Relationship of hierarchical structure to mechanical properties

The mechanical properties in complex systems are explained based on the hie rarchical structures present in the system. Hierarchical structures designe d for specific mechanical responses are best exemplified by examples from b iology. Collagen, a main component in soft connective tissues, is organized into hierarchical structures in the form of tendons or intervertebral disc s as examples. Understanding these structures is vital in relating the stru ctures to the intended properties. This approach is also used to characteri ze organic/inorganic natural composites such as human bone, reindeer antler and nacre. Another example of a hierarchical structure in biology with exc ellent mechanical properties is that of cellulose, when organized into wood . The importance of hierarchical structures also applies to synthetic polym ers for a clearer understanding of the structure-property relationships. So lid-state biaxially oriented polypropylene has excellent tensile and impact properties, which are explained by the hierarchical structure induced duri ng the processing. Thermotropic liquid crystalline polymers develop a hiera rchical structure during injection molding that influence the final propert ies. Furthermore, the impact modification of polycarbonate is more easily u nderstood when the system is explained in a hierarchical manner. It is also now possible to create or force hierarchical structures in synthetic polym ers by microlayering technology. Several systems are outlined in which a hi erarchical structure is created to enhance specific properties. SAN, a brit tle polymer, can be microlayered with PC to create tough materials due to t he scale, interaction and architecture of the microlayered composite. Anoth er example is the effect of microlayered composite of PC/SAN on the interfa cial adhesion mechanisms. Furthermore, toughening mechanisms in filled micr olayers are examined based on the hierarchical structure.