Prof. Dr. Axel Müller

Optimizing the reinforcement of polymers with nanoplatelets requires optimization of the aspect ratio and the moduli of the filler while providing a complete stress transfer. Employing a novel shear-stiff nano-mica with large aspect ratio, we focus on maximizing the interfacial interaction between filler and matrix. External surfaces of the nano-mica were selectively modified by a polycationic macro-initiator and two PMMA-polymer brushes of length below and above critical entanglement length, respectively, and the mechanical properties of the three PMMA composites were measured. The multiple electrostatic anchoring groups of the macro-initiator not only provide reliable adhesion but at the same time allow the variation of the degree of protonation providing a local match between the charge densities of the clay surface and the adsorbed macro-initiator. PMMA-coating of the nano-mica yielded long-term stable suspensions in THF that showed birefringence of a nematic phase.

Solution blending of the PMMA coated nano-mica allows for dispersing single tactoids in the translucent PMMA-composites at 5 wt% clay loading as determined by transmission electron microscopy (TEM). Although significantly improved mechanical properties could be achieved as compared to nanocomposites made with conventional clay fillers, the full potential – as expressed by Halpin-Tsai equations - of the PMMA coated nano-mica can still not be utilized. This is attributed to the non-wetting character of the densely packed PMMA brushes attached to planar nanoplatelets.