Nanocellulose (NC), which consists of a bundled structure of cellulose chains, is a class of shape-anisotropic materials with a diameter of several tens of nanometers and a length of over several hundreds of nanometers. Because the production of NC is based on the extraction process from natural resources in water, NC is generally obtained in an aqueous dispersion state. Thus, an aqueous dispersion of NC can be regarded as a precursor for NC-reinforced polymer composites. The objective of this study is to gain a better understanding of (1) local rheological properties of the NC aqueous dispersion at various length scales using a particle tracking measurement and (2) how they affect the mechanical properties of the resultant NC-reinforced polymer composite. The observation of individual particles at different positions revealed that the NC aqueous dispersion was spatially heterogeneous at the characteristic length of a few tens of micrometers. When the NC aqueous dispersion was sonicated and subsequently left undisturbed at room temperature, the length scale of the heterogeneity decreased. From the NC aqueous dispersion with and without the sonication treatment, having smaller and larger length scale of heterogeneity, respectively, poly(vinyl alcohol) (PVA) films containing NC were fabricated. The dispersion state of NC in the PVA matrix was improved by the prior sonication treatment. The addition of NC remarkably enhanced the ductility of the PVA film. However, this was the case only after the sonication treatment. The knowledge here obtained should provide a novel concept for polymer composites with tunable mechanical properties, based on the precursor-based design.
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