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Nacre-mimetic bionanocomposites ofhigh montmorillonite (MTM) claycontent, prepared from hydrocolloidal suspensions, suffer from reducedstrength and stiffness at high relative humidity. We address thisproblem by chemical modification of xyloglucan in (XG)/MTM nacre-mimeticnanocomposites, by subjecting the XG to regioselective periodate oxidationof side chains to enable it to form covalent cross-links to hydroxylgroups in neighboring XG chains or to the MTM surface. The resultingmaterials are analyzed by FTIR spectroscopy, thermogravimetric analysis,carbohydrate analysis, calorimetry, X-ray diffraction, scanning electronmicroscopy, tensile tests, and oxygen barrier properties. We comparethe resulting mechanical properties at low and high relative humidity.The periodate oxidation leads to a strong increase in modulus andstrength of the materials. A modulus of 30 GPa for cross-linked compositeat 50% relative humidity compared with 13.7 GPa for neat XG/MTM demonstratesthat periodate oxidation of the XG side chains leads to cruciallyimproved stress transfer at the XG/MTM interface, possibly throughcovalent bond formation. This enhanced interfacial adhesion and internalcross-linking of the matrix moreover preserves the mechanical propertiesat high humidity condition and leads to a Young’s modulus of21 GPa at 90%RH. [ABSTRACT FROM AUTHOR]

Nacre-mimetic clay/polymer nanocomposites with clay platelet orientation parallel to the film surface show interesting gas barrier and mechanical properties. In moist conditions, interfacial adhesion is lowered and mechanical properties are reduced. Molecular dynamic simulations (MD) have been performed to investigate the effects of counterions on molecular adhesion at montmorillonite clay (Mnt)-xyloglucan (XG) interfaces. We focus on the role of monovalent cations K(+), Na(+), and Li(+) and the divalent cation Ca(2+) for mediating and stabilizing the Mnt/XG complex formation. The conformation of adsorbed XG is strongly influenced by the choice of counterion and so is the simulated work of adhesion. Free energy profiles that are used to estimate molecular adhesion show stronger interaction between XG and clay in the monovalent cation system than in divalent cation system, following a decreasing order of K-Mnt, Na-Mnt, Li-Mnt, and Ca-Mnt. The Mnt clay hydrates differently in the presence of different counterions, leading to a chemical potential of water that is highest in the case of K-Mnt, followed by Na-Mnt and Li-Mnt, and lowest in the case of Ca-Mnt. This means that water is most easily displaced from the interface in the case of K-Mnt, which contributes to the relatively high work of adhesion. In all systems, the penalty of replacing polymer with water at the interface gives a positive contribution to the work of adhesion of between 19 and 35%. Our work confirms the important role of counterions in mediating the adsorption of biopolymer XG to Mnt clays and predicts potassium or sodium as the best choice of counterions for a Mnt-based biocomposite design.

The development of clay bionanocomposites requires processing routes with nanostructural control. Moreover, moisture durability is a concern with water-soluble biopolymers. Here, oriented bionanocomposite coatings with strong in-plane orientation of clay platelets are for the first time prepared by continuous water-based processing. Montmorillonite (MTM) and a "new" unmodified biological polymer (xyloglucan (XG)) are combined. The resulting nanocomposites are characterized by FE-SEM, TEM, and XRD. XG adsorption on MTM is measured by quartz crystal microbalance analysis. Mechanical and gas barrier properties are measured, also at high relative humidity. The reinforcement effects are modeled. XG dimensions in composites are estimated using atomistic simulations. The nanostructure shows highly oriented and intercalated clay platelets. The reinforcement efficiency and effects on barrier properties are remarkable and are likely to be due to highly oriented and well-dispersed MTM and strong XG-MTM interactions. Properties are well preserved in humid conditions and the reasons for this are discussed.