Tailoring the Mechanical Properties of Montmorillonite-Nanocomposites via Surface-Bound RAFT-Polymer

Abstract

The successful preparation of Montmorillonite--polymer-nanocomposites with tailorable mechanical properties via surface grafting of RAFT polymers was presented. Strategies to precisely tune the properties of filler--matrix and matrix-free composites were developed and characterized via tensile testing. The possible application for composites with enhanced gas barrier properties was explored. Naturally occurring MMT was modified with a monomer for radical polymerization via an ion exchange procedure and subsequently employed in a grafting through RAFT polymerization of methyl acrylate. The successful surface modification with monomer and polymer was demonstrated with a variety of methods. TGA and EA both showed a significant increase of organic content after modification. via ATR-FTIR the presence of all components (MMT, surface grafted monomer and polymer) was proven in the resulting composites. The combination of IR-spectroscopy and AFM using s-SNOM allowed for the prove that the polymer can be bound firmly and that any free polymer can be separated from the sample, allowing for the production of hybrid nanosheets consisting only of MMT and surface-tethered polymer. SAXS was employed to show the successful intercalation of monomer and polymer in between the MMT layers resulting in exfoliation of the nanoclay and enabling access to a polymer modified filler with high aspect ratio. This was further supported by TEM which gave an optical impression of the change in particle morphology induced by the polymer modification. The first class of nanocomposites that was studied were composites consisting of a PMA matrix filled with nanosheets to reinforce the material. First of all, by comparing composites filled with nanosheets with either no modification, monomer modification, or polymer modification it could be shown that the strongest reinforcement is indeed observed when polymer coated nanosheets are used. Subsequently, the effect of the nanosheet content on the mechanical properties was studied. A nearly linear dependence of Young’s modulus and tensile strength on the filler content could be shown. The strain at break behaved anti-proportionally while the toughness stayed rather constant. In comparison with DMA this effect could be attributed to a reduced chain mobility of the polymer as consequence of the formation of a glassy layer around the particles. Additionally, the effect of the grafted polymers chain length was explored. This strategy also resulted in an enhanced modulus that linearly depends on the grafted chain length. The tensile strength was found to behave in the same way, however the effect is less pronounced as for the variation of the filler content. Characteristic for these composites was, that the plastic deformation stayed unmodified resulting a significantly enhanced overall toughness. The observed effects were attributed to enhanced entanglement of the surface bound chains and the matrix chains resulting in better stress propagation. Microscopy studies yielded further insight on the behavior of the nanosheets under external stress, revealing that individual particles break in tensile direction. It could also be shown that this has no negative impact on the materials performance. The second class of studied nanocomposites are matrix-free composites consisting only of nanosheets and surface-bound polymer without any free polymer chains. Using this technique, much higher nanosheet contents are possible. It could be shown, that for the formation of coherent and mechanically loadable composites, a minimal grafted chain length that allows chains to entangle with each other is required. Variation of the grafted polymer chain length could be used as a tool to adjust the MMT content and to tune the composites properties. The same dependency as described before was observed. Cross-linking of the polymer via addition of star polymer or the introduction of hydrogen bonding moieties in the polymer allowed for further reinforcement of the composite. To explore possible applications of the described techniques it was demonstrated that poly ethylene foils coated with PMA decorated nanosheets exhibit enhanced mechanical properties as well as enhanced gas barrier properties.