Self-assemblies bridging the length scales for biomimetic and functional materials
Olli Ikkala, Aalto University, Espoo, Finland
Self-assemblies driven by competing interactions allow structures and soft functional materials inspired by biological materials. A challenge is to bridge the self-assemblies at different length scales to useful macroscopic materials properties. This talk addresses selected examples of self-assemblies at nanometers to two orders of magnitude larger ones, i.e., optical dimensions, to control mechanical, ionic, and optical functionalities. Inspired by the attractive mechanical properties of nacre, we first show that aligned clay/polymer self-assemblies at a periodicity of 2.6 nm allow nacre-mimetic bulk materials with high strength of 220 MPa combined with high fracture toughness of 3.4 MPa m1/2, approaching those of nacre (1). The fracture processes at the progressing crack tip can be followed by laser speckle methods (2). Upon forcing ionic liquid molecules with long range ionic interactions into ultra-confined ca. 2 nm thick ionic layers upon surfactant complexation driven by their repulsive alkyl tails, we show promoted ionic transport in comparison to the corresponding bulk ionic liquids (3). On the other hand, the long range of ionic interactions leads to block copolymer micelles with highly stretched polymeric corona in solvent media, leading to periodic structures with lattice sizes in the optical wavelength, i.e. 200-300 nm (4). This allows tunable structural colors by selecting the concentrations. On the other hand, structural white color can be obtained using anomalous light scattering from nanocellulose aerogels (5), where nanocelluloses with their a few nm thick mechanically strong fibrils allow a platform for functional self-assemblies (6). Combination of structural units of different sizes allows a generic route for hierarchical self-assemblies.
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