Silica/polymer composite materials: synthesis, characterization and applications
Zhao, Yue; Möller, Martin (Thesis advisor); Albrecht, Markus (Thesis advisor)
Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2022
This dissertation deals with silica-based particular composite materials, which are prepared via a sol-gel process in combination with a surfactant-free emulsion technique using various derivatives of hyperbranched polyethoxysiloxane (PEOS) as both silica precursor and emulsifier. The resulting materials are thoroughly investigated concerning their chemical composition, morphology and properties. Chapter 2 provides a literature survey on advanced silica-based materials including microcapsules, aerogel, and interpenetrating polymer networks. Their state-of-the-art preparation methods and applications are summarized, and challenges have been identified. Chapter 3 reports on a facile one-step approach to enclosing fragrances in mechanically robust silica capsules, where a wax, myristyl myristate, is used as matrix to accommodate the fragrance molecules for further retarding their evaporation. An oil phase consisting of wax, fragrance and PEOS is emulsified in water owing to the hydrolysis-induced amphiphilicity of PEOS. The wax and fragrance are then quantitatively microencapsulated in silica converted from PEOS. The resulting microcapsules exhibit excellent controlled releasing properties as well as outstanding reloading properties. In Chapter 4, a new strategy for preparation of SiO2/poly(methyl methacrylate) hybrid particles with a semi-interpenetrating polymer network structure is reported, which utilizes intrinsically amphiphilic PEGylated PEOS as emulsifier in heterophase polymerization of the corresponding monomer. After removing the polymer via calcination, nanoporous silica particles with a large specific surface area can be obtained. The influence of radical initiator, PEGylation degree of the silica precursor as well as its fraction in the oil phase on the polymerization mechanism and the morphology of the products has been investigated. Chapter 5 describes the preparation of thermal insulating coatings with closed nanopores for textile surfaces by adding nanoporous silica microparticles (aerogel particles) to commercially available film-forming agents. By co-condensation of tetraethoxysilane and methyltrimethoxysilane followed by PEGylation of the condensation product, a silica precursor is obtained, which is converted to silica aerogel particles with a hydrophobic interior that are dispersible not only in water, but also in polar organic liquids. Aqueous formulations with polyacrylates or polyurethanes are prepared by mixing aerogel particles into corresponding aqueous polymer dispersions. Aerogel-containing poly(vinyl chloride) plastisol is prepared by dispersing the polymer latex and aerogel particles in a plasticizer. Coating of a polyester fabric is carried out by a roller doctor blade and subsequent thermal fixation. It is found that the aerogel particles are distributed homogeneously in the coatings, while the nanoporous structure is retained. The particles added to the coating agent significantly influence the thermal radiation, as manifested not only in reduced radiation transmission and absorption but also in increased radiation reflection. This effect increases with the increased amount of the incorporated particles. Moreover, by addition of 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane, strong water and oil repellency of the coatings is achieved. A surfactant-free miniemulsion polymerization technique for preparation of hexadecane@polystyrene@SiO2 microcapsules is shown in Chapter 6. In this process, a homogeneous oil phase consisting of styrene, hexadecane, PEOS or its phenyl-substituted derivatives and a radical initiator is emulsified in water to form an oil-in-water miniemulsion, and polymerization then takes places upon heating. Different strategies have been adopted to achieve the targeted morphology. It has been demonstrated that hexadecane@polystyrene@SiO2 microcapsules are formed at a low styrene/hexadecane ratio or when the aqueous phase contains a small amount of radical inhibitor. Such kind of microcapsules can also be obtained by using phenyl-substituted PEOS derivatives prepared via co-condensation of tetraethoxysilane and trialkoxyphenylsilane instead of non-modified PEOS.