Hyperuniform anodic aluminium oxide (hAAO): a 2D metamaterial with improved mechanicalproperties for hard-soft bilayer composite actuators

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Hyperuniform disordered (HuD) structures are statistically isotropic without translational symmetry but exhibit a hidden symmetry by suppressing long-range density fluctuations. 2D HuD structures contain no grain boundaries or specific crystallographic directions along which crack propagation can occur. Thus, improved 2D mechanical metamaterials with superior fracture strength may be accessible by hyperuniformity design. Porous anodic aluminum oxide (AAO) membranes are produced by anodization of aluminum. They contain arrays of straight and parallel cylindrical pores oriented normal to the membrane plane with diameters ranging from a few 10 nm to a few 100 nm. Under appropriate conditions, self-ordering of the growing pores into hexagonal domains occurs. So far, research on AAO has predominantly aimed at the improvement of the pore ordering. Here, we consider the AAO pores as discrete elements enabling 2D in-plane hyperuniformity design. Thus, we plan to produce AAO with 2D hyperuniform pore arrangements (hAAO) resulting in improved resistance to fracture propagation and, therefore, improved fracture strength. For this purpose, rational design of disorder in AAO pore arrays will be achieved by departures from the narrow parameter windows (self-ordering regimes) in which mild anodization of aluminum results in self-ordered pore growth. In our preliminary work we already obtained nearly hyperuniform AAO, suggesting that AAO is a promising candidate for an effectively hyperuniform 2D mechanical metamaterial that nearly perfectly matches the ideal model theoretically devised by Torquato. Using hAAO as model system, we aim at the experimental validation of 2D hyperuniformity as a generic concept to optimize the mechanical properties of freestanding thin hard layers. By marrying the concepts “mechanical metamaterial” and “shape-changing material”, we will establish hAAO as platform for the design of hard-soft bilayer composites that can reversibly or permanently change their shape in response to triggers even under extreme conditions, such as high operating temperatures. As shown in preliminary experiments, AAO-polystyrene (PS) bilayer composites show pronounced reversible shape changes in response to temperature changes caused by the different thermal expansion behavior of PS and AAO. Adhesion and mechanical coupling between hAAO and polymer layer will be enhanced because the polymer partially infiltrates the hAAO pores. Also, hAAO enhances the hardness of the bilayer composites under operating conditions and reduces energy dissipation by unwanted local deformations during shape changes. In an exploratory activity, all-porous bilayer composites consisting of hAAO and a block copolymer (BCP) layer bent by volume expansion of the BCP caused by solvent swelling will be evaluated as curved crossflow ultrafiltration membranes with enhanced mechanical stability and improved anti-fouling behaviour.