•Metallosiloxane nanoparticles enhance the AO erosion resistance of copolyimides.•Siloxane-copolyimide is able to resist the AO attack on a par with nanoparticles.•The protection of metallosiloxane nanoparticles is not affected by the AO fluence.•The combined protection of the filled copolyimide is achieved at the high AO fluence.

This article presents the results of our study on the direct mechanochemical synthesis of tetramethoxysilane in a high-pressure reactor using CuO and CuCl as catalysts and some promoters. The rate of the reaction of silicon with methanol in the presence of CuO is significantly lower than that in the presence of CuCl. A method for studying the influence of promoters (SnCl2, Zn, and PbCl2) on different process steps (formation of an active silicide and the reaction of silicon with methanol) has been proposed. It has been shown that the addition of 5000 ppm of SnCl2 [or the same amount of Sn as Sn(0)] improves the rate of the reaction between silicon and methanol in the presence of a low-active copper source (CuO) to the level provided by CuCl. Scanning electron microscopy, powder X-ray diffraction, and X-ray photoelectron spectroscopy studies of the contact masses (CM) carried out at different steps of the process showed that tin promotes the reduction of CuO to Cu by silicon. The term "mechanochemical contact mass" was proposed because in traditional systems without in situ milling (fixed bed, slurry, and fluidization), the CMs are static, resulting in uneven formation of active reaction sites.

In order to expand the range of materials that can be used in outer space and in development of small spacecraft, ladder polyphenylsilsesquioxanes with different molar weights and the Nb-siloxane composites based on them were studied. The properties of the polymer films were studied, including tests in an oxygen plasma flow. Both initial and filled ladder polymers feature extremely low erosion coefficients in the region of 10–26 cm3/atom O at a high fluence of atomic oxygen of 1.0 × 1021 atom O/cm2. Ladder polyphenylsilsesquioxane films irradiated with atomic oxygen (AO) retain their integrity, do not crack, and exhibit good optical properties, in particular, a high transmittance. The latter slightly decreases during AO exposure. The Nb-siloxane filling retains the AO resistance and slight decrease in optical transmission due to diffuse scattering on the formed Nb-[(SiO)x] nanoparticles. Ladder polyphenylsilsesquioxanes demonstrate their suitability for creating protective, optically transparent coatings for small spacecraft that are resistant to the erosive effects of incoming oxygen plasma.

Condensation of alkoxysilanes in the active medium, which means acetic or trifluoroacetic acids, is a convenient and versatile method for preparing silsesquioxane copolymers with various functional groups. The method does not require the selection of special conditions, the use of additional solvents and catalysts. The obtained co-polymers from poorly compatible monomers are homogeneous in composition without the use of additional solvents. Starting from commercially available functional triethoxisilanes and triethoxysilyl derivatives with branched tert-perfluorohexyl substituents (RF = CF3CF2CF2C(CF3)2(CH2)3-) under condensation in an active medium new functional silsesquioxane polymers with various groups (-NH2, -SH, or -COOH) were obtained. The synthesis techniques used provided a high yield of the resulting copolymers (91-94%). Despite the low compatibility of perfluoroalkyl organosilicon compounds with functional alkoxysilanes, the resulting copolymers are homogeneous in composition, which was confirmed by NMR and GPC. Copolymers are effective modifiers for the surface of materials. In particular, they were used as water repellents for cotton and polyamide fabrics. Modified surfaces of cotton and polyamide fabrics acquired highly hydrophobic properties (alpha > 120). The presence of -NH2, -SH or -COOH groups in the chemical structure of fluorinated silsesquioxane copolymers provided high washing-resistant of modified cotton textile. For polyamide fabric, a similar effect was achieved where copolymers with -NH2 or -SH groups were used. The COOH-containing copolymer coating on the poly -amide textile do not provide resistance of the modified polyamide textile to repeated washing. Thus, we demonstrated that the new functional silsesquioxane polymers with branched perfluoroalkyl substituents and -NH2 or -SH groups are versatile and efficient hydrophobic agents for both cotton and polyamide textiles.

In this paper, a new kind of space-durable nanocomposites has been prepared using one-step in situ method of filling organosoluble polyimides (PI). Three types of metalloalkoxysiloxanes (tris-(diethoxymethylsiloxy) aluminum, tetrakis-(diethoxymethylsiloxy) zirconium, pentakis-(diethoxymethylsiloxy) niobium) were used as filler precursors. It is demonstrated that the nanosized filler particles formed in the polymer volume have a hybrid chemical structure and contain М-О-Si and Si–O–Si bonds. The in situ filling of PI preserves its unique thermal properties, increases the glass transition temperature, and maintains high stability to the thermal oxidation of the matrix PI. The atomic oxygen (AO) erosion resistance of the filled PI has been investigated by exposing its surface to a variety of AO fluences. The introduction of nanosized filler in PI contributes to a sharp, order-of-magnitude decrease in the AO erosion coefficient of the nanocomposite. The valence of the central metal atom of the precursor predetermines the silica-block content in the forming filler and its atomic oxygen AO-protective function. During the transition from a precursor with a trivalent metal atom to a precursor with a tetra- or pentavalent metal atom, the erosion coefficient of filled PI decreases at a constant fluence of AO. Taking into account that the M-O bond energy in a M-O-Si group is higher than the Si–O bond energy, the presence of such atoms as Zr or Nb in the chemical structure of the filler can act as a “reinforcing” element that increases the resistance of the protective layer against AO action.

The effect of branched functional metallosiloxane oligomers (BFMSO) used as precursors on the morphology and atomic oxygen resistance of filled polyimide films was studied. Nanocomposites are characterized by elevated glass transition temperatures and greater erosion resistance to atomic oxygen than the original polymer. In situ filling of polyimide with BFMSO-based nanoparticles allows one to reduce the rate of film mass loss under the influence of atomic oxygen by an order of magnitude and reduce the erosion yield by more than 70%. To correctly compare the effectiveness of fillers based on precursors of different chemical structures as protective elements of the polymer surface from the effects of atomic oxygen, the specific erosion yields calculated as the ratio of the erosion yield to the filler concentration expressed in moles of filler per unit mass of polymer were used. This approach made it possible to identify the main factors of the directional regulation of erosion resistance of in situ filled PI to the effects of atomic oxygen. Primarily they include the chemical structure of the organic frame of the central metal atom of the precursor, which determines the effectiveness of the filler in increasing the erosion resistance of nanocomposites. The "replacement" of a phenyl substituent with a methyl substituent at a silicon atom in the composition of BFMSO and, accordingly, particles based on it, reduces the specific erosion yields of nanocomposites by 2.2-2.6 times. The nature of the central metal atom of the precursor is a minor factor. Its change allows reducing the specific erosion yields of coatings based on polyimide and branched functional metallosiloxane oligomers by no more than 1.5 times. (C) 2020 Elsevier Ltd. All rights reserved.

The structure and properties of nanocomposites based on organosoluble polyimide (PI) and branched functional metallosiloxane oligomers with different types of central metal atoms (Al, Cr, Fe, Zr, Hf and Nb) were investigated. Under the same weight content of the filler, the geometric parameters of the nanoparticles and thermal properties of the nanocomposites did not exhibit a direct relationship with the ability of the materials to withstand the incident flow of oxygen plasma. The atomic oxygenerosion resistance of the filled PI films was influenced by the composition of the hybrid fillerand the type of metal atom in the hybrid filler in the base metallosiloxane oligomer. To determine the effectiveness of the nanoparticles as protective elements of the polymer surface, the nanocomposite erosion yields pertaining to the concentration of the crosslinked organo–inorganic polymer forming the dispersed phase were determined and expressed in mmol per gram PI. The filler concentration in the polymer, expressed in these units, allows for comparison of the efficiency of different nanosize fillers for use in fabricating space survivable coatings. This can be important in the pursuit of new precursors, fillers for fabricating space survivable polymer composites.