Effect of the concentration ratios of organosiloxane/initiator and treatment temperature on the characteristics of hydrophobic products obtained by modification of surface of fumed silica with poly(methylphenylsiloxane) (PMPS) in the presence of dimethyl carbonate has been studied. Morphology, particle size, surface area and coating microstructure of modified silicas were analyzed by methods of transmission electron and atomic force microscopies, nitrogen adsorption-desorption data. Carbon contents in the grafted modifying layer of organosilicas were determined using IR spectroscopy and elemental analysis. Hydrophilic-hydrophobic properties of surface of the obtained modified silicas were estimated by measurements of contact angles of wetting. It was shown that modification of pyrogenic silicas with mixtures of poly(methylphenylsiloxane) and dimethyl carbonate allows to obtain the homogeneous hydrophobic products and serve their nanodispersity.
Modified disperse silicas with grafted phenyl groups are applied as the stationary phases and carriers in chro- matography, and as the fillers for manufacturing adhesives, coatings and materials with high heat resistance [
It is known from the experiment [
Dimethyl and diethyl carbonates as it was earlier established are effective reagents in the reaction of siloxane bond cleavage in poly(dimethylsiloxanes) [
In this work, such approach was tested in the simultaneous application of dimethyl carbonate and poly(methylphenylsiloxane) (PMPS) for modification of the silica surface. Special attention was paid for investigation of structural and surface characteristics of the products obtained during modification of fumed silica with the mixtures of different composition.
Fumed silica characterized by a specific surface area of 260 m2/g (A-300, Kalush, Ukraine), synthesized via high-temperature hydrolysis of silicon tetrachloride was taken for surface modification. As a modifying reagent poly(methylphenylsiloxane) fluid PMPS-4 (Zaporizhzhya, Ukraine, GOST 15866-70) with degree of polymerization n = 8 - 10 was used. The applied organosilicon polymer is characterized by a wide temperature (from −20˚С to +350˚С) range of stability, chemical inertness and hydrophobic properties. Dimethyl carbonate (DMC) was supplied by Sigma Aldrich.
Synthesis of Composite MaterialsModification of fumed silica surface with PMPS was performed at the different temperatures for 2 hrs with or without the addition of dimethyl carbonate (DMC). The modification process was performed in a glass reactor with a stirrer at rotational speed from 20 to 300 ppm. After loading with fumed silica, all air volume in the reactor was filled with nitrogen and the reactor was heated up to a defined temperature. Then, nitrogen supply was interrupted and the modifying reagent was added into reactor by means of its aerosol spraying through a nozzle.
Four series of the samples were synthesized.10 g of fumed silica were treated with 2.2 g of PMPS in the first series. In these second series the surface treatment was carried out with the mixture of PMPS and DMC at the weight ratios SiO2:PMPS:DMC = 10:2.2:1.5. The weight ratios SiO2:PMPS:DMC = 10:2.2:1.0 were used in the third series. Modification of the surface was carried out with the mixture of the PMPS and DMC in the weight ratios SiO2:PMPS:DMC = 10:2.2:0.5 in the fourth series. Modification of the fumed silica surface was performed at three different temperatures: 200˚C, 250˚C and 300˚C. 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. Then, the washed sample was dried at 80˚C for 2 hrs.
In order to control the flow of surface reactions, IR spectra were recorded using a Specord M-80 spectrophotometer in a range of wave numbers 4000 - 200 cm−1. The silica samples were pressed into rectangular 28 × 8 mm plates of 25 mg weight.
Surface microstructure and morphology as well as coating homogeneity for the modified samples were analyzed by transmission electron microscopy, TEM (Tecnai G2T20 X-TWIN, USA) and atomic force microscopy, AFM (Nanoscope V Digital Instruments, USA, with a Tapping Mode technique). AFM data processing was performed using the SPIP program (version 5.0.6).
Porous structure (specific surface area, pore volume and pore size distribution function) was characterized using nitrogen adsorption-desorption data measured using Accelerated Surface Area and Porosimetry analyzers ASAP 2020 and 2420 (Micromeritics, USA). Before the measurements, the samples were outgassed at 110˚C. The specific surface area, SBET, was calculated using the BET method [
Hydrophilic-hydrophobic properties of the surface of obtained modified silicas were estimated by measurements of contact angles of water drops. The contact angle data were measured using a commercial Contact Angle Meter (GBX Scientific Instruments, France) equipped with a temperature and humidity controlled measuring chamber and a digital camera (T = 20˚C; RH = 50%).
To measure the content of grafted organic groups in the synthesized samples, the Perkin-Elmer 2400 CHN- analyzer (USA) was used. The modifying layer was oxidized to produce Н2О and СО2 during the samples heating in the oxygen flow at 750˚С.
Control of the surface reactions under the surface modification of the fumed silica with poly(methylphenylsi- loxane) and dimethyl carbonate was performed by IR spectroscopy. IR spectra of the silicas modified with PMPS are shown in
IR spectra are also characterized by the presence of a broad absorption band at 3600 - 3000 cm−1 corresponding to O−H vibrations in adsorbed water molecules and in silanol groups which formed hydrogen bonds with molecules of adsorbate. The band at 3750 cm−1 corresponding to O−H stretching vibrations of the free silanol groups [
Carrying out the process of modification in the simultaneous presence of PMPS and dimethyl carbonate in the reaction mixture results in both the increase of intensity of the bands of stretching vibrations of C−H bonds in the methyl groups in the wave number range 3000 - 2900 cm−1 [
Т, ˚С | Component ratios | SBET, m2/g | Vp, cm3/g | Rave nm | Carbon content, wt.% | Contact angle of wetting Q, degrees |
---|---|---|---|---|---|---|
Pristine SiO2 | 260 | 0.539 | 11 | 0 | 0 | |
200 | SiO2 + PMPS (10 g + 2 ml) | 102 | 0.024 | 40 | 7.3 | 133 |
SiO2 + PMPS + 0.5 ml DMC | 151 | 0.021 | 15 | 6.0 | 130 | |
SiO2 + PMPS + 1.0 ml DMC | 158 | 0.021 | 15 | 6.0 | 130 | |
SiO2 + PMPS + 1.5 ml DMC | 171 | 0.027 | 31 | 7.4 | 130 | |
250 | SiO2 + PMPS | 168 | 0.027 | 31 | 7.9 | 133 |
SiO2 + PMPS + 0.5 ml DMC | 168 | 0.022 | 16 | 7.3 | 130 | |
SiO2 + PMPS + 1.0 ml DMC | 173 | 0.021 | 14 | 6.6 | 129 | |
SiO2 + PMPS + 1.5 ml DMC | 191 | 0.024 | 17 | 6.5 | 130 | |
300 | SiO2 + PMPS | 190 | 0.028 | 28 | 6.0 | 134 |
SiO2 + PMPS + 0.5 ml DMC | 160 | 0.023 | 17 | 7.0 | 130 | |
SiO2 + PMPS + 1.0 ml DMC | 170 | 0.022 | 15 | 6.7 | 130 | |
SiO2 + PMPS + 1.5 ml DMC | 178 | 0.022 | 16 | 7.0 | 130 |
(
It should be noted that there is no absorption band О−Н in the free silanol groups at 3750 cm−1 in the spectra of the modified products, indicating their full participation in the reactions with the mixture of PMPS and DMC.
in comparison with the pristine silica.
Structural and surface characteristics of the pristine silica and the samples modified with poly(methylpheny- lsiloxane) are given in
The samples synthesized by modification of the silica surface with PMPS have large number of voids with the average radius Rave > 28 nm. For the samples modified with the mixture PMPS and DMC size-range of voids is Rave < 17 nm.
Materials obtained by treatment with poly(methylphenylsiloxane) and mixtures of PMPS and DMC with different ratios of the components have similar contact angles of wettings (from 129˚ to 134˚). It indicates the high hydrophobic properties of surface of the synthesized organosilicas (
According to the elemental analysis data (
Extreme character of dependence of the carbon content on the modification process temperature is a result both the chemisorption kinetics and the depth of thermal depolymerisation. During modification with the mixture of PMPS and DMC two processes are competing: a thermal and chemical (with participation of DMC) depolymerisation. The degree of modification depends on kinetic features of chemisorption and the process of splitting of polysiloxane chains. At 200˚C the rate-limiting step of modification is splitting the siloxane bonds in the reaction with dimethyl carbonate with following attachment of oligomers to the silica surface. However, at 250˚C and 300˚С the thermal depolymerisation prevails, the quantity of initiator slightly affects the surface carbon content that controls the degree of the siloxane depolymerisation. Generally, with increasing amounts of initiator the degree of the polysiloxane grafting increases, however the temperature 250˚C is optimal for maximum anchoring of the functional groups to the silica surface.
In
It is known that the atomic force microscopy is crucial in measuring the characteristics of surface topography. In this paper, the microstructure of the surface layer of modified silica was studied by applying the AFM technique. The AFM images obtained for the pristine silica and after its modification with PMPS and PMPS/DMC mixture are presented in
the structure of the surface layer is relatively homogeneous. The silica modified with the mixture of DMC/ PMPS is characterized by a narrow size distribution and smallest grain size (
The properties of modified fumed silicas were studied using the methods of IR spectroscopy, elemental analysis, transmission electron microscopy, nitrogen adsorption-desorption data, atomic force microscopy and measurements of contact angles of wetting. According to IR spectroscopy data, mixtures of PMPS with DMC were shown to provide an increase of the concentration of grafted organic groups in the surface layer and full participation of the free silanol groups in the chemisorptions process at the moderate temperatures (200˚С). At 250˚C, the highest concentrations of grafted organic groups in the synthesized samples (the average of carbon content ~7 wt.%) were reached. The average of carbon content is smaller (≥6.6 wt.% carbon) for the samples synthesized at 200 and 300˚C. These data indicate that the temperature 250˚C is optimal for maximum attachment of the functional groups on the silica surface. The quantity of initiator slightly influenced the surface carbon content in the surface modifying layer.
The type of nitrogen adsorption isotherm is common for all modified silica, namely they are characterized by the fourth type according to the Brunauer’s classification and the third type of the hysteresis loop in the range of high values of relative pressures. Coating microstructure and morpholology of the modified samples is relatively homogeneous under modification of the silica surface with the mixture of PMPS and DMC. Their structure consists of particles of size 10 - 12 nm. After the modification of silica with pure PMPS a wider size distribution is observed. Particles of silica modified with PMPS form the aggregates of the size 20 - 40 nm.
The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme FP7/2007-2013/under REA grant agreement No PIRSES-GA-2013-612484.
Iryna S.Protsak,Valentyn A.Tertykh,Yulia M.Bolbukh,DariuszSternik,AnnaDerylo-Marczewska, (2015) Synthesis and Properties of Fumed Silicas Modified with Mixtures of Poly(methylphenylsiloxane) and Dimethyl Carbonate. World Journal of Nano Science and Engineering,05,152-160. doi: 10.4236/wjnse.2015.54017