 4. Initial approach scheme of probable attack of DMC on PDMS.

occur with the formation of a four-membered activated complex involving the carbon atom in the carbonyl group (CO) and the oxygen atom of the methoxy group (O− CH3) which can attack the atoms of Si and O in the Si−O bond, respectively (Figure 5). As a result of such chemical interactions, a siloxane bond in PDMS is broken and a carbon dioxide molecule is eliminated, ultimately resulting in the replacement of the Si−O−Si bond with two Si−O−Me groups (Figure 5). SEC Study of the Depolymerization of PMPS. After confirming the viscosity reduction of poly(dimethylsiloxane) upon reaction with dimethyl carbonate, the products of the reaction with poly(methylphenylsiloxane) were characterized by size-exclusion chromatography. Poly(methylphenylsiloxane) was chosen for this experiment due to the absorption of the phenyl rings at 237 nm, rendering the compounds readily detectable with the UV detector. Figure 6 shows representative SEC chromatograms of neat PMPS and depolymerized PMPS at 50 and 150 °C. The largest peak (1) on the chromatogram of PMPS depolymerized at 50 °C (b) represents compounds with the highest molecular weight. The intensity of this peak (1) is much higher in comparison to the intensity of the same peak (1) on the chromatogram of the unreacted control (curve a). Looking at the chromatogram of PMPS depolymerized at 150 °C (c), the intensity of this peak is higher in comparison to the unreacted control. Note that with the increasing temperature (curve c, peak 2) the amount of lower molecular weight species is drastically increased in comparison to curve b, peak 2. This can be explained by the higher depolymerization rate of PMPS at higher temperatures. Such results are expected on the basis of the data on the thermal depolymerization of organosiloxanes given in ref 29. To summarize, after the interaction of PMPS with DMC at both 50 and 150 °C (curves b and c respectively), two methylphenyl-containing products are formed: molecular weight fractions with the apparent molecular masses 1846−1855 Da, and lower molecular weight fractions with the apparent molecular masses 282−289 Da,

Figure 6. SEC chromatograms of molecular weight distribution for neat PMPS fraction (a), for depolymerized PMPS fraction at 50 °C (b) and 150 °C (c).

down from 2183.057 Da calculated as the apparent molecular mass of the initial PMPS using experimental data. Overall, the formation of new weight species seen in both chromatograms (b, c, peak 2) in high concentration represent the partial depolymerization of PMPS upon chemical interaction with DMC. Surface Structure of Modified Silica Nanoparticles. After depolymerization of organosiloxane, the modification of silica surfaces with depolymerized polymers was carried out. As can be seen in Figure 7, in 1:1 water/THF, the ζ-potential of

Figure 7. ζ-Potential measurements of neat SiO2, SiO2 modified with neat PMPS and SiO2 modified with a mixture of PMPS + DMC (1.8 mmol) at 200 °C for 2 h.

Figure 5. Probable scheme of interaction between PDMS and DMC. 9723

DOI: 10.1021/acs.langmuir.8b01580 Langmuir 2018, 34, 9719−9730

Article

Langmuir unfunctionalized particles is strongly negative (−9.0 (±1.4 mV)), indicating a high density of acidic silanol surface groups (Si−OH) on the silica surface. According to the result obtained (Figure 7), silica nanoparticles which were modified with neat PMPS at 200 °C exhibit a ζ-potential of ζ = −4207 (±1.0 mV), indicating that, although there may be some physical adsorption or thermal addition of PMPS to the silica particle, there are still some unreacted silanols remaining on the SiO2 surface. In contrast, the silica nanoparticles functionalized with the PMPS/DMC mixture showed ζ = −0.512 (±0.2 mV), indicating that there were much lower amounts of unreacted free silanols on the particle surface in comparison to silica/PMPS and unmodified silica. This is in a good agreement with the 29Si CP/MAS NMR data (discussed below). In the 29Si CP/MAS NMR (Figure 8) spectrum of the crude SiO2 nanoparticles, the chemical shifts of the Q2, Q3, and Q4

(where superscript indicates the number of siloxane bonds) silicon nuclei are observed at −91, −101, and −109 ppm, respectively.49−52 The peak at Q2 is assigned to silicon with two hydroxyl groupsgerminal silanols (−91 ppm), the peak at Q3 represents the silicon with one unreacted hydroxyl groupisolated silanols (−101 ppm), and the peak at Q4 assigned to the silicon atoms bound to four other OSi moieties (−109 ppm).49−52 29Si CP/MAS NMR spectrum of SiO2 modified with a mixture of PMPS/DMC shows a resonance at −109 ppm which is characterized by a broader shoulder and higher intensity in comparison to the same resonance of SiO2 modified with neat PMPS (Figure 8). Simultaneously the signal attributed to the SiOH decreased for SiO2/PMPS + DMC compared to SiO2/PMPS, which clearly indicates that more silanols were involved in the reaction with a modifier agent. The fact that the resonance of isolated silanols (−101 ppm) is decreased for SiO2/PMPS + DMC in comparison to neat SiO2 (Figure 8) but not disappear completely indicates that some of the OH groups were inaccessible to depolymerized PMPS. It is well-known that on a fully hydroxylated surface all the hydroxyls cannot be reacted even with the molecule of hexamethyldisilazane (HMDS),53 and therefore, the reaction of SiO2 hydroxyls with depolymerized PMPS, which consists of much bigger molecules in size in comparison to HMDS, will not occur completely as not all of the silica surface is accessible to the reagent. Note that the intensity of the signal at −101 ppm of the isolated silanols for SiO2 modified with neat PMPS is virtually the same as it is for neat SiO2. It can be explained by the fact that neat PMPS is a big molecule with very long chains compared with depolymerized PMPS, which consists of shorter polymer chains, and logically the big molecule of neat PMPS is not accessible to the SiO2 surface hydroxyls. This is in a good agreement with the ζ-potential data (Figure 7). The attachment of organosiloxane on the silica surfaces (SiO2/PMPS + DMC and SiO2/PMPS) is shown by the appearance of a signal at −18 ppm, which corresponds to −OSi(CH3C6H5)− moieties. 13 C CP/MAS NMR spectra (Figure S8) show higher peak intensity of methyl (δ = −0.6 ppm) and phenyl groups (δ = 127 and 134 ppm) for SiO2/PMPS + DMC in comparison with SiO2/PMPS, which is in good agreement with the elemental analysis data. Table 1 shows that silicas modified with mixtures of PMPS/DMC, after washing them in n-hexane at its boiling point, are characterized by a higher carbon content (6.2−7.9 wt %) in comparison to SiO2/PMPS (4.1− 5.3 wt %). It is worth mentioning that the carbon content of silica nanoparticles modified with neat PMPS (Table 1) after

Figure 8. 29Si CP/MAS NMR spectra of unmodified SiO2, silica modified with neat PMPS and its mixture of DMC (1.8 mmol) at 200 °C for 2 h.

Table 1. Hydrophobic Surface Properties of Silicas Modified with Neat PMPS and Mixtures of PMPS/DMC T, °C 25 200

250

300

sample SiO2 (A-300) SiO2/PMPS SiO2/PMPS + SiO2/PMPS + SiO2/PMPS SiO2/PMPS + SiO2/PMPS + SiO2/PMPS SiO2/PMPS + SiO2/PMPS +

DMC (0.6) DMC (1.8) DMC (0.6) DMC (1.8) DMC (0.6) DMC (1.8)

carbon content before washing in n-hexane, wt % (SD ± 0.3%)

carbon content after washing in n-hexane, wt % (SD ± 0.3%)

0 8.1 6.4 8.2 8.7 8.1 7.3 6.5 7.7 7.8

0 5.3 6.2 7.9 5.2 7.9 7.2 4.1 7.5 7.6 9724

bonding density ([Si(CH3C6H5O]) groups/nm2 0 1.61 1.92 2.52 1.58 2.52 2.27 1.22 2.39 2.41

± ± ± ± ± ± ± ± ±

0.005 0.006 0.008 0.005 0.008 0.007 0.004 0.007 0.007

DOI: 10.1021/acs.langmuir.8b01580 Langmuir 2018, 34, 9719−9730

Article

Langmuir

the presence of DMC (curve 1), compared with SiO2 modified with neat PMPS at the same temperature (curve 2). The disappearance of the 3742 cm−1 band from the spectra, as well as the reduction of intensity of adsorbed water (a broad band at 3700−2700 cm−1) after silica modification (Figure 9, curves 2, 3), but with the appearance of the bands at 3079− 2968 and 2909 cm−1, are all evidence that good methylphenylsiloxane coverage on the silica surface has been obtained. The reduction of the adsorbed water intensity (band at 3700−2700 cm−1) is due to heating of the silica nanoparticles during the modification at 200 °C for 2 h. Nevertheless, silica nanoparticles still contain interfacial water in the contact zones between primary SiO2 nanoparticles,56,57 removal of which requires higher temperatures. However, despite this interfacial water inside the silica, the modified silicas cannot be wetted in aqueous media and are characterized by hydrophobic surface properties (average contact angles 128−130°) (Figure S9), whereas unmodified silica nanoparticles show good solubility in water. From the IR spectra (Figure 10, curves 2, 3), it is seen that an increase in the temperature of the silica nanoparticle’s

being washed in boiling solvent is characterized by a decreased value of carbon, which indicates partial desorption of the grafted organic layer into the solvent. Simultaneously, for silicas which were modified in the presence of DMC, the value of carbon remained the same in the grafted modifier layer, indicating that depolymerized organosiloxanes chemically bonded with the silica surface sites. Table 1 also shows that the bonding densities are higher for the surfaces prepared by the reaction of PMPS/DMC, and silicas modified with neat PMPS are characterized by a lower grafting density. High values of grafting density for SiO2 modified in the presence of DMC indicate the formation of closely packed organic monolayers and may suggest ordering in the monolayers.54 Grafting densities in this range have been reported for silica surfaces prepared by the reaction of dichlorodimethylsilane/ siloxane in toluene with a reaction duration of 14 days,54 while we propose a method of modification of SiO2 with PMPS/ DMC for only 2 h. The higher concentration of grafted siloxanes for these samples may be due to the partial depolymerization of PMPS by DMC: the concentration of the resulting lower-weight siloxane species with lower viscosity increases and this facilitates an increase in the density of contacts between them and the surface OH groups. In contrast, neat PMPS molecules can form a helix structure due to the corresponding rotations around the Si−O bonds.21,22,26 Therefore, only a portion of segments of PMPS molecules can interact with the surface. This can also be explained by the fact that the structure of the adsorption complexes of the organosiloxane changes after its treatment with DMC. For example, in the concentrated solutions of organosiloxane in hexane, the fraction of unfolded molecules increases,55 and this can result in an increase in the density of contacts between the siloxane molecules and the surface OH groups which in turn promotes an increase in the concentration of grafted siloxane on the silica surface. As indicated by the inset of Figure 9, the intensity of C−H stretching vibration in the methyl and phenyl groups at 3079− 2968 cm−1 and at 2909 cm−1 is increased for SiO2 modified in

Figure 10. IR spectra of neat fumed silica (1), fumed silica modified with mixtures of PMPS and DMC (1.8 mmol) at 200 °C (θMPS = 81 ± 0.081%) (2) and at 300 °C (θMPS = 84 ± 0.084%) (3). The spectra of modified silicas were normalized to the intensity of the band at 1869 cm−1.

modification is accompanied by an increase in the intensity of C−H stretching vibrations in the phenyl and methyl groups at 3079−2968 cm−1 and the accompanying band at 2909 cm−1. This may be explained if one assumes that at 200 °C the degree of depolymerization of PMPS is lower in comparison to that occurring at the higher temperaturethe latter promoting chemisorption of the oligomers with relatively higher molecular weight and with weaker contacts with the SiO2 surface sites.58 At 300 °C, by contrast, an increased number of low-weight oligomers is formed which interacts intensively with the silica sites, resulting in an increase in the degree of surface grafting. In addition to the presence of the C−H stretching vibration of the methyl and phenyl groups on the spectra of silica modified with PMPS/DMC (Figure 9, curve 2), we can also see the band at 2852 cm−1 and this corresponds to C−H stretching vibration in the methoxy groups (O−CH3). On the silica surface, there are two types of active sites for attack by dimethyl carbonate molecules: the structural silanol groups

Figure 9. IR spectra of fumed silica modified with mixtures of PMPS and DMC (1.8 mmol) (1) (θMPS = 81 ± 0.081%) and with neat PMPS at 200 °C (2) (θMPS = 75 ± 0.075%). The spectra of modified silicas were normalized to the intensity of the band at 1869 cm−1, the inset shows the intensity of C−H stretching vibration in the methyl and phenyl groups at 3079−2968 cm−1 and at 2909 cm−1 for SiO2/ PMPS + DMC (1) and SiO2/PMPS (2). 9725

DOI: 10.1021/acs.langmuir.8b01580 Langmuir 2018, 34, 9719−9730

Article

Langmuir

Figure 11. Attack by dimethyl carbonate (DMC) on the silica silanol group (I) and attack by DMC on the silica Si−O bond (II).

Figure 12. TGA data for silicas modified with neat PMPS (a), modified with the mixture of PMPS/DMC (1.2 mol) (b) and silica modified with the mixture of PMPS/DMC (1.8 mol) (c).

(Figure 11, scheme I) and the siloxane bridges (Figure 11, scheme II). In both cases, the products of the chemical reaction are carbon dioxide, grafted methoxy groups, and the free silanol group (Figure 5). Grafted methoxy groups on the SiO2 surface serve as additional “active centers” for further PMPS chemisorption. Due to the higher chemisorption of the PMPS/DMC mixture at the SiO2 surface, the thermal stability of these silica nanoparticles is increased (discussed in the next section).16 The TGA results (Figure 12a,b, red curves) show that, in comparison with SiO2/PMPS, the initial degradation temperature for SiO2/PMPS + DMC (1.2) shifts from 71 to 62 °C, and the shift of the temperature of the maximum rate of weight loss is from 346 to 433 °C and from 453 to 550 °C. The same

is true for SiO2/PMPS + DMC (1.8) (Figure 12a,c red curves): the initial degradation temperature is decreased (71 vs 66 °C) and the temperature of maximum weight loss is increased (453 vs 563 °C, respectively). A decrease in the initial degradation temperature for both silicas modified in the presence of DMC is related to the evaporation of a small amount of very low-boiling methylsiloxanes formed as a result of PMPS depolymerization with DMC, adsorbed water, DMC, and probably methanol, as a result of some methoxy group’s hydrolysis by air moisture as well as the reaction between the neighboring methoxy and silanol groups and water. The split of the DTG peak at the main degradation stage for SiO2/PMPS + DMC (1.8) (Figure 12c red curve) may be attributed to the oxidation of different organic groupsmethoxysilyl groups originating from the interaction of DMC with PMPS (the left 9726

DOI: 10.1021/acs.langmuir.8b01580 Langmuir 2018, 34, 9719−9730

Article

Langmuir

Figure 13. Effective viscosity dependence of the shear rate over time for fumed silica suspension modified with a mixture of DMC and PDMS-50 (a) and silica modified with neat PDMS-50 (b) in industrial oil I-40A with the concentration of solid phase 5 wt %.

Figure 14. SEM images and particle size distribution of bare fumed silica (a, d), silicas modified with neat PMPS (b, e) and mixtures of PMPS/ DMC (c, f).

one) and the PMPS methyl and phenyl groups (the right one). The second change appears to be related to the formation of methylsiloxane coverage, which is more thermally stable (than that formed without the addition of DMC) because of higher chemisorption through esterification of the surface silanol groups. The weight loss for silicas modified in the presence of DMC is accompanied by the less pronounced exo effect in comparison to silica modified with neat PMPS (Figure 12a−c blue curve). The less intense differential thermal analysis (DTA) peak at the main stage of thermal degradation and the smaller amount of weight loss for silicas modified in the presence of DMC confirms the reduction of an amount of oxidized PMPS compared with SiO2/PMPS degradation. Exo effects at temperatures higher than 500 °C for all resulting samples are not accompanied by notable weight loss and are presumably related to restructuring of the surface layer.

Overall, the addition of dimethyl carbonate to the modifying mixture improves the thermal stability of the grafted organosiloxane layer at the SiO2 surface. The viscosity of modified silicas in organic dispersion media is dependent on the nature of the interaction between the polymer (modifier agent) and the surface of the silica. Thus, the viscosity increases if polymers interact with the maximum number of solid particles. If a fraction of polymer interacts with a smaller number of nanoparticles (per a macromolecule), or a fraction of polymer is desorbed and dissolved (in the dispersion medium), then the viscosity increases in part. As can be seen from Figure 13, SiO2 modified with a mixture of PDMS/DMC (Figure 13a) is characterized by a fully reversible inner structure as the viscosity increases to the initial values following the mechanical action (effective viscosity decreases rapidly at the higher shear rates compared with lower shear 9727

DOI: 10.1021/acs.langmuir.8b01580 Langmuir 2018, 34, 9719−9730

Article

Langmuir

Figure 15. TEM images of bare fumed silica (a), silicas modified with neat PMPS (b) and mixtures of PMPS/DMC (c).

the ζ-potential and solid-state 29Si NMR data which increases bonding between particles and increases the tendency to agglomeration. The TEM images of SiO2, modified with a mixture of PMPS/DMC, show no sign of visible coating (Figure 15c), indicating that the resulting modifier mixture is uniformly dispersed on the SiO2 matrix. This can be explained by assuming that the chemisorption of depolymerized PMPS is accompanied by the grafting of shorter polymer chains with lower viscosity on the SiO2 surface, which improves the distribution of the depolymerized PMPS on the SiO2 surface. Ultimately, uniform coverage of the methylsiloxane layer on the SiO2 surface may improve the interaction of the polymermodified SiO2 nanoparticles (filler). This also results in better filler dispersion. On the other hand, it is possible to see some part of the polymer on the SiO2 surface which has been modified with neat PMPS (Figure 15b). Such random grafting of neat PMPS at the SiO2 surface can be explained by the high viscosity of the initial PMPS molecules which impedes the polymer spreading over the SiO2 surface during the synthesis, and simultaneously shows another advantage of using a mixture of PMPS/DMC for SiO2 surface modification.

rates, and gradual recovery of the structure is then observed and viscosity begins to increase again). SiO2 modified with neat PDMS-50 (Figure 13b) is characterized by incomplete recovery of viscosity, which means that some of the oligomer on the SiO2 surface is physically adsorbed, and in the organic media (I-40A) desorption of the adsorbed polymer has occurred. Note that silica modified in the presence of DMC requires a shorter time for structure recovery after the shear force has been applied, whereas SiO2 which has been modified using neat PDMS-50 is characterized by a longer recovery time as far as its nanostructure is concerned. Morphology of Modified Silicas. Initial fumed silica in this study is composed of nonporous nanoparticles with a true density of amorphous nanosilica ρo = 2.2 g/cm3 forming aggregates and agglomerates of aggregates (Figure 14a). After silica modification, the sphere-like morphology of the resulting samples was retained (Figure 14a−c). The particles obtained following the modification of SiO2 by mixtures of PMPS/DMC (Figure 14c) look similar to the unmodified silica, forming aggregates with voids between the primary nanoparticles in the secondary structures which are responsible for the textural porosity of the silica.22 After silica modification, the aggregation was aggravated with neat PMPS (Figure 14b). This can be explained if one assumes that the reaction of neat polymers with the SiO2 surface can run through the island-like polymer distribution onto the silica surface (as the chains of neat polymers are very long and it is difficult to spread them out over the surface), and the fact that bonded molecules favoring other molecules to be bound nearby may cause more intensive aggregation of SiO2 nanoparticles. Treatment of the SEM image (Figure 14d−f) using the specific software (ImageJ with Granulometry Plugin59 based on gray level mathematical morphology operations) allows us to calculate the size distribution functions of primary particles and their aggregates for the samples presented. SiO2 nanoparticles modified with pure PMPS (Figure 14e) were shown to form beads with sizes which are larger (44 nm) than unmodified nanosilica (Figure 14d) and silica modified with the mixture of PMPS/DMC (Figure 14f). The fumed silica modified with the mixture of PMPS/DMC consists mostly of smaller nanoparticles with an average size of 37 nm (Figure 14f). This is very promising when using such silicas as thickeners in organic media, as the higher dispersity of the nanosilicas suggests that higher compatibility with the polymer matrix will be achieved. Additionally, the smaller size of the nanoparticles of silicas which were modified with a mixture of PMPS/DMC may be due to the lower amount of H-bonded silanols after the silica surface modification. This is in good agreement with the ζpotential and solid-state 29Si NMR data (discussed above). In contrast, silica surfaces which were modified with neat PMPS show a higher level of H-bonded silanols in comparison with

■

CONCLUSIONS Siloxane bond cleavage in organosiloxanes was demonstrated using the “green” reagent dimethyl carbonate in an alcohol solution. The viscosity study shows a reduction in the viscosity of depolymerized poly(dimethylsiloxane) in comparison to neat PDMS-100, whereas the 1H NMR study confirmed the formation of methoxy and ethoxy end groups of the depolymerized PDMS-100 polymer. On the basis of endgroup analysis, the degree of polymerization of PDMS-100 reacting with DMC in alcohol (room temperature, 2 h) was found to be 52.09, down from an estimated average degree of polymerization of 67.6 for PDMS-100 (assuming an Mn of 5000 Da). The results obtained from SEC shows that, after the reaction of dimethyl carbonate with organosiloxane, a new molecular weight species is formed. Perhaps most importantly, the partially depolymerized product was endowed with reactive alkoxy end groups by the cleavage reaction with DMC, making the resulting oligomers a candidate for surface functionalization. To test the surface-functionalizing capability of the cleavage reaction products, we investigated the reaction of the PMPS/DMC mixtures with fumed silica. According to IR, elemental analysis and 29Si and 13C solid-state NMR data, silica nanoparticles modified with a mixture of PMPS/DMC are characterized by greater grafting density (average 2.2 groups/ nm2) and more stable grafts than those achievable by physisorption of PMPS alone onto fumed silica. TGA data show a higher thermal stability (up to 500 °C in air) for SiO2/ PMPS + DMC in comparison to SiO2/PMPS, a property 9728

DOI: 10.1021/acs.langmuir.8b01580 Langmuir 2018, 34, 9719−9730

Article

Langmuir

spinning nuclear magnetic resonance; δ, chemical shift; ρ, bonding density; θ, modification degree; DTA, differential thermal analysis; TG, curve of weight loss; DTG, derivative thermogravimetric curve; IR, Infrared spectroscopy; SEM, scanning electron microscopy; TEM, transmission electron microscopy

which is very promising for potential applications at elevated temperatures. Scanning electron microscopy data reveal a spherical morphology for all the resulting samples. However, the aggregate size of the silica nanoparticles which were obtained in the presence of DMC is smaller (37 nm) than that of initial silica (41 nm) and silica modified with neat PMPS (44 nm), which makes them promising candidates for thickeners in organic media where high dispersity of nanoparticles is required. Rheology studies show a thixotropic behavior in industrial oil (I-40A) for modified silicas and fully reversible nanostructure and shorter structure recovery time for nanosilica modified in the presence of DMC. We believe that the new grafting approach proposed in this paper can be extended to other types of silica substrates and various poly(organosiloxanes), giving it greater potential in the development of new coatings, sealants, and adhesives.

■

■

(1) Helmy, R.; Wenslow, R. W.; Fadeev, A. Y. Reaction of Organosilicon Hydrides with Solid Surfaces: An Example of SurfaceCatalyzed Self-Assembly. J. Am. Chem. Soc. 2004, 126, 7595−7600. (2) Krumpfer, J. W.; Fadeev, A. Y. Displacement Reactions of Covalently Attached Organosilicon Monolayers on Si. Langmuir 2006, 22, 8271−8272. (3) Kazakevich, Y. V.; Fadeev, A. Y. Adsorption Characterization of Oligo(dimethylsiloxane)-Modified Silicas: An Example of Highly Hydrophobic Surfaces with Non-Aliphatic Architecture. Langmuir 2002, 18, 3117−3122. (4) Graffius, G.; Bernardoni, F.; Fadeev, A. Y. Covalent Functionalization of Silica Surface Using “inert” poly(dimethylsiloxanes). Langmuir 2014, 30, 14797−14807. (5) Fadeev, A. Y.; McCarthy, T. J. Self-Assembly Is Not the Only Reaction Possible between Alkyltrichlorosilanes and Surfaces: Monomolecular and Oligomeric Covalently Attached Layers of Dichloro- and Trichloroalkylsilanes on Silicon. Langmuir 2000, 16, 7268−7274. (6) Shirono, H.; Amano, Y.; Kawaguchi, M.; Kato, T. Characteristics of Alkyltrimethoxysilane-Treated Fumed Silicas and Rheological Behavior of Fumed Silica Suspensions in an Epoxy Resin. J. Colloid Interface Sci. 2001, 239, 555−562. (7) Litvinov, V. M.; Barthel, H.; Weis, J. Structure of a PDMS Layer Grafted onto a Silica Surface Studied by Means of DSC and SolidState NMR. Macromolecules 2002, 35, 4356−4364. (8) Rao, A. V.; Kulkarni, M.; Amalnerkar, D. P.; Seth, T. Surface chemical modification of silica aerogels using various alkyl-alkoxy/ chloro silanes. Appl. Surf. Sci. 2003, 206, 262−270. (9) Park, S. E.; Prasetyanto, E. A. Morphosynthesis and Catalysis by Organofunctionalized Mesoporous Materials. In Organosilanes, Properties, Performance, and Applications; Wyman, E. B., Skief, M. C., Eds.; Nova Science Publishers: New York, NY, 2010; pp 101−131. (10) Daoud, W. A.; Xin, J. H.; Xiaoming, T. Synthesis and characterization of hydrophobic silica nanocomposites. Appl. Surf. Sci. 2006, 252, 5368−5371. (11) Bernardoni, F.; Kouba, M.; Fadeev, A. Y. Effect of curvature on the packing and ordering of organosilane monolayers supported on solids. Chem. Mater. 2008, 20, 382−387. (12) Moitra, N.; Ichii, S.; Kamei, T.; Kanamori, K.; Zhu, Y.; Takeda, K.; Nakanishi, K.; Shimada, T. Surface Functionalization of Silica by Si−H Activation of Hydrosilanes. J. Am. Chem. Soc. 2014, 136, 11570−11573. (13) Fadeev, A. Y.; McCarthy, T. J. Binary monolayer mixtures: Modification of nanopores in silicon-supported tris(trimethylsiloxy)silyl monolayers. Langmuir 1999, 15, 7238−7243. (14) Li, Y.-F.; Xia, Y.-X.; Xu, D.-P.; Li, G.-L. Surface Reaction of Particulate Silica with Polydimethylsiloxanes. J. Polym. Sci., Polym. Chem. Ed. 1981, 19, 3069−3079. (15) Gun’ko, V. M.; Pakhlov, E. M.; Goncharuk, O. V.; Andriyko, L. S.; Marynin, A. I.; Ukrainets, A. I.; Charmas, B.; Skubiszewska-Zięba, J.; Blitzd, J. P. Influence of hydrophobization of fumed oxides on interactions with polar and nonpolar adsorbates. Appl. Surf. Sci. 2017, 423, 855−868. (16) Protsak, I. S.; Kuzema, P. O.; Tertykh, V. A.; Bolbukh, Y. M.; Kozakevich, R. B. Thermogravimetric analysis of silicas chemically modified with products of deoligomerization of polydimethylsiloxane. J. Therm. Anal. Calorim. 2015, 121, 547−557. (17) Guba, G. Y.; Bogillo, V. I.; Chuiko, A. A. Kinetics and mechanism of the reaction of organosiloxanes with the surface of pyrogenic silica. Theor. Exp. Chem. 1993, 28, 146−150.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b01580. Viscosity measurements (Figure S1); apparatus for the synthesis of high-disperse materials with high content of the grafted organic groups (Figure S2); modification degree (θ) of the silica (eq S3); bonding density of the attached layers at the SiO2 surface (eq S4); 90 MHz 1H NMR spectrum of neat PDMS-100 (Figure S5); endgroup analysis of depolymerized PDMS-100 (eq S6); 90 MHz 1H NMR spectrum of depolymerized PMPS at 200 °C (Figure S7); 13C CP/MAS NMR spectra of silica modified with neat PMPS (Figure S8); contact angle of SiO2 nanoparticles, modified with a mixture of PMPS/ DMC at 200 °C for 2 h (Figure S9) (DOCX)

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wen Dong: 0000-0003-4386-5111 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by the Special Funding of the “Belt and Road” International Cooperation of Zhejiang Province (2015C04005). This work was carried out, in part, at the Center for Integrated Nanotechnologies, an Office of the Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Los Alamos National Laboratory (Contract DE-AC52-06NA25396) and Sandia National Laboratories (Contract DE-NA-0003525). The authors thank Marek Studziński for assistance with SEC.

■

ABBREVIATIONS SiO 2 , fumed silica; PDMS-100 and PDMS-50, poly(dimethylsiloxane); PMPS, poly(methylphenylsiloxane); DMC, dimethyl carbonate; CDCl3, deuterated chloroform; SEC, size-exclusion chromatography; I-40A, industrial oil; ζ, zeta potential; 1H NMR, proton nuclear magnetic resonance; CP/MAS NMR, solid-state cross-polarization−magic angle 9729

DOI: 10.1021/acs.langmuir.8b01580 Langmuir 2018, 34, 9719−9730

Article

Langmuir

(40) Tundo, P.; Perosa, A.; Zecchini, F. Methods and Reagents for Green Chemistry: An Introduction, 1st ed.; Willey: Venezia, Italy, 2007. (41) Selva, M.; Fabrisa, M.; Perosa, A. Decarboxylation of dialkyl carbonates to dialkyl ethers over alkali metal-exchanged faujasites. Green Chem. 2011, 13, 863−872. (42) Tundo, P.; Selva, M. The Chemistry of Dimethyl Carbonate. Acc. Chem. Res. 2002, 35, 706−716. (43) Ono, Y. Catalysis in the Production and Reactions of Dimethyl Carbonate, an Environmentally Benign Building Block. Appl. Catal., A 1997, 155, 133−166. (44) Lamoureux, G.; Agüero, C. A Comparison of Several Modern Alkylating Agents. ARKIVOC 2009, 2009, 251−264. (45) Tundo, P.; Rossi, L.; Loris, A. Dimethyl Carbonate as an Ambident Electrophile. J. Org. Chem. 2005, 70, 2219−2224. (46) Protsak, I.; Tertykh, V.; Pakhlov, E.; Derylo-Marczewska, A. Modification of Fumed Silica Surface with Mixtures of Polyorganosiloxanes and Dialkyl Carbonates. Prog. Org. Coat. 2017, 106, 163− 169. (47) Protsak, I.; Pakhlov, E.; Tertykh, V.; Le, Z.-C.; Dong, W. A New Route for Preparation of Hydrophobic Silica Nanoparticles Using a Mixture of Poly(dimethylsiloxane) and Diethyl Carbonate. Polymers 2018, 10, 116. (48) Demianenko, E. M.; Grebenyuk, A. G.; Lobanov, V. V.; Protsak, I. S.; Kozakevych, R. B.; Bolbukh, Y. M.; Tertykh, V. A. Quantum chemical study on interaction of dimethyl carbonate with polydimethylsiloxane. Chem., Phys. Technol. Surf. 2014, 5, 473−479. (49) Liu, C. C.; Maciel, G. E. The Fumed Silica Surface: A Study by NMR. J. Am. Chem. Soc. 1996, 118, 5103−5119. (50) Maciel, G. E.; Sindorf, W. D. Silicon-29 NMR study of the surface of silica gel by cross polarization and magic-angle spinning. J. Am. Chem. Soc. 1980, 102, 7606−7607. (51) Sindorf, D. W.; Maciel, G. E. 29Si NMR Study of Dehydrated/ Rehydrated Silica Gel Using Cross Polarization and Magic-Angle Spinning. J. Am. Chem. Soc. 1983, 105, 1487−1493. (52) Iuliucci, R.; Taylor, C.; Hollis, K. W. 1H/29Si crosspolarization NMR experiments of silica-reinforced polydimethylsiloxane elastomers: probing the polymer−filler interface. Magn. Reson. Chem. 2006, 44, 375−384. (53) Brei, V. V. 29Si Solid-state NMR Study of the Surface Structure of Aerosil Silica. J. Chem. Soc., Faraday Trans. 1994, 90, 2961−2964. (54) Fadeev, A. Y.; Kazakevich, Y. V. Covalently attached monolayers of oligo(dimethylsiloxane)s on silica: a siloxane chemistry approach for surface modification. Langmuir 2002, 18, 2665−2672. (55) Lipatov, Y. S.; Sergeeva, L. M. Adsorption of Polymers; Halsted Press: New York, NY, 1974. (56) Tertykh, V. A.; Belyakova, L. A. Chemical Reactions Involving Silica Surface; Naukova Dumka: Kiev, 1991. (57) Zhuravlev, L. T. The surface chemistry of amorphous silica. Zhuravlev model. Colloids Surf., A 2000, 173, 1−38. (58) Kučera, I.; Lanikova, J. Thermal Stability of Polydimethylsiloxane I. Deactivation of Basic Active Centers. J. Polym. Sci. 1961, 54, 375−384. (59) Devaux, M. F.; Legland, D. Grey level granulometry for histological image analysis of plant tissues. In Microscopy: Advances in Scientific Research and Education; Méndez-Vilas, A., Ed.; Formatex Research Center: Badajoz, Spain, 2014; Vol. 2, pp 681−688.

(18) Xiao, D.; Zhang, H.; Wirth, M. Chemical Modification of the Surface of Poly(dimethylsiloxane) by Atom-Transfer Radical Polymerization of Acrylamide. Langmuir 2002, 18, 9971−9976. (19) Barthel, H.; Nikitina, E. INS and IR study of Intermolecular Interactions at the Fumed Silica-Polydimethylsiloxane Interphase, Part 3. Silica-Siloxane Adsorption Complexes. Silicon Chem. 2002, 1, 261−279. (20) Smith, J. S.; Borodin, O.; Smith, G. D.; Kober, E. M. A Molecular Dynamics Simulation and Quantum Chemistry Study of Poly(dimethylsiloxane)-Silica Nanoparticle Interactions. J. Polym. Sci., Part B: Polym. Phys. 2007, 45, 1599−1615. (21) Gun’ko, V. M.; Borysenko, M. V.; Pissis, P.; Spanoudaki, A.; Shinyashiki, N.; Sulim, I. Y.; Kulik, T. V.; Palyanytsy, B. B. Polydimethylsiloxane at the interfaces of fumed silica and zirconia/ fumed silica. Appl. Surf. Sci. 2007, 253, 7143−7156. (22) Sulym, I. Y.; Borysenko, M. V.; Goncharuk, O. V.; Terpilowski, K.; Sternik, D.; Chibowski, E.; Gun’ko, V. M. Structural and hydrophobic−hydrophilic properties of nanosilica/zirconia alone and with adsorbed PDMS. Appl. Surf. Sci. 2011, 258, 270−277. (23) Krumpfer, J. W.; McCarthy, T. J. Rediscovering Silicones: “Unreactive” Silicones React with Inorganic Surfaces. Langmuir 2011, 27, 11514−11519. (24) Qiang, Z.; Wadley, M. L.; Vogt, B. D.; Cavicchi, K. A. Facile Non-Lithographic Route to Highly Aligned Silica Nanopatterns Using Unidirectionally Aligned Polystryrene-Block-Polydimethysiloxane Films. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1058−1064. (25) Palacios-Pineda, L. M.; Perales-Martinez, I. A.; LozanoSanchez, L. M.; Martínez-Romero, O.; Puente-Córdova, J.; SeguraCárdenas, E.; Elías-Zúñiga, A. Experimental Investigation of the Magnetorheological Behavior of PDMS Elastomer Reinforced with Iron Micro/Nanoparticles. Polymers 2017, 9, 696. (26) Voronkov, M. G.; Meleshkevich, V. P.; Yuzhelevsky, Y. A. Siloxane Bond; Nauka: Novosibirsk, 1976. (27) Chang, C. L.; Lee, H. S.; Chen, C. K. Aminolysis of cured siloxane polymers. Polym. Degrad. Stab. 1999, 65, 1−4. (28) Hsiao, Y. C.; Hill, L. W.; Pappas, S. P. Reversible amine solubilization of cured siloxane polymers. J. Appl. Polym. Sci. 1975, 19, 2817−2820. (29) Clarson, S. J.; Semlyen, J. A. Studies of Cyclic and Linear Poly(dimethylsiloxanes): 21. High Temperature Thermal Behavior. Polymer 1986, 27, 91−95. (30) Thomas, T. H.; Kendrick, T. C. Thermal Analysis of Polydimethylsiloxanes. I. Thermal Degradation in Controlled Atmospheres. J. Polym. Sci., Part A-2 1969, 7, 537−549. (31) Brook, M. A.; Zhao, S.; Liu, L.; Chen, Y. Surface etching of silicone elastomers by depolymerization. Can. J. Chem. 2012, 90, 153−160. (32) Tertykh, V. A.; Pavlov, V. V.; Chuiko, A. A. Nucleophilic substitution reactions at the silicon atom on silica surfaces. Theor. Exp. Chem. 1975, 11, 823−827. (33) Aristova, V. G.; Zimmer, I. M.; Gorbunov, A. I.; Zhilenkov, I. V.; Saushkin, V. V. Mechanism of Siloxane Bond Splitting on a Hydrated Aerosil Surface. Dokl. Akad. Nauk SSSR 1980, 255, 131− 134. (34) Tertykh, V. A.; Pavlov, V. V. Reactivity of molecules attacking a functional center. Adsorpt. Adsorbt. 1978, 6, 67−75. (35) Okamoto, M.; Suzuki, S.; Suzuki, E. Polysiloxane Depolymerization with Dimethyl Carbonate Using Alkali Metal Halide Catalysts. Appl. Catal., A 2004, 261, 239−245. (36) Okamoto, M.; Miyazaki, K.; Kado, A.; Suzuki, S.; Suzuli, E. Deoligomerization of cyclooligosiloxanes with dimethyl carbonate over solid-base catalysts. Catal. Lett. 2003, 88, 115−118. (37) Okamoto, M.; Miyazaki, K.; Kado, A.; Suzuki, S. Deoligomerization of siloxanes with dimethyl carbonate over solid-base catalysts. Chem. Commun. 2001, 0, 1838−1839. (38) Ono, Y. Dimethyl Carbonate for Environmentally Benign Reactions. Pure Appl. Chem. 1996, 68, 367−375. (39) Tundo, P. New Developments in Dimethyl Carbonate Chemistry. Pure Appl. Chem. 2001, 73, 1117−1124. 9730

DOI: 10.1021/acs.langmuir.8b01580 Langmuir 2018, 34, 9719−9730