PhD Thesis
Hybrids Based on Layered Silicates
Stephan Weiß (01/2013-06/2013)
Support: Axel H. E. Müller
Summary
Novel hybrid nanoparticles were synthesized based on combinations of various layered silicates as inorganic core and well-defined polymer chains as a shell. In all cases positively charged 2-(dimethylamino)ethyl methacrylate (DMAEMA) was incorporated into the polymeric structure to serve as a firm anchor onto the negatively charged clay surface via electrostatic adsorption. First, hybrid nanofillers were synthesized to improve the mechanical properties of a homopolymer matrix by combining a shear-stiff synthetic K-hectorite with a tailored surface activity. For this, the synthetic fluorohectorite with very high aspect ratios was organophilized with a specifically designed macroinitiator created by statistical Reversible Addition Fragmentation Chain Transfer (RAFT) copolymerization of DMAEMA and the initator-monomer 2-(2-bromoisobutyryloxy)ethyl methacrylate (BIEM). The copolymer was firmly anchored through multiple cationic charges distributed over the chain while the multiple initiating functions were used to polymerize the monomer of choice via Atom Transfer Radical Polymerization (ATRP). The final hybrid was equipped with a hydrophobic polymeric shell of poly(methyl methacrylate) (PMMA), which enables dispersion in organic solvents. The hybrid particles were compounded into a polymeric matrix of commercial PMMA and tested with regard to its reinforcing properties. The similarity of the polymeric shell to the homopolymer matrix of the chosen sample composite combined with the inherent stiffness of the inorganic core lead to an increase in tensile modulus of up to 84 % at 5% filler content. Further, patchy hybrid nanodiscs based on natural montmorillonite as core and a shell made from compartments of two different polymers were evaluated as cheap and versatile compatibilizers in an immiscible polymer blend. In a simple one-step modification process a shell comprising patches of either of two polymer species (PMMA and polystyrene, PS), each chosen to be similar in polarity to one of the matrix polymers, was attached to the inorganic core via Coulomb interaction. The behaviour of these particles in a solvent-cast blend of 2:1 PS/PMMA was investigated via transmission electron microscopy (TEM) and dynamic-mechanical analysis (DMA). Particles were found distributed in both of the blend’s domains and at the interface and an improvement of the storage module of 17% was found. Finally, kaolinite was used as a core to create true hybrid Janus nanodiscs, which were applied for compatibilizing an immiscible polymer blend of 2:1 PS/PMMA. It was possible to create two chemically distinct surfaces on the clay particle by addressing each of its two basal surfaces individually via simple, but selective, surface modification. Two diblock copolymers were used to create the Janus structure, each one with a first block consisting of monomer units bearing the anchoring group for the respective surface and a second block, PS or PMMA, tailored to the polarity of the respective matrix polymer. Thus it was possible to direct the Janus particles straight into the interface between the polymeric domains, visualized by TEM images taken from solvent-cast nanocomposite films.