Olli Ikkala2 André Gröschel1 Esko Kauppinen2 Orlando Rojas3

2, Aalto University, Espoo, , Finland
1, University of Duisburg-Essen, Essen, , Germany
3, Aalto University, Espoo, , Finland

Biological materials provide inspiration for lightweight but mechanically excellent materials, suggesting energy efficiency especially in transportation applications. Nacre is a prototypical natural structural material combining high strength and toughness with lightweight construction due to its aligned self-assembled aragonite sheets glued by thin protein layers, thus serving as an inspiration for materials scientists. We show a facile route for nacre-mimetic composites by first coating clay nanosheets by polymers in aqueous dispersion, followed by their packing to films and their subsequent lamination for thicker sheets. This results in bulk clay/polymer nacre-mimetic nanocomposites with high clay alignment, showing strength 220 MPa and fracture toughness 3.4 MPa m1/2, approaching those of nacre (1). The plastic phenomena at the propagating crack tip are responsible for the toughening. Such fracture processes can be followed by laser speckle methods to get information on the microscopic void formation before the major fracture (2). Cellulose nanofibers are colloidal fibers with high mechanical properties due to their internal native crystalline structure. We show a facile way for porous films thereof using a sequence of solvent exchanges. Unlike classic aerogels, whose mechanical properties do not allow tensile testing, the present aerogels/xerogels show high tensile strength of 100 MPa and Young's modulus of 6 GPa under tension. With its low density (0.6 g/cm3), this leads to high specific strength. The films were coated with single-wall carbon nanotubes via aerosol filtration from the nanotube synthesis processes. Such routes provides highly conducting transparent films, which can have potential in flexible devices.
[1] M. Morits, T. Verho, J. Sorvari, V. Liljeström, M. Kostiainen, A. Gröschel, O. Ikkala, Toughness and Fracture Properties in Nacre-Mimetic Clay/Polymer Nanocomposites, Adv. Funct. Mater., 27, 1605378, 2016.
[2] T. Verho, P. Karppinen, A. Gröschel, O. Ikkala, Imaging Inelastic Fracture Processes in Biomimetic Nanocomposites and Nacre by Laser Speckle for Better Toughness, Adv. Sci, 2017, in press.
[3] Ambient Dried Cellulose Nanofibril Aerogel Membranes with High Tensile Strength and Their Use for Aerosol Collection and Templates for Transparent, Flexible Devices, M. S. Toivonen, A. Kaskela, O. J. Rojas, E. I. Kauppinen, O. Ikkala, Adv. Funct. Mater., 25, 6618, 2015.