Relationship of hierarchical structure to mechanical properties

The mechanical properties in complex systems are explained based on the hierarchical structures present in the system. Hierarchical structures designed for specific mechanical responses are best exemplified by examples from biology. Collagen, a main component in soft connective tissues, is organized into hierarchical structures in the form of tendons or intervertebral discs as examples. Understanding these structures is vital in relating the structures to the intended properties. This approach is also used to characterize organic/inorganic natural composites such as human bone, reindeer antler and nacre. Another example of a hierarchical structure in biology with excellent mechanical properties is that of cellulose, when organized into wood. The importance of hierarchical structures also applies to synthetic polymers for a clearer understanding of the structure‐property relationships. Solid‐state biaxially oriented polypropylene has excellent tensile and impact properties, which are explained by the hierarchical structure induced during the processing. Thermotropic liquid crystalline polymers develop a hierarchical structure during injection molding that influence the final properties. Furthermore, the impact modification of polycarbonate is more easily understood when the system is explained in a hierarchical manner. It is also now possible to create or force hierarchical structures in synthetic polymers by microlayering technology. Several systems are outlined in which a hierarchical structure is created to enhance specific properties. SAN, a brittle polymer, can be microlayered with PC to create tough materials due to the scale, interaction and architecture of the microlayered composite. Another example is the effect of microlayered composite of PC/SAN on the interfacial adhesion mechanisms. Furthermore, toughening mechanisms in filled microlayers are examined based on the hierarchical structure.