Artificial nacre-like bionanocomposite films from the self-assembly of chitosan-montmorillonite hybrid building blocks.

In the last decade, there has been a trend in chemistry to reduce the human impact on the environment. Special attention has been paid to the replacement of conventional petroleum-based plastics by materials based on biopolymers. However, the mechanical and thermal properties and functionalities of these biopolymers have to be enhanced to be competitive with the petroleum-based plastics from the viewpoint of practical applications. One of the most promising solutions to overcome these drawbacks is the elaboration of bionanocomposite, namely the dispersion of nanosized filler into a biopolymer matrix. Because of their functional properties, bionanocomposites as green nanocomposites based on biopolymers and layered silicates (clays) have received intensive attention in materials science. 4] Chitosan and montmorillonite (MTM), an abundant polysaccharide and a natural clay respectively, have been widely used as the constituents of bionanocomposites. The intercalation of chitosan into MTM and the dispersion of MTM nanosheets in the chitosan matrix have been systematically investigated. Bionanocomposites based on chitosan intercalation into MTM can be used as a sensor applied in the potentiometric determination of several anions. Bionanocomposite films formed through the dispersion of MTM nanosheets in the chitosan matrix have shown enhancement of the mechanical and thermal properties compared with the pure chitosan film. Unfortunately, the enhancement of the tensile strength and thermal stability of the chitosan–MTM bionanocomposite film is still low far from the expectations in industry. Systematic studies are carried out in materials science on natural materials with the objective of duplicating their properties in artificial materials. Natural nanocomposites provide prime design models of lightweight, strong, stiff, and tough materials due to the hierarchical organization of the micro and nanostructures. One attractive biological model for artificial material design is nacre (mother-of-pearl). The microscopic architecture of nacre has been classically illustrated as a “brick-and-mortar” arrangement that plays an important role in the amazing mechanical properties of the nacre. This arrangement is constituted of highly aligned inorganic aragonite platelets surrounded by a protein matrix, which serves as a glue between the platelets. Recently, the microstructure of the nacre has been mimicked by several innovative techniques to fabricate the artificial nacre-like materials with high mechanical performance. For example, layer-by-layer (LBL) deposition combining with cross-linking yielded poly(vinyl alcohol)/MTM nacre-like nanocomposites with a tensile strength of up to 400 MPa; the ice-crystal templates of the microscopic layers were designed to form a brick-and-mortar microstructured Al2O3/poly(methyl methacrylate) composite that is 300 times tougher than its constituents; the assembly of Al2O3 platelets on the air/water interface and sequent spincoating was developed into the fabrication of lamellar Al2O3/ chitosan hybrid films with high flaw tolerance and ductility; the self-assembly of nanoclays with polymers coating by a paper-making method resulted in the nacre-mimetic films; and nacre-like structural MTM–polyimide nanocomposites were fabricated by centrifugation deposition-assisted assembly. Our group has also fabricated nacre-like chitosanlayered double hydroxide hybrid films with a tensile strength of up to 160MPa by sequential dipping coating and the LBL technique. The concept of mimicking nacre and recently developed innovative techniques inspired us to fabricate the highly sustainable artificial nacre-like chitosan–MTM bionanocomposite film with high performance to seek a promising material for the replacement of conventional petroleumbased plastics. Herein, we introduce a novel approach to fabricate artificial nacre-like chitosan–MTM bionanocomposite films by self-assembly of chitosan–MTM hybrid building blocks (Scheme 1). The chitosan molecules are very easily coated onto exfoliated MTM nanosheets to yield the hybrid building blocks by strong electrostatic and hydrogen-bonding interactions. These hybrid building blocks can be dispersed in distilled water and then aligned to a nacre-like lamellar microstructure by vacuum-filtrationor water-evaporationinduced self-assembly because of the role that the orientation of the nanosheets and linking of the chitosan play. The fabrication process is simple, fast, time-saving, and easily scaled up compared with the LBL, ice-crystal-template, and other techniques. [*] H. B. Yao, Z. H. Tan, H. Y. Fang, Prof. Dr. S. H. Yu Division of Nanomaterials and Chemistry Hefei National Laboratory for Physical Sciences at Microscale Department of Chemistry National Synchrotron Radiation Laboratory University of Science and Technology of China Hefei, Anhui 230026 (P.R. China) Fax: (+ 86)551-360-3040 E-mail: shyu@ustc.edu.cn Homepage: http://staff.ustc.edu.cn/~ yulab/