Bioinspired Assembly of Inorganic Nanoplatelets for Reinforced Polymer Nanocomposites

Steel and metal alloys have long been used for structural applications because they are both strong and flaw-tolerant. By contrast, ceramics, which are strong but not tolerant to surface flaws and cracks, and polymers, which are flaw-tolerant and can deform under applied stresses, are less favourable as structural materials. However, nature has resolved this dilemma by millions of years of biological evolution. For example, the nacreous layer of mollusk shells(Jackson, Vincent et al. 1988; Aksay, Trau et al. 1996; Smith, Schaffer et al. 1999), which is made up of relatively weak components (consisting of 95 volume % of brittle aragonite platelets and 5 volume % of soft biological macromolecules), shows unexpected toughness (resistance to cracking) and stiffness (resistant to deformation)(Barthelat 2007). The oriented assembly of aragonite platelets and the intricate brick-and-mortar nanostructure found in the nacreous layer have been attributed to the major reasons for the exceptional toughness and stiffness of nacres. This unusual combination of different mechanical properties inspires scientists to create strong and flaw-tolerant artificial materials that mimic the mechanical design principles found in nacres by combining platelet-like ceramic building blocks with polymeric matrices(Tang, Kotov et al. 2003; Podsiadlo, Kaushik et al. 2007; Bonderer, Studart et al. 2008). Various bottom-up self-assembly techniques have been developed for creating biomimetic reinforced nanocomposites. Layer-by-layer (LBL) assembly of inorganic nanoplatelets and polyelectrolytes has recently been demonstrated as an efficient methodology in making reinforced polymer nanocomposites(Podsiadlo, Kaushik et al. 2007; Bonderer, Studart et al. 2008; Podsiadlo, Michel et al. 2008). The sequential adsorption of anionic montmorillonite clay platelets and poly(diallydimethylammonium) chloride polycation results in an organic– inorganic hybrid material that has ultimate tensile strength (~100 MPa) approaching to that of nacre (~130 MPa)(Tang, Kotov et al. 2003). By treating with glutaraldehyde to improve the bonding and load transfer between clay platelets and polymer matrices, nanocomposites made by LBL assembly of montmorillonite clay platelets and poly(vinyl alcohol) show even higher tensile strength (~400 MPa)(Podsiadlo, Kaushik et al. 2007). Ice-templated crystallization, gravitational sedimentation, centrifugation, as well as spin-coating have also been explored to assemble inorganic nanoplatelets into ordered structures(Almqvist, Thomson et al. 1999; Chen, Wang et al. 2008; Liu, Chen et al. 2008; Munch, Launey et al.