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

超均匀无序（HuD）结构在统计上是各向同性的，没有平移对称性，但通过抑制远程密度波动表现出隐藏的对称性。二维HuD结构不包含晶界，也不包含裂纹扩展的特定晶体方向。因此，通过超均匀性设计可以获得具有优异断裂强度的改进的二维机械超材料。多孔阳极氧化铝（AAO）膜是通过铝的阳极氧化制备的。它们包含垂直于膜平面的直的和平行的圆柱孔阵列，直径从10纳米到100纳米不等。在适当的条件下，生长的孔隙会自有序地形成六边形结构域。到目前为止，对AAO的研究主要是为了改善孔隙的有序性。在这里，我们将AAO孔隙视为可实现二维平面内超均匀性设计的离散元素。因此，我们计划生产具有二维超均匀孔隙排列（hAAO）的AAO，从而提高抗裂缝扩展能力，从而提高断裂强度。为此，合理设计AAO孔阵列的无序性将通过偏离窄参数窗口（自有序区）来实现，在窄参数窗口（自有序区）中，轻微的铝阳极氧化会导致自有序的孔生长。在我们的初步工作中，我们已经获得了接近超均匀的AAO，这表明AAO是一个有希望的有效超均匀二维机械超材料的候选者，它几乎完全符合Torquato理论设计的理想模型。以hAAO为模型系统，通过实验验证二维超均匀性作为一个通用概念，以优化独立薄硬层的力学性能。通过结合“机械超材料”和“变形材料”的概念，我们将建立hAAO作为设计硬-软双层复合材料的平台，即使在极端条件下，如高温下，也可以根据触发可逆或永久地改变其形状。初步实验表明，由于聚苯乙烯（PS）和AAO的热膨胀行为不同而引起的温度变化，AAO-聚苯乙烯（PS）双层复合材料的形状发生了明显的可逆变化。聚合物部分渗透到hAAO孔隙中，增强了hAAO与聚合物层之间的粘附和力学耦合。此外，hAAO提高了双层复合材料在工作条件下的硬度，并减少了形状变化过程中不必要的局部变形造成的能量耗散。在一项探索性活动中，由hAAO和嵌段共聚物（BCP）层组成的全孔双层复合材料将被评估为具有增强机械稳定性和改善抗污染性能的弯曲横流超滤膜。

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.