Structure characterization of organic-inorganic nanocomposites formed by the sol-gel polycondensation
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Structure characterization of organic-inorganic nanocomposites formed by the sol-gel polycondensation Libor Matějka Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovský Sq.2, 162 06 Prague 6, Czech Republic
ABSTRACT The organic-inorganic (O-I) hybrid networks were prepared from well-defined O-I precursors (a) alkoxysilane-endcapped oligomers (ASO) and (b) epoxy-functional silsesquioxane (SSQO) clusters (EC). ASO precursors were crosslinked by sol-gel polycondensation and EC by the reaction of the epoxy groups with oligomeric diamine Jeffamine D2000. Formation, structure and mechanical properties of the networks were followed by SEC, NMR, SAXS and DMA. The O-I block copolymer network is composed of SSQO domains forming network junctions linked by an oligomer chain. Structure of the SSQO crosslink “points” ranges from compact polyhedral cages to large loose SSQO clusters in dependence on the precursor type and catalysis of the sol-gel process. Interphase interaction and mechanical properties of the O-I networks are discussed.
INTRODUCTION Nanocomposite organic-inorganic (O-I) materials show excellent properties in many respects and O-I polymers with an inorganic phase formed in-situ by the sol-gel polycondensation are typical nanocomposite systems. The sol-gel process involves hydrolysis and subsequent condensation of alkoxysilane derivatives used often to build silicon-containing inorganic phase. Silica or silsesquioxane (SSQO) structures are thus formed within an organic matrix. A large interface surface is an important characteristic of nanocomposites due to a small, nanoscale size, of the inorganic filler dispersed in an organic polymer. Interphase interaction is a parameter affecting to a great extent morphology and mainly mechanical properties. Therefore, coupling agents, such as organotrialkoxysilanes, assuring a covalent bonding between organic and inorganic phases are often applied to improve interphase interaction. Strong bonding could be achieved by polymerizing O-I macromonomers possessing both organic and inorganic functionalities as precursors of formation of covalently bonded organic and inorganic structures. As a result, a crosslinked O-I block copolymer is built-up. The morphological model by Wilkes et al.1 assumes the presence of three phases in such an O-I polymer – organic-rich, inorganic-rich and a mixed phase. However, a detailed characterization of the inorganic structure within network and its effect on interphase interaction and final nanocomposite properties is still lacking. We have used two types of the silicon based O-I precursors to synthesize the O-I block copolymer networks (a) alkoxysilane-endcapped oligomers (ASO) and (b) SSQO oligomer clusters with epoxy functionalities (EC). ASO were crosslinked by sol-gel polycondensation of the alkoxysilane groups at the chain extremities and EC were reacted by their epoxy groups with oligomeric diamine (Jeffamine D2000) to produce a network. The characterization of the network structure, interphase interac
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