A 29Si, 1H, and 13C Solid-State NMR Study on the Surface Species of Various Depolymerized Organosiloxanes at Silica Surface
Three poly(organosiloxanes) (hydromethyl-, dimethyl-, and epoxymethylsiloxane) of different chain lengths and pendant groups and their mixtures of dimethyl (DMC) or diethyl carbonates (DEC) were applied in the modification of fumed silica nanoparticles (FSNs). The resulting modified silicas were studied in depth using 29Si, 1H, and 13C solid-state NMR spectroscopy, elemental analysis, and nitrogen adsorption-desorption (BET) analysis. The obtained results reveal that the type of grafting, grafting density, and structure of the grafted species at the silica surface depend strongly on the length of organosiloxane polymer and on the nature of the “green” additive, DMC or DEC. The spectral changes observed by solid-state NMR spectroscopy suggest that the major products of the reaction of various organosiloxanes and their DMC or DEC mixtures with the surface are D (RR’Si(O0.5)2) and T (RSi(O0.5)3) organosiloxane units. It was found that shorter methylhydro (PMHS) and dimethylsiloxane (PDMS) and their mixtures with DMC or DEC form a denser coverage at the silica surface since SBET diminution is larger and grafting density is higher than the longest epoxymethylsiloxane (CPDMS) used for FSNs modification. Additionally, for FSNs modified with short organosiloxane PMHS/DEC and also medium organosiloxane PDMS/DMC, the dense coverage formation is accompanied by a greater reduction of isolated silanols, as shown by solid-state 29Si NMR spectroscopy, in contrast to reactions with neat organosiloxanes. The surface coverage at FSNs with the longest siloxane (CPDMS) greatly improves with the addition of DMC or DEC. The data on grafting density suggest that molecules in the attached layers of FSNs modified with short PMHS and its mixture of DMC or DEC and medium PDMS and its mixture of DMC form a “vertical” orientation of the grafted methylhydrosiloxane and dimethylsiloxane chains, in contrast to the reaction with PDMS/DEC and epoxide methylsiloxane in the presence of DMC or DEC, which indicates a “horizontal” chain orientation of the grafted methyl and epoxysiloxane molecules. This study highlights the major role of solid-state NMR spectroscopy for comprehensive characterization of solid surfaces.
KeywordsSilicones Dialkyl carbonates 1H solid-state NMR spectroscopy 29Si solid-state NMR spectroscopy 13C solid-state NMR spectroscopy Surface modification Fumed nanosilica Bonding density
Carbon weight percentage
- CP/MAS NMR
Cross-polarization magic-angle spinning nuclear magnetic resonance
Fumed silica nanoparticles
Hydrophobized fumed silica nanoparticles (FSNs) are of interest from a practical point of view because these materials can be better fillers of nonpolar or weakly polar polymers or more appropriate hydrophobic materials for other practical applications than unmodified hydrophilic silica [1, 2, 3, 4]. Functionalization of FSNs can be performed using various traditional types of modifying agents such as alkoxy-, halo-, and aminosilanes and organosilazanes [3, 4, 5, 6, 7, 8]. However, due to the high reactivity and moisture sensitivity of the modifying agents, purification is often critical for these hydrolyzable precursors. Organosiloxanes with methyl-terminated groups provide a viable and environmentally benign alternative to the chemical functionalization of oxides, taking into account three aspects of their structure that set it apart from carbon-based polymers: the bond lengths of Si–O and Si–C (1.63 and 1.90 Å) in organosiloxane are longer than the C–C (1.53 Å) bonds of most polymers; the S–O–Si bond angle (143°) is significantly greater than the C–C–C bond angles (109°) in the main chain of carbon-based polymers; and the differences in Pauling electronegativity values between silicon (1.8) and oxygen (3.5) and between silicon (1.8) and carbon (2.5) impart ionic character to both the Si–O backbone bonds (51% ionic) and the Si–C bonds (12% ionic). These three structural differences allow rotational and vibrational degrees of freedom in organosiloxane that are not available to carbon-based polymers and are the basis for unusual and unique properties: high thermal stability; excellent dielectric properties; and resistance to oxygen, water, and UV irradiation and so on [5, 8, 9, 10, 11]. Linear organosiloxanes are generally not considered to be reactive with inorganic oxide surfaces, and an enormous research effort has been made over the last 50 years to develop other silicon-containing reagents with reactive functional groups . One of the likely ways to increase the reactivity of a silicone polymer is partial depolymerization of high molecular poly(organosiloxanes), followed by grafting formed oligomers (with terminated alkoxy groups) on silica surfaces. Complete depolymerization of poly(dimethylsiloxanes) can be achieved by treatment of siloxanes with such toxic agents as various amines [13, 14]; thermal degradation (300–400 °C); and treatment with sulfuric acids, thionyl chloride, and mixtures of alkali (NaOH, KOH) or with alcohols (methanol, ethanol) [15, 16, 17, 18]. In our previous work, we found that dimethyl carbonate, which is an environmentally friendly reagent [19, 20] that meets all the requirements of green chemistry, promotes partial depolymerization of organosiloxanes, making the resultant oligomers a candidate for surface functionalization . However, no systematic characterization on the surface species of various depolymerized organosiloxanes on silica surface has been performed. Useful but limited information on the bonded species of silylated silica surfaces can be obtained through zeta potential, infrared spectroscopy, scanning, and transmission electron microscopy. One of the problems often met with these methods concern the difficulty in the identification of different OH and Si–O bonds. More specific information can be obtained by high-resolution 13C and 29Si cross-polarization magic-angle spinning NMR (CP-MAS NMR) and 1H MAS NMR. Indeed, only the use of abovementioned method allows a full characterization of the surface species on silylated silica. Some solid-state NMR studies have been already performed on gel and fumed silicas modified with different alkoxysilanes [22, 23, 24, 25, 26, 27, 28], mesoporous silica modified with cetyltrimethylammonium bromide , and 3-metacryloxypropyltrimethoxysilane (MPS) deposited in various solvents onto porous silica .
Therefore, the aim of this work is to study the surface species of various organosiloxanes and their mixtures with dimethyl or diethyl carbonate at a fumed silica surface depending on the polymer chain length of siloxane used as a modifying agent and on the nature of dimethyl or diethyl carbonate applied as an initiator for partial organosiloxane deoligomerization.
For preparation of the modified silica surfaces, poly(methylhydrosiloxane) (code name PMHS, linear, –CH3 terminated, viscosity of ca. 3 cSt at 25 °C), poly(dimethylsiloxane) (code name PDMS, linear, –CH3 terminated, viscosity of ca. 100 cSt at 25 °C), and poly[dimethylsiloxane-co-(2-(3,4-epoxycyclohexyl)ethyl)methylsiloxane] (code name CPDMS, linear, –CH3 terminated, viscosity of ca. 3,300 cSt at 25 °C) were purchased from Sigma Aldrich, USA. Commercial, dimethyl carbonate (DMC), diethyl carbonate (DEC), and fumed silica (SiO2, SBET = 278 m2/g) were purchased from Aladdin Reagents, China. The purity of the reagents, as reported by the manufacturers, was ≥ 99.0 %. The reagents were used as received.
Modification of Fumed Silica Surfaces
Organosiloxanes were chosen as non-toxic and environmentally benign modifying reagents with high carbon content. FSNs were applied as a matrix for modification because of the high regularity hydroxyl groups on the surface and good dispersibility. In addition, the main advantage of these FSNs over larger monodisperse particles is the fact that they provide a large surface area and thus high sensitivity for solid-state NMR. The modification of the fumed silica surface was performed with PMHS, PDMS, and CPDMS at 180–200 °C for 2 h with or without addition of DMC or DEC, which does not contribute to the weight of modified silica due to the reaction mechanism in gaseous (nitrogen) dispersion media (i.e., without a solvent). The amount of modifier agent was determined to be 17 wt% of silica weight. The modification process was performed in a glass reactor with a stirrer with a rotational speed of 350–500 rpm. The modifying agent was added by means of aerosol-nozzle spray. The samples were subsequently cooled to room temperature after the synthesis.
Carbon content, bonding density, and surface area of grafted neat organosiloxanes and their mixtures with DMC or DEC at the SiO2 surface
Carbon content, wt%
Bonding density ([Si(CH3R1O)), groups/nm2 where R1 = CH3, H, CH2CH2C6H9
2.42 ± 0.08
2.60 ± 0.04
2.17 ± 0.11
5.96 ± 1.17
6.04 ± 0.01
2.28 ± 0.20
0.57 ± 0.04
1.77 ± 0.01
2.98 ± 0.28
where Mw is the molecular weight of the grafted group, %C is the carbon weight percentage of the modified silica, S(BET) is the surface area of the original silica (m2/g), and nс is the number of carbon atoms in the grafted group in each silicone used for modification. Equation 1 gives the number of [–Si(RR1)O–] repeat units per 1 nm2 of the surface (ρ) (where R is methyl group (CH3); R1 is hydro (H) or methyl (CH3) or epoxy(cyclohexylethyl) group (CH2CH2C6H9)).
29Si, 1H, and 13C CP/MAS NMR Measurements
Solid-state 1H MAS NMR spectra were recorded on a Bruker Avance 400 III HD spectrometer (Bruker, USA, magnetic field strength of 9.3947 T) at resonance frequency of 79.49 MHz. The powder samples were placed in a pencil-type zirconia rotor of 4.0 mm o.d. The spectra were obtained at a spinning speed of 10 kHz, with a recycle delay of 1 s. The adamantane was used as the reference of 1H chemical shift.
Solid-state 29Si CP/MAS NMR spectra were recorded on a Bruker Avance 400 III HD spectrometer (Bruker, USA, magnetic field strength of 9.3947 T) at resonance frequency of 79.49 MHz for 29Si using the 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 4.0 mm o.d. The spectra were obtained at a spinning speed of 8 kHz (4 μs 90° pulses), a 8-ms CP pulse, and a recycle delay of 4 s. The Si signal of tetramethylsilane (TMS) at 0 ppm was used as the reference of 29Si chemical shift.
Solid-state 13C CP/MAS NMR spectra were recorded on a spectrometer (Bruker, USA, with a magic field strength of 9.3947 T) at a resonance frequency of 100.61 MHz for 13C using the 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 4.0 mm o.d. The spectra were obtained at a spinning speed of 5 kHz (4 μs 90° pulses), a 2-ms CP pulse, and a recycle delay of 4 s. The methylene signal of glycine at 176.03 ppm was used as the reference of 13C chemical shift.
1H Liquid NMR Spectroscopy
1H NMR spectra of each initial organosiloxane (PMHS (Additional file 1: Figure S1), PDMS (Additional file 1: Figure S2), CPDMS (Additional file 1: Figure S3); see Additional file 1) were recorded at 90 MHz with an Anasazi Eft–90 spectrometer (Anasazi Instruments, USA). Each polymer was dissolved in deuterated chloroform CDCl3, and the resulting solution was analyzed by 1H NMR spectroscopy.
To analyze the surface area (SBET, m2/g) of the silicas, the samples were degassed at 150 °C for 300 min. Low-temperature (77.4 K) nitrogen adsorption–desorption isotherms were recorded using a Micromeritics ASAP 2420 adsorption analyzer (Micromeritics Instrument Corp., USA). The specific surface area (Table 1, SBET) was calculated according to the standard BET method.
Results and Discussion
The intense resonance in the 3.5–5.0 ppm range has been studied widely by different researchers. Liu and Maciel , for example, by using CRAMPS observed a peak at 4.1 ppm in humidified fumed silica (Cab–O–Sil) which they reported as intermediate between that of liquid water protons (4.9 ppm) and that of the physisorbed water peak (3.5 ppm). According to their studies, a resonance at 3.5 ppm assigned to physisorbed water could easily be desorbed by evacuation at 25 °C. Moreover, evacuation at 100 or 225 °C led to further decrease in the intensity of this resonance, and it was attributed to “rapidly exchanging weakly hydrogen bonded hydroxyls, including those of both water and silanols” [25, 29]. On the other hand, the 1H MAS NMR investigation of silicas by Turov et al. [31, 32] reported the chemical shift of water at around 5 ppm at 25 °C. Several other studies of silicas have also attributed the resonances at 4.5–5.0 ppm to water on strongly hydrated surfaces and chemical shift near 3 ppm to water on significantly dehydrated surfaces, as reported by Turov et al. .
29Si Chemical shifts (δ) of grafted neat and depolymerizied organosiloxane species at SiO2 surface
(≡SiO)2SiR, R-attached polymer chain
(≡SiO)3SiR, R-attached polymer chain
Si(CH3)(R)(O–)2 in D4, R = CH3, C2H4C6H8
Si(CH3)(O–)2 in linear MD4M
As can be seen in Fig. 3, after silica surface modification with low viscous poly(methylhydrosiloxane) and DEC (curve d), a significant decrease in the signals Q3 and Q2 is accompanied by an increase in the intensity growth of the signal Q4. Additionally, the signal at − 35 ppm appears, and this can be identified with the methylhydrosiloxane species (D1), (Fig. 4d and Table 2). This implies that a reaction has occurred between the silica surface and the PHMS/DEC mixture.
Overall, from the solid-state NMR data obtained, it is evident that the addition of DMC to the modifying mixture has a significant effect on the chemical interaction of organosiloxane of a medium length of polymer chain (PDMS) used for modification at the silica surface, while DEC addition has practically no influence on the chemical interaction of SiO2 with PDMS. In contrast, the DEC has a great effect on the chemical interaction of short organosiloxane (PMHS) used for modification at the SiO2 surface, while DMC has minimal impact on the chemical interaction of SiO2 with PMHS.
Note that all the presented surfaces generally exhibit the grafting density decrease as the size of the polymer increases, used for surfaces functionalization. Similar results were also presented for silicas functionalized by different bis-fluoroalkyl disiloxanes . This can be due to steric hindrance from the long polymer chains on the macromolecule, as discussed above.
An in-depth solid-state NMR study of FSNs functionalized with organosiloxanes of various lengths of polymer chains and their mixtures of DMC or DEC is presented. For better analysis of the length of polymer chain effects, the organosiloxanes studied here are much longer and with a larger difference in the viscosity as well as pendant groups than the organosiloxanes studied before [12, 35, 48, 49, 50, 51, 52, 53, 54]. The obtained results reveal that the structure of the grafted species, type of grafting, and grafting density at the SiO2 surface depend strongly on the length of organosiloxane polymer and on the nature of the “green” additive, DMC or DEC. Spectral changes observed by solid-state NMR spectroscopy suggest that the major products of the reaction of various organosiloxanes and their DMC or DEC mixtures with the FSNs were D (RR’Si(O0.5)2) and T (RSi(O0.5)3) organosiloxane units. The appearance of grafted siloxane units at SiO2/PHMS+DEC and SiO2/PDMS+DMC surfaces is accompanied by a significant reduction of Q3 signals, while for neat organosiloxanes and some of their mixture with alkyl carbonate used for SiO2 modification, a reduction of Q3 is hardly observable. The small amounts of residual silanols (hardly accessible for modifier reagents used) and physisorbed water remain in all the samples of modified silicas (note that the crude silica was not preheated at high temperatures).
Addition of DMC to the modifying mixture facilitates the passage of chemical reaction between medium (PDMS) or long (CPDMS) polymer and the SiO2 surface. Diethyl carbonate addition somewhat worsens the chemical reaction between medium organosiloxane (PDMS) and SiO2 surface but greatly facilitates the reaction when organosiloxanes at short (PMHS) and long polymer chain (CPDMS) are applied for FSNs modification. Thus, from the technological point of view, for FSNs modification with short organosiloxanes, it is reasonable to use DEC; at medium organosiloxane, the application of DMC is necessary; and at long organosiloxane, it is beneficial to use both DMC and DEC.
The data for CP/MAS NMR, BET, and chemical analysis suggest the “vertical” orientation of grafted organosiloxane chains when short and medium polymer or its mixture with DMC (ρ = 7.2–7.4 groups/nm2) are applied for FSNs modification. The reaction of FSNs with medium and long polymer and its mixture with DEC (PDMS/DEC or CPDMS/DEC) leads to the formation of the “horizontal” chains at the surface (ρ = 0.1–2.5 groups/nm2). The findings open new ways for the preparation of similar materials of the same quality using different substrates such as various silicas—silica gels, porous silicas, and precipitated silica. The comparison of the influence of substrate nature on poly(organosiloxane)/alkyl carbonate modification is of undoubted interest for future study.
This research was supported by the Special Funding of the ‘Belt and Road’ International Cooperation of Zhejiang Province under grant 2015C04005 and China Postdoctoral Science Foundation grant Z741020001. Partly, this research was supported by the Center for Integrated Nanotechnologies, an Office of the Science User Facility operated for the US Department of Energy (DOE), Office of Science by Los Alamos National Laboratory (Contract DE-AC52-06NA25396), and Sandia National Laboratories (Contract DE-NA-0003525).
Availability of Data and Materials
The datasets supporting the conclusions of this work are included within the article. Any raw data generated and/or analyzed in the present study are available from the corresponding author on request.
ISP, YMM, IMH, and DZ conceived and designed the experiments; ISP and YMM performed all the experiments; ISP, IMH, and YMM analyzed and interpreted the data; ISP wrote the manuscript; and ZL and WD contributed reagents/materials/analysis tools. All the authors revised and approved the final version of the manuscript.
The authors declare that they have no competing interests.
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