Web Release Date: April 25,
Counterintuitive Effect of Molecular Strength and Role of Molecular Rigidity on Mechanical Properties of Layer-by-Layer Assembled Nanocomposites
and
Departments of Chemical Engineering, Materials Science and Engineering, and Biomedical Engineering, University of Michigan, Ann Arbor, Michigan 48109, and Center for Nanoscience and Nanotechnology, Beijing, China
Received January 10, 2007
Revised February 22, 2007
Abstract:
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) (
UTS ~12 MPa, E ~0.2 GPa) with a much stronger polysaccharide polycation, chitosan (CH, UTS ~108 MPa, E ~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, ~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: UTS 
80 MPa and E 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 ~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.
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