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2 fragments in structure
Modern Electronic Materials (2016) 2, 18–22
Finely divided methylsilsesquioxane particles with SiO4/2 fragments in structure Pavel A. Averichkina, Yuri B. Andrusova, Igor A. Denisova,n, Tursunbek B. Klychbaevb, Yuri N. Parkhomenkoa, Natalia A. Smirnovaa a
Joint Stock Company «Giredmet», 5-1 B. Tolmachevsky Lane, Moscow 119017, Russia Chemical and Metallurgical Holding «Metal», 40 Sovetskaya Str., Bishkek 720055, Kyrgyzstan
b
n
KEYWORDS
Abstract
Methylsilsesquioxanes; Silicon-organic compounds; Siloxane particles; Hydrolysis; Suspension; Laser correlation spectroscopy; Electron scanning microscopy; Crystalline nanoparticles
Results of structural and morphological investigations of methylsilsesquioxane finely divided particles synthesized by hydrolytic co-condensation of in situ alkoxylated methyltrichlorosilane and tetrachlorous silicon are presented. The silica-type hydrophobe particles have been obtained using the abovementioned method with an output of 98.7–99.4 wt% of the load. These particles have been identified as crystalline formations with a lattice period of 5.5– 8.0 nm and an interplane distance of 0.92–1.04 nm in the near order. We show that unlike for well-known crystalline methylsilsesquioxanes, introduction of chemically bonded SiO4/2 fragments to the material structure leads to the formation of 2–3 μm-sized spherical particles from the initial 10–15 nm particles; these particles are responsible for the spherical morphology of the surface. A method of extracting siloxane nanoparticles (2.5–280 nm) from the reaction products has been described, the behavior of particles in suspension has been studied and particle dimensional parameters have been determined using helium neon 0.6328 nm laser correlation spectroscopy. & 2016 The National University of Science and Technology MISiS. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Corresponding author. E-mail addresses: [email protected] (P.A. Averichkin), [email protected] (Y.B. Andrusov), [email protected] (I.A. Denisov), [email protected] (T.B. Klychbaev), [email protected] (N.A. Smirnova). Peer review under responsibility of the National University of Science and Technology MISiS.
Introduction Organic silsesquioxane (OSSO) class siliconorganic compounds are the unit structural elements which consist of tetrameric (for silicon) siloxane cycles with hydrocarbon substituents at the silicon corners of the cage known as oligomeric and polymeric products of cyclic-linear, cyclonetwork and closed polyhedral structure [1]. It is commonly believed that cyclic-linear (ladder)
http://dx.doi.org/10.1016/j.moem.2016.08.002 2452-1779/& 2016 The National University of Science and Technology MISiS. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Finely divided methylsilsesquioxane particles oligomers have a regular structure with residual silanol groups at the ends (1.5–3.5 wt%) and a narrow molecular weight distribution [2]. In the form of dry powders these products are thermosetting resins having the phase transition from solid to viscous flow state at about Tg =50–80 1C. Their solubility in organic solvents [3] allows their use in electronics technologies as functional or service thin protective films. Unlike cycliclinear oligomer structures, cyclonetwork ones are synthesized by additionally modifying silsesquioxane structures with chemically bonded spirane SiO4/2 fragments using co-hydrolysis of organotrichlorsilans with tetrachlor- or tetralkoxysilane [4–6]. Cyclonetwork oligomers also exhibit reactivity and solubility in organic solvents. Due to the increase in the rigidity of the main siloxane chain by SiO4/2 fragments, oligomer resins, mainly with aliphatic hydrocarbon substituents (—H; —CH3; —C2H3; —C2H5) at the silicon edges, do not undergo the phase transition from resin-like solid to viscous flow state under normal conditions. The resin melting point increases above the organic radical destruction point and tends to reach the silicon oxide glass transition point. The absence of softening and melting in oligomer resins allows their use for forming moldable thin nanosized films and protective coatings which retain almost constant structural parameters (setting, surface texture and morphology, internal stresses) and shape (thickness, topological size etc.) during inter-operation processing, e.g. thermal anneals, ion plasma etching, lithography etc. Polyhedral OSSO were obtained and identified as octahedral (closed) methyl- and vinylsilsesquioxanes (Me T-8 and Vin T-8 spatial “cubes”) synthesized from methyl- and vinyltrichlorsilane, respectively, with a low output, only 6– 19 wt% [7]. These products are in the form of finely divided crystalline powders insoluble in organic solvents and, according to IR spectroscopy, not containing hydroxyl groups. When separated from reaction products in the form of discrete particles, they were first identified using X-ray analysis (CuKα radiation) as crystalline formations with certain crystallographic parameters [7,8]. Spatial silsesquioxane structures with phenyl radicals at the corners (phenylsilsesquioxanes) can form, in the course of polymeric transformations, polyhedral shapes, e.g. Ph T-8, Ph T-10, Ph T-12 etc. Their large size and aromatic nature of the substituents at the silicon corners favor the formation of polyhedral polytypes [9]. The reason for the interest to polyhedral OSSO is the possibility of obtaining discrete hydrophobic particles that replace hydrophilic aerosils and do not contain pores. Of the abovementioned silsesquioxane compounds, of greatest scientific and practical interest are polyhedral methylsilsesquioxanes (MSSO) modified with SiO4/2 structural fragments. The small size of the organic radical at the corner, porosity and relatively well-developed surface, as well as good moldability allow these compounds to be considered as promising for the synthesis of finely divided silica-type hydrophobic siloxane particles. The synthesized particles can be used as reinforcing fillers and binders for the development and fabrication of siliconorganic compounds and sealants for the protection and sealing of p–n junctions in high-voltage (to 6 kV) semiconductor devices and ICs. Experimental specimens of polyhedral MSSO were successfully tested at Giredmet as soft abrasives for chemical mechanical polishing of surfaces of cadmium telluride semiconductor wafers [10].
19 The aim of this work is to provide polyhedral MSSO with spirane SiO4/2 structural fragments (to 60 mol%) by hydrolysis and co-condensation of in situ alkoxylated methyltrichlorosilane (MTCS) and tetrachlorous silicon (TCS), and develop methods for separation of finely divided siloxane particles from reaction products, their structural identification and size assessment.
Experimental We synthesized MSSO with chemically bonded SiO4/2 fragments by hydrolysis and co-condensation of an in situ alkoxylated [11] water/alcohol mixture of MTCS and TCS in a 2:3 M ratio, respectively [12]. The reaction products were synthesized as solid discrete particles and separated on a Schott filter glass, washed from HCl in water, dried and stored in an air proof cabinet. The output of the powdered product was 99.0–99.4 wt% of the load. The structure of the chemical bonds in the MSSO was studied using IR spectroscopy in the 400–4000 cm 1 region on a Bruker Equinox 55/S spectrophotometer (Bruker, Germany). The specimens for IR spectroscopy were prepared in the form of pressed tablets mixed with KBr using a conventional method. The IR spectral lines were characterized in accordance with well-known reference and literary data [13]. X-ray analysis of the powders was carried out on a URD-6 diffractometer (Germany) in CuKα radiation and a Ni filter by reflection technique in the 6 arc dego2θo50 arc deg angular range. The curves were processed (smoothing, normalization) and corrected for parasitic scattering and collimation using special software [14]. The sizes of the crystalline regions were determined using the Selyakov– Scherrer method. The shapes of the discrete siloxane particles, their surface texture and morphology were studied under a Vega II XMU-Tescan (Tescan, Czech Republic) scanning electron microscope (SEM) with a 10,000 magnification. The sizes of the siloxane nanoparticles in the prepared samples of suspension solutions were assessed using laser correlation spectroscopy (LCS) based on the determination of the spectral characteristics of quasielastically scattered light (in Rayleigh mode) after transmission through a solution of the finely divided 1–2000 nm sized particles [15,16]. The oscillations of the scattered light relative to the reference determinate radiation of far greater intensity were recorded from one helium-neon laser source (LGN207A) at 0.6328 μm with an output power of 2.5 mW. The measurements were made on an LCS-03 spectrometer developed at the Moscow Institute for Physical Engineering (State University). The frequency shifts of radiation scattered by the microparticles was 100 106 Hz. The correlation spectra of light fluctuation on moving particles were processed after approx. 1000 measurements. During sample preparation we separated narrow particle fractions of the synthesized products as follows. We prepared acetone and water/acetone (1:1) suspensions of 1 mg particles in 1 ml of liquid, separated large micrometer-sized particles by sedimentation and filtered the finely divided particles through a 0.45 μm pore nylon membrane. We chose acetone as the suspension media for its good
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compatibility with MSSO and strong solvating properties making it capable of forming solvate shells on the surface of siloxane nanoparticles. Strongly polarized water was preferred for its ability to rapidly change the solvating properties of acetone. In an equimolar acetone/water mixture, polar water molecules wedge the solvate shells of the nanoparticles until destruction. Suspension was prepared from highly pure acetone and deionized water with a resistivity of approx. 17 MOhm.cm. When determining the parameters of particle size distribution functions for acetone and for water/acetone solution we accepted maximum relative systematic error to be 720% and the maximum relative random error to be 710%.
Results and discussion Water hydrolysis of MTCS is known to cause the substitution of Si—Cl for Si—OH groups followed by their homocondensation and the formation, depending on synthesis conditions, of cyclic-linear (ladder), octahedral and gelled MSSO. The resultant cyclic-linear oligomers have a broad molecular weight distribution and are unstable (labile) during processing and storage. Octahedral MSSO form as by-products of cyclic-linear oligomer synthesis with an output of within 10 wt% of the load. Gelled MSSO form during uncontrolled MTCS hydrolytic polycondensation and contain up to 20 wt% of accompanying mole fractions. Synthesis of regular MSSO structures and high-quality products on their basis can be achieved using our method of hydrolysis of in situ alkoxylated MTCS with C1–C2 primary alcohols by boiling the reaction mixture. Gradual substitution of alkyl groups for silanol ones and their subsequent condensation provide for a controlled stepwise process resulting in the formation of defect-free structures. The use of TCS with MTCS causes hydrolysis of monomers and their condensation or embedding into the sesquioxane (—SiO3/2) structure of spirane (SiO4/2) fragments which drastically change the siloxane structure of the macromolecules. The joint action of the —SiO3/2 and SiO4/2 chains under these conditions forms either cyclonetwork or polyhedral (closed) siloxane skeletons. The SiO4/2 spirane fragments increase the rigidity of the siloxane structure of the macromolecules. The primary polyhedral siloxane structures separated from the reaction products agglomerate to discrete particles typical of silica. The methyl substituents at the silicon corners provide for the organogenic nature of the siloxane structure thus noticeably modifying its surface and ensuring good hydrophobicity, organophilicity and compatibility of the forming discrete particles in organic or inorganic composites. As can be seen from the IR spectra (Figure 1), octahedral and structurally modified MSSO do not contain Si—OH groups whose chemical bonds are typically strongly expressed in the valence (3200–3800 cm 1) and deformation (approx. 860 cm 1) regions. The oscillation pattern of the Si—O—Si bonds in the characteristic region suggests that the siloxane skeletons have a symmetrical structure corresponding to the oscillations of silicon oxide compounds with the absorption peak at 1060 cm 1. The Si—O—Si bond oscillation peak halfwidth (dII) (Figure 1, curve 2)
Figure 1 MSO IR spectra: (1) octahedral structure, (2) polyhedral structure with chemically bonded SiO4/2 fragments, (3) structures polymer gel and (4) cyclic-liner (ladder) oligomer structure.
increases almost twofold compared to octahedral MSSO (dI) if inorganic SiO4/2 fragments are introduced. A decrease in the percentage of the organic fraction causes suppression and reduction of the intensity of the absorption lines near 605, 1165 and 2900 cm 1 corresponding to organic methylsilil ( Si—CH3) radicals. Noteworthy, an increase in the percentage of SiO4/2 fragments in the structure to above 80 mol% triggers uncontrolled condensation of three- and four-functional monomers and the formation of a gelled structure with a high content of silanol groups. In structured polymer gel the siloxane bonds have an unordered (random) structure, and the oscillations of longitudinal and lateral Si—O—Si bonds (Figure 1, curve 3) make statistically similar contributions to the IR spectrum. The Si—O—Si bond oscillation peak splits and the intensities of the shortand long-wave absorption bands become almost equal. The IR spectra of MSSO with a cyclic-linear (ladder) oligomer structure (Figure 1, curve 4) and structured polymer gel (Figure 1, curve 3) suggest that the shapes of the spectral lines of Si—O—Si bond oscillation in their respective characteristic region are not similar. The asymmetry of the ladder structure shows itself in the splitting of the characteristic peak into the short-wave region (1100 cm 1) and the long-wave region (1020 cm 1). The absorption intensity in the short-wave spectral region corresponding to longitudinal Si—O—Si bonds in cyclic-linear structures is higher [17]. MTCS based compounds and MTCS/TCS mixtures synthesized using an earlier method [12] are in the form of finely divided agglomerated particles which after separation from the reaction products and drying aggregate to loose powders. A general schematic of the structure of unit particles
Finely divided methylsilsesquioxane particles
Figure 2
21
Primary MSSO nanoparticles of (a) I and (b) II structural types.
Figure 3 Images of discrete MSSO particles of different compositions and structure: (a) a colony of [CH3SiO1.5]8 crystalline particles (structural type I), and (b) and (c) [(CH3SiO1.5)8.(SiO4/2)]12 spherical particles (structural type II).
corresponding to spectra 1 and 2 in Figure 1 (hereinafter, particle types I and II, respectively) is illustrated in Figure 2 as polyhedral (closed skeleton) structures not containing Si—OH groups. X-ray diffraction showed that the synthesized powdered products are single-phase systems consisting of quasicrystalline formations with a period of 5.5–8.0 nm. The diffraction patterns also suggest that the crystalline regions are restricted by amorphous ones that presumably are structures with deformed valence angles of Si—O—Si bonds that may vary over a wide range [18]. The interplane spacing of I and II type structures was 0.92–1.04 nm which suggests the similarity of their structures in the shortrange order. Figure 3 shows SEM images of primary agglomerated I and II structural type particles, magnification 10,000. Primary structural type I discrete particle agglomerates differ noticeably from primary structural type II discrete particle agglomerates not only in composition, but also in structure. During synthesis and separation from the reaction products, type I octahedral structures agglomerate to cube-shaped crystals (Figure 3a) with a wide size range (5–20 μm). The primary 10–15 nm sized type II discrete nanoparticles agglomerate during separation to 2–3 μm sized spherical particles (Figure 3b). Furthermore, these primary nanoparticles form the texture and morphology of the microparticle spherical surfaces and predetermine the system of surface pores (Figure 3c). The size and behavior of the separated type II nanosized particle fractions were characterized using dynamic light scattering in acetone and water/acetone (1:1) suspensions. After processing of the results the distributions of
Figure 4 Hydrodynamic size distribution histograms of structural type II light-scattering MSSO particles after normalization of LC spectra for (a) acetone and (b) water/acetone solution.
22 light-scattering particles were presented as histograms (Figure 4) where the Y axis shows the percentage of particles in the suspension and the X axis shows the hydrodynamic size of the particles. The acetone suspension histograms (Figure 4a) suggest that siloxane particles form 55–280 nm agglomerates with solvate shells on the surfaces. After the addition of strongly polarized water (Figure 4b) the solvate acetone shells degrade and decompose and the particles divide in two groups sized 2.5–2.8 and 25–57 nm, respectively. Addition of more acetone (until a certain threshold) causes regrouping and reagglomeration of the nanoparticles into a single group. These transformations of siloxane particles in suspension media over a certain concentration range may repeat multiply. Thus, the variable behavior of siloxane nanoparticles in suspension media is of scientific and practical interest. Controlled nanoparticle manipulation in suspensions can be used for preparation purposes and in technologies, e.g. in the development of hi-tech composite recipes or synthesis of materials with preset properties for electronics.
Summary A polyhedral MSSO particle synthesis technique has been developed and finely divided hydrophobic MSSO particles have been synthesized in the form of aggregated powders with an output of 99.0–99.4 wt% of the load. The structure of the siloxane particles has been examined, and they proved to be crystals with a period of 5.5–8.0 nm and an interplane spacing of 0.92–1.04 nm in the short-range order. We have found that the introduction of up to 60 mol% of spirane (SiO4/2) fragments to MSSO during synthesis leads to the formation of polyhedral structures not containing silanol groups. The 2–3 μm sized discrete spherical particles forming during synthesis and separation from the reaction products are agglomerates of 10–15 nm sized primary nanoparticles. These primary nanoparticles form the texture and morphology of the microparticle spherical surfaces and predetermine the system of surface pores. A method has been developed for separating siloxane nanoparticles (2.5–280 nm) from the reaction products and preparing samples for LCS dispersion analysis. The behavior of nanoparticles in acetone and water/acetone suspensions and changes of their sizes have been studied using dynamic light scattering.
Acknowledgment The work was performed with financial support from the Ministry of Education and Science of the Russian Federation (State Contract No. 02.513.12.3059). The authors wish to thank V.G. Pevgov and A.R. Abarenkov for help in the size measurements of siloxane particles by laser correlation spectroscopy.
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References [1] Andrianov, K. A. Polimery s neorganicheskimi glavnymi tsepyami makromolekul [Polymers of Inorganic Main Micromolecular Chains]. Moscow: Izd. AN SSSR, p. 360. [2] Andrianov, K. A., Vasil'eva T. V., D'yachenko, B. I. Hydrolytic polycondensation of methyltrichlorsilane, Izv. SSSR. Ser. Khimiya. 1973, vol. 1, pp. 76-79. [3] Vasil'eva, T. V., Andrianov, K. A., Katashuk, N. M. Products of hydrolytic polycondensation of methyltrichlorsilane, Vysok. Soedin. – Polym. Sci. 1976, vol. 17, no. 6, pp. 1270–1275. [4] Andrianov, K. A., Vasil'eva, T. V., D'yachenko, B. I. Izv. SSSR. Ser. Khim. 1977, vol. 2, pp. 420–422. [5] Andrianov, K. A., Vasil'eva, T. V., D'yachenko, B. I. Hydrolytic polycondensation of tetrachlorous silicon in mixture with different functional organochlorsilanes, Zh. Obshchei Khimii. 1973, vol. 43, pp. 2458–2462. [6] Vasil'eva, T. V., Andrianov, K. A., D'yachenko, B. I. Hydrolytic polycondensation of tetrachlorous silicon in mixture with organochlorsilanes, Zh. Obshchei Khimii. 1973, vol. 43, pp. 768–771. [7] Andrianov, K. A., Petrovnina, N. M., Vasil'eva, T. V. Hydrolytic polycondensation of organochlorsilanes with tetrachlorous silicon or silicon acids in high functional mixtures, Zh. Obshchei Khimii. 1973, vol. 48, no. 12, pp. 2454–2458. [8] Larsson, K. Ark. kemi. 1960, no. 16, pp. 203-206. [9] Barry, A. J., Daudt, W. H., Domicone, J. J., Gilkey, J. М. Cristalline organosilsesqioxanes, J. Am. Chem. Soc. 1955, vol. 77, pp. 4248–4252. [10] Averichkin, P. A., Andrusov, Yu. B., Grishechkin, M. B., Denisov, I. A., Smirnova, N. A. Fabrication of CdZnTe-based substrates using polyhedral methylsilsesquioxanes, Izv. Vuzov. Mater. Elektronnoi Tekhniki = Mater. Electron. Eng. 2011, vol. 1, pp. 12–15. [11] Averichkin, P. A., Chernyshev, E. A. Gidroliticheskaya polikondensatsiya alkoksilirovannykh (in situ) organokhlorsilanov nepreryvnym sposobom [Continuous Hydrolytic Polycondensation of Alkoxylated (in situ) Organochlorsilanes]. Tez. dokl. «I Kremniiorganicheskii mikrosimpozium» Moscow, 1994. pp. 22-23. [12] Patent RF No. 2444539. Sposob polucheniya poliedricheskikh organosilseskvioksanov [Method of Synthesis of Polyhedral Organosilsesquioxanes]. P. A. Averichkin, Yu. B. Andrusov, I. A. Denisov, V. A. Kal’kov, Yu. N. Parkhomenko, N.A. Smirnova. (Appl. 05.10.2010, Publ. 10.03.2012, Bull. No. 7). [13] Koptyug, V.A. IK-spektry kislorodsoderzhashchikh kremniiorganicheskikh Soedin. [IR Spectra Oxyg.-Contain. Siliconorganic Compd.], Novosibirsk, 1986. 182 p. [14] Svergun, D. I., Feigin, L. A. Rentgenovskoe i neitronnoe malouglovoe rasseyanie [Low-Angle Scattering of X-Rays and Neutrons] Moscow: Nauka, p. 230. [15] Lebedev A.D., Lomakin A.V., Noskin V.A. Primenenie lazernoi korrelyatsionnoi spektroskopii dlya izucheniya EF biologicheskikh ob"ektov v rastvorakh. Instrumental'nye metody v fiziologii i biofizike [Laser Correlation Spectroscopy in Study of Biological Objects in Solutions. Instrumental Methods in Physiology and Biophysics]. Leningrad, Nauka, 1987, pp. 90–95. [16] Kaszuba, M., McKnight, D., Connah, M.T., McNeil-Watson, F.K., Nobbmann U. Measuring sub nanometre sizes using dynamic light scattering, J. Nanopart. Res. 2008, vol. 10, no. 5, pp. 823-829. http://dx.doi.org/10.1007/s11051-007-9317-4. [17] Vol'chek, B. Z., Purkina, A. V., Nikitin, N. N. An IR spectroscopic investigation of the structure of polyphenylsilsesquioxane, Polym. Sci. USSR. 1976, vol. 18, no. 6, pp. 1379–1385, http://dx.doi.org/10.1016/0032-3950(76)90329-4. [18] Voronkov, M.G., Mileshkevich, V.P., Yuzhelevskii, Yu. A.Siloksanovaya svyaz' [Siloxane Bond] Novosibirsk: Nauka, p, 413.
Finely divided methylsilsesquioxane particles with SiO4/2 fragments in structure Pavel A. Averichkina, Yuri B. Andrusova, Igor A. Denisova,n, Tursunbek B. Klychbaevb, Yuri N. Parkhomenkoa, Natalia A. Smirnovaa a
Joint Stock Company «Giredmet», 5-1 B. Tolmachevsky Lane, Moscow 119017, Russia Chemical and Metallurgical Holding «Metal», 40 Sovetskaya Str., Bishkek 720055, Kyrgyzstan
b
n
KEYWORDS
Abstract
Methylsilsesquioxanes; Silicon-organic compounds; Siloxane particles; Hydrolysis; Suspension; Laser correlation spectroscopy; Electron scanning microscopy; Crystalline nanoparticles
Results of structural and morphological investigations of methylsilsesquioxane finely divided particles synthesized by hydrolytic co-condensation of in situ alkoxylated methyltrichlorosilane and tetrachlorous silicon are presented. The silica-type hydrophobe particles have been obtained using the abovementioned method with an output of 98.7–99.4 wt% of the load. These particles have been identified as crystalline formations with a lattice period of 5.5– 8.0 nm and an interplane distance of 0.92–1.04 nm in the near order. We show that unlike for well-known crystalline methylsilsesquioxanes, introduction of chemically bonded SiO4/2 fragments to the material structure leads to the formation of 2–3 μm-sized spherical particles from the initial 10–15 nm particles; these particles are responsible for the spherical morphology of the surface. A method of extracting siloxane nanoparticles (2.5–280 nm) from the reaction products has been described, the behavior of particles in suspension has been studied and particle dimensional parameters have been determined using helium neon 0.6328 nm laser correlation spectroscopy. & 2016 The National University of Science and Technology MISiS. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Corresponding author. E-mail addresses: [email protected] (P.A. Averichkin), [email protected] (Y.B. Andrusov), [email protected] (I.A. Denisov), [email protected] (T.B. Klychbaev), [email protected] (N.A. Smirnova). Peer review under responsibility of the National University of Science and Technology MISiS.
Introduction Organic silsesquioxane (OSSO) class siliconorganic compounds are the unit structural elements which consist of tetrameric (for silicon) siloxane cycles with hydrocarbon substituents at the silicon corners of the cage known as oligomeric and polymeric products of cyclic-linear, cyclonetwork and closed polyhedral structure [1]. It is commonly believed that cyclic-linear (ladder)
http://dx.doi.org/10.1016/j.moem.2016.08.002 2452-1779/& 2016 The National University of Science and Technology MISiS. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Finely divided methylsilsesquioxane particles oligomers have a regular structure with residual silanol groups at the ends (1.5–3.5 wt%) and a narrow molecular weight distribution [2]. In the form of dry powders these products are thermosetting resins having the phase transition from solid to viscous flow state at about Tg =50–80 1C. Their solubility in organic solvents [3] allows their use in electronics technologies as functional or service thin protective films. Unlike cycliclinear oligomer structures, cyclonetwork ones are synthesized by additionally modifying silsesquioxane structures with chemically bonded spirane SiO4/2 fragments using co-hydrolysis of organotrichlorsilans with tetrachlor- or tetralkoxysilane [4–6]. Cyclonetwork oligomers also exhibit reactivity and solubility in organic solvents. Due to the increase in the rigidity of the main siloxane chain by SiO4/2 fragments, oligomer resins, mainly with aliphatic hydrocarbon substituents (—H; —CH3; —C2H3; —C2H5) at the silicon edges, do not undergo the phase transition from resin-like solid to viscous flow state under normal conditions. The resin melting point increases above the organic radical destruction point and tends to reach the silicon oxide glass transition point. The absence of softening and melting in oligomer resins allows their use for forming moldable thin nanosized films and protective coatings which retain almost constant structural parameters (setting, surface texture and morphology, internal stresses) and shape (thickness, topological size etc.) during inter-operation processing, e.g. thermal anneals, ion plasma etching, lithography etc. Polyhedral OSSO were obtained and identified as octahedral (closed) methyl- and vinylsilsesquioxanes (Me T-8 and Vin T-8 spatial “cubes”) synthesized from methyl- and vinyltrichlorsilane, respectively, with a low output, only 6– 19 wt% [7]. These products are in the form of finely divided crystalline powders insoluble in organic solvents and, according to IR spectroscopy, not containing hydroxyl groups. When separated from reaction products in the form of discrete particles, they were first identified using X-ray analysis (CuKα radiation) as crystalline formations with certain crystallographic parameters [7,8]. Spatial silsesquioxane structures with phenyl radicals at the corners (phenylsilsesquioxanes) can form, in the course of polymeric transformations, polyhedral shapes, e.g. Ph T-8, Ph T-10, Ph T-12 etc. Their large size and aromatic nature of the substituents at the silicon corners favor the formation of polyhedral polytypes [9]. The reason for the interest to polyhedral OSSO is the possibility of obtaining discrete hydrophobic particles that replace hydrophilic aerosils and do not contain pores. Of the abovementioned silsesquioxane compounds, of greatest scientific and practical interest are polyhedral methylsilsesquioxanes (MSSO) modified with SiO4/2 structural fragments. The small size of the organic radical at the corner, porosity and relatively well-developed surface, as well as good moldability allow these compounds to be considered as promising for the synthesis of finely divided silica-type hydrophobic siloxane particles. The synthesized particles can be used as reinforcing fillers and binders for the development and fabrication of siliconorganic compounds and sealants for the protection and sealing of p–n junctions in high-voltage (to 6 kV) semiconductor devices and ICs. Experimental specimens of polyhedral MSSO were successfully tested at Giredmet as soft abrasives for chemical mechanical polishing of surfaces of cadmium telluride semiconductor wafers [10].
19 The aim of this work is to provide polyhedral MSSO with spirane SiO4/2 structural fragments (to 60 mol%) by hydrolysis and co-condensation of in situ alkoxylated methyltrichlorosilane (MTCS) and tetrachlorous silicon (TCS), and develop methods for separation of finely divided siloxane particles from reaction products, their structural identification and size assessment.
Experimental We synthesized MSSO with chemically bonded SiO4/2 fragments by hydrolysis and co-condensation of an in situ alkoxylated [11] water/alcohol mixture of MTCS and TCS in a 2:3 M ratio, respectively [12]. The reaction products were synthesized as solid discrete particles and separated on a Schott filter glass, washed from HCl in water, dried and stored in an air proof cabinet. The output of the powdered product was 99.0–99.4 wt% of the load. The structure of the chemical bonds in the MSSO was studied using IR spectroscopy in the 400–4000 cm 1 region on a Bruker Equinox 55/S spectrophotometer (Bruker, Germany). The specimens for IR spectroscopy were prepared in the form of pressed tablets mixed with KBr using a conventional method. The IR spectral lines were characterized in accordance with well-known reference and literary data [13]. X-ray analysis of the powders was carried out on a URD-6 diffractometer (Germany) in CuKα radiation and a Ni filter by reflection technique in the 6 arc dego2θo50 arc deg angular range. The curves were processed (smoothing, normalization) and corrected for parasitic scattering and collimation using special software [14]. The sizes of the crystalline regions were determined using the Selyakov– Scherrer method. The shapes of the discrete siloxane particles, their surface texture and morphology were studied under a Vega II XMU-Tescan (Tescan, Czech Republic) scanning electron microscope (SEM) with a 10,000 magnification. The sizes of the siloxane nanoparticles in the prepared samples of suspension solutions were assessed using laser correlation spectroscopy (LCS) based on the determination of the spectral characteristics of quasielastically scattered light (in Rayleigh mode) after transmission through a solution of the finely divided 1–2000 nm sized particles [15,16]. The oscillations of the scattered light relative to the reference determinate radiation of far greater intensity were recorded from one helium-neon laser source (LGN207A) at 0.6328 μm with an output power of 2.5 mW. The measurements were made on an LCS-03 spectrometer developed at the Moscow Institute for Physical Engineering (State University). The frequency shifts of radiation scattered by the microparticles was 100 106 Hz. The correlation spectra of light fluctuation on moving particles were processed after approx. 1000 measurements. During sample preparation we separated narrow particle fractions of the synthesized products as follows. We prepared acetone and water/acetone (1:1) suspensions of 1 mg particles in 1 ml of liquid, separated large micrometer-sized particles by sedimentation and filtered the finely divided particles through a 0.45 μm pore nylon membrane. We chose acetone as the suspension media for its good
20
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compatibility with MSSO and strong solvating properties making it capable of forming solvate shells on the surface of siloxane nanoparticles. Strongly polarized water was preferred for its ability to rapidly change the solvating properties of acetone. In an equimolar acetone/water mixture, polar water molecules wedge the solvate shells of the nanoparticles until destruction. Suspension was prepared from highly pure acetone and deionized water with a resistivity of approx. 17 MOhm.cm. When determining the parameters of particle size distribution functions for acetone and for water/acetone solution we accepted maximum relative systematic error to be 720% and the maximum relative random error to be 710%.
Results and discussion Water hydrolysis of MTCS is known to cause the substitution of Si—Cl for Si—OH groups followed by their homocondensation and the formation, depending on synthesis conditions, of cyclic-linear (ladder), octahedral and gelled MSSO. The resultant cyclic-linear oligomers have a broad molecular weight distribution and are unstable (labile) during processing and storage. Octahedral MSSO form as by-products of cyclic-linear oligomer synthesis with an output of within 10 wt% of the load. Gelled MSSO form during uncontrolled MTCS hydrolytic polycondensation and contain up to 20 wt% of accompanying mole fractions. Synthesis of regular MSSO structures and high-quality products on their basis can be achieved using our method of hydrolysis of in situ alkoxylated MTCS with C1–C2 primary alcohols by boiling the reaction mixture. Gradual substitution of alkyl groups for silanol ones and their subsequent condensation provide for a controlled stepwise process resulting in the formation of defect-free structures. The use of TCS with MTCS causes hydrolysis of monomers and their condensation or embedding into the sesquioxane (—SiO3/2) structure of spirane (SiO4/2) fragments which drastically change the siloxane structure of the macromolecules. The joint action of the —SiO3/2 and SiO4/2 chains under these conditions forms either cyclonetwork or polyhedral (closed) siloxane skeletons. The SiO4/2 spirane fragments increase the rigidity of the siloxane structure of the macromolecules. The primary polyhedral siloxane structures separated from the reaction products agglomerate to discrete particles typical of silica. The methyl substituents at the silicon corners provide for the organogenic nature of the siloxane structure thus noticeably modifying its surface and ensuring good hydrophobicity, organophilicity and compatibility of the forming discrete particles in organic or inorganic composites. As can be seen from the IR spectra (Figure 1), octahedral and structurally modified MSSO do not contain Si—OH groups whose chemical bonds are typically strongly expressed in the valence (3200–3800 cm 1) and deformation (approx. 860 cm 1) regions. The oscillation pattern of the Si—O—Si bonds in the characteristic region suggests that the siloxane skeletons have a symmetrical structure corresponding to the oscillations of silicon oxide compounds with the absorption peak at 1060 cm 1. The Si—O—Si bond oscillation peak halfwidth (dII) (Figure 1, curve 2)
Figure 1 MSO IR spectra: (1) octahedral structure, (2) polyhedral structure with chemically bonded SiO4/2 fragments, (3) structures polymer gel and (4) cyclic-liner (ladder) oligomer structure.
increases almost twofold compared to octahedral MSSO (dI) if inorganic SiO4/2 fragments are introduced. A decrease in the percentage of the organic fraction causes suppression and reduction of the intensity of the absorption lines near 605, 1165 and 2900 cm 1 corresponding to organic methylsilil ( Si—CH3) radicals. Noteworthy, an increase in the percentage of SiO4/2 fragments in the structure to above 80 mol% triggers uncontrolled condensation of three- and four-functional monomers and the formation of a gelled structure with a high content of silanol groups. In structured polymer gel the siloxane bonds have an unordered (random) structure, and the oscillations of longitudinal and lateral Si—O—Si bonds (Figure 1, curve 3) make statistically similar contributions to the IR spectrum. The Si—O—Si bond oscillation peak splits and the intensities of the shortand long-wave absorption bands become almost equal. The IR spectra of MSSO with a cyclic-linear (ladder) oligomer structure (Figure 1, curve 4) and structured polymer gel (Figure 1, curve 3) suggest that the shapes of the spectral lines of Si—O—Si bond oscillation in their respective characteristic region are not similar. The asymmetry of the ladder structure shows itself in the splitting of the characteristic peak into the short-wave region (1100 cm 1) and the long-wave region (1020 cm 1). The absorption intensity in the short-wave spectral region corresponding to longitudinal Si—O—Si bonds in cyclic-linear structures is higher [17]. MTCS based compounds and MTCS/TCS mixtures synthesized using an earlier method [12] are in the form of finely divided agglomerated particles which after separation from the reaction products and drying aggregate to loose powders. A general schematic of the structure of unit particles
Finely divided methylsilsesquioxane particles
Figure 2
21
Primary MSSO nanoparticles of (a) I and (b) II structural types.
Figure 3 Images of discrete MSSO particles of different compositions and structure: (a) a colony of [CH3SiO1.5]8 crystalline particles (structural type I), and (b) and (c) [(CH3SiO1.5)8.(SiO4/2)]12 spherical particles (structural type II).
corresponding to spectra 1 and 2 in Figure 1 (hereinafter, particle types I and II, respectively) is illustrated in Figure 2 as polyhedral (closed skeleton) structures not containing Si—OH groups. X-ray diffraction showed that the synthesized powdered products are single-phase systems consisting of quasicrystalline formations with a period of 5.5–8.0 nm. The diffraction patterns also suggest that the crystalline regions are restricted by amorphous ones that presumably are structures with deformed valence angles of Si—O—Si bonds that may vary over a wide range [18]. The interplane spacing of I and II type structures was 0.92–1.04 nm which suggests the similarity of their structures in the shortrange order. Figure 3 shows SEM images of primary agglomerated I and II structural type particles, magnification 10,000. Primary structural type I discrete particle agglomerates differ noticeably from primary structural type II discrete particle agglomerates not only in composition, but also in structure. During synthesis and separation from the reaction products, type I octahedral structures agglomerate to cube-shaped crystals (Figure 3a) with a wide size range (5–20 μm). The primary 10–15 nm sized type II discrete nanoparticles agglomerate during separation to 2–3 μm sized spherical particles (Figure 3b). Furthermore, these primary nanoparticles form the texture and morphology of the microparticle spherical surfaces and predetermine the system of surface pores (Figure 3c). The size and behavior of the separated type II nanosized particle fractions were characterized using dynamic light scattering in acetone and water/acetone (1:1) suspensions. After processing of the results the distributions of
Figure 4 Hydrodynamic size distribution histograms of structural type II light-scattering MSSO particles after normalization of LC spectra for (a) acetone and (b) water/acetone solution.
22 light-scattering particles were presented as histograms (Figure 4) where the Y axis shows the percentage of particles in the suspension and the X axis shows the hydrodynamic size of the particles. The acetone suspension histograms (Figure 4a) suggest that siloxane particles form 55–280 nm agglomerates with solvate shells on the surfaces. After the addition of strongly polarized water (Figure 4b) the solvate acetone shells degrade and decompose and the particles divide in two groups sized 2.5–2.8 and 25–57 nm, respectively. Addition of more acetone (until a certain threshold) causes regrouping and reagglomeration of the nanoparticles into a single group. These transformations of siloxane particles in suspension media over a certain concentration range may repeat multiply. Thus, the variable behavior of siloxane nanoparticles in suspension media is of scientific and practical interest. Controlled nanoparticle manipulation in suspensions can be used for preparation purposes and in technologies, e.g. in the development of hi-tech composite recipes or synthesis of materials with preset properties for electronics.
Summary A polyhedral MSSO particle synthesis technique has been developed and finely divided hydrophobic MSSO particles have been synthesized in the form of aggregated powders with an output of 99.0–99.4 wt% of the load. The structure of the siloxane particles has been examined, and they proved to be crystals with a period of 5.5–8.0 nm and an interplane spacing of 0.92–1.04 nm in the short-range order. We have found that the introduction of up to 60 mol% of spirane (SiO4/2) fragments to MSSO during synthesis leads to the formation of polyhedral structures not containing silanol groups. The 2–3 μm sized discrete spherical particles forming during synthesis and separation from the reaction products are agglomerates of 10–15 nm sized primary nanoparticles. These primary nanoparticles form the texture and morphology of the microparticle spherical surfaces and predetermine the system of surface pores. A method has been developed for separating siloxane nanoparticles (2.5–280 nm) from the reaction products and preparing samples for LCS dispersion analysis. The behavior of nanoparticles in acetone and water/acetone suspensions and changes of their sizes have been studied using dynamic light scattering.
Acknowledgment The work was performed with financial support from the Ministry of Education and Science of the Russian Federation (State Contract No. 02.513.12.3059). The authors wish to thank V.G. Pevgov and A.R. Abarenkov for help in the size measurements of siloxane particles by laser correlation spectroscopy.
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