Counterintuitive effect of molecular strength and role of molecular rigidity on mechanical properties of layer-by-layer assembled nanocomposites.

Molecular engineering of multilayered composites by layer-by-layer assembly (LBL) made possible easy replication of mechanical properties of nacre. Taking advantage of the ability of LBL to finely control the structure of the composite, one can further improve the mechanical properties of the multilayers, e.g., increase the strength and stiffness, and gain better understanding of the nanoscale and molecular scale mechanics of the materials critical for a variety of advanced technologies. In this study, we have replaced poly(diallyldimethylammonium chloride) (PDDA) (sigmaUTS approximately 12 MPa, E approximately 0.2 GPa) with a much stronger polysaccharide polycation, chitosan (CH, sigmaUTS approximately 108 MPa, E approximately 2 GPa), considering that its superior molecular strength will improve the macroscale mechanical properties of the nanocomposite: strength and stiffness. Free-standing films of the CH and montmorillonite (MTM) have been successfully made, and the resulting films revealed high uniformity with very high loading of MTM closely comparable to that in the natural nacre, approximately 80 wt %. Contrary to our expectations and theoretical predictions, the CH-MTM composite revealed lower strength and stiffness than those of PDDA-MTM and lower strength than CH polymer itself: sigmaUTS approximately 80 MPa and E approximately 6 GPa. Analysis of the morphology of adsorbing CH chains with atomic force microscopy revealed highly elongated molecules, which is opposite to the observations made for PDDA. Plane-to-plane adhesion showed a factor of approximately 4 lower strength when compared to PDDA-MTM nanocomposite. Altogether these facts support the conclusion that CH lacks flexibility necessary for strong adhesion and efficient load transfer between the organic matrix and MTM platelets. High rigidity of the CH chains does not allow them to acquire a conformation necessary for maximizing the interfacial attraction with nanoscale component of the composite. These observations create an important foundation in the experimental design of the high-performance nanocomposite materials.