Chemically Tailored Interfaces in Nanocomposites

New material properties of inorganic-organic nanocomposites are obtained by the incorporation of inorganic building blocks on the nanometer length scale in a polymer matrix. Two crucial parameters in the preparation of these materials are the synergistic combination of the properties of the single components and their homogeneous distribution in the material. Based on the size of the nanobuilding blocks and the resulting large surface, the chemical control of the interface to the polymer matrix represents an important factor for the nanocomposite properties. Chemical surface-modification of the inorganic components is crucial for a homogeneous distribution in the polymer and allows a stable chemical linkage between the constituents, which results in an improvement of the mechanical properties and increased materials stability. In the proposed project the surface of silica and zirconia nanoparticles is chemically functionalized and the influence of this surface-modification on the distribution of the components in the polymer matrix and on the mechanical properties of the formed nanocomposites is investigated. Anchor groups of the coupling agents, their chemical composition and reactive groups which allow a linkage to the polymer matrix are systematically varied. Variations of the inorganic building block size and the relative composition of the nanocomposites represent further parameters in the studies of these materials. The obtained nanobuilding blocks are incorporated in different polymers by copolymerization and the resulting materials are investigated in an interdisciplinary approach by physicists concerning their internal nanostructure and its change with respect to mechanical stress, as well as by materials scientists regarding their mechanical properties. Both properties will be correlated with changes at the interface as well as with the size of the nanoparticles to obtain novel insights in structure-property relationships of interfaces in nanocomposites.

The chemical design of the interface in nanocomposites plays a crucial role for the macroscopic properties of these materials. Systematic studies on different particle types with various modifications on their surface revealed that changes in the monomolecular layer at the interface between the inorganic particles and the polymer matrix can lead to mechanical property changes of the bulk materials. In this research project a systematic study was carried out on the impact of surface functionalization of metal oxide nanoparticles on their dispersion behavior in polymer matrices, internal materials structure and macroscopic mechanical properties of nanocomposites. A bottom up chemical approach, starting from molecular precursors resulting in nanocomposites, was applied to control the properties of the materials on every length scale. Two nanoparticle types were studied to cover different properties of these building blocks, namely spherical amorphous SiO2 nanoparticles and ZrO2 nanocrystallites. Both nanobuilding blocks were synthesized in different dimensions to investigate the size effects on the properties of the final nanocomposites. In addition different coupling agents were used to modify the surfaces of the nanoparticles and thus control the compatibility as well as the interaction of the particles with the polymer matrix. Interface active compounds with long alkyl chains were tested on their ability to self-assemble to well-ordered aggregates on the surface of the nanoparticles. It could be shown that the formation of so called self- assembled monolayers (SAMs) on the nanoparticle surface strongly depends on the nanoscopic curvature of the particles and influences nanopowder agglomeration and dispersion behavior in the polymer matrix. Controlled disturbance of the ordering at the interface can help to improve dispersion behavior of the particles. Surfaces that were modified with poly(dimethylsiloxanes) resulted in significantly altered dispersion behavior of the nanoparticles compared to the other studied systems. Different types of nanoparticles were incorporated in the polymer matrix applying in situ polymerization methods and thus poly(methyl methacrylate)-, polystyrene-, and epoxy resin nanocomposites with different amounts of inorganic nanofiller were produced and their structural as well as mechanical properties were investigated. As a result of all the systematic studies it can be concluded that it is crucial for the preparation of homogeneous nanocomposites with strong mechanical reinforcement to chemically tailor the interfacial compatibility and interfacial adhesion between the components.