Cellulose nanofibrils-reinforced polymer nanocomposite via physical adsorption at fluid/fluid interfaces

Abstract

Polymer nanocomposites have been extensively used from daily supplies to advanced technologies including aerospace fields, and play an important role in improving the properties required by industries. Uniform dispersion of nanoparticles in a polymer matrix is essential for the fabrication of the polymer nanocomposite to exhibit its superior performance. However, uniform dispersion of nanoparticles in the polymer matrix is difficult because of the intermolecular interactions between nanoparticles and incompatibility between the nanoparticle and the polymer matrix. To overcome the poor compatibility between nanoparticles and polymer matrices, surface modifications of nanofillers are generally applied to change the surface of nanofillers into hydrophobic surfaces. However, it often results in the change of original properties of the surface-modified nanoparticles during the surface modification process. Therefore, a dispersion technology capable of dispersing nanoparticles in a polymer matrix is needed without any surface-modification approaches. In this dissertation, research on cellulose nanofibrils-polymer nanocomposites through physisorption is introduced. Dispersion of hydrophilic cellulose nanofibrils in a hydrophobic polymer matrix is attainable from particle adsorption at the interface, which enables cellulose nanofibrils to adsorb on the surface of hydrophobic polymer particles. The key principle of the dispersion technology is to adsorb cellulose nanofibrils onto the interface stably. In order to understand the principle of adsorption of cellulose nanofibrils to the interface, the dynamic process of particle adsorption was analyzed using a model system composed of an oil drop and a cellulose nanofibrils monolayer. The model system showed that the drainage time of the thin film between the oil droplet and the monolayer plays a critical role in determining whether or not particles adsorb onto interfaces, and that the drainage time can be controlled by modulating the hydrodynamic interaction. Based on a correlation between particle adsorption and hydrodynamic interactions, the well-dispersed polymer nanocomposite film was prepared based on particle adsorption making cellulose nanofibrils adsorb onto the surface of hydrophobic polymer particles: polymethyl methacrylate (PMMA) and polypropylene (PP), respectively. The mechanical stiffness of both PMMA/CNFs and PP/CNFs was increased by 80%. The increased mechanical stiffness was consistent with the theoretically predicted value. Despite the substantial increase of the modulus, the brittleness of the composite became notably worse due to the incompatibility between CNFs and PP. To solve the incompatibility, the PP matrix and CNFs were the bridged via an esterification, thereby the elongation at break of the bridged PP/CNFs composite was improved by 10-fold compared to PP/CNFs composites. This study shows that the dynamic process of particle adsorption is a critical process to determine particle adsorption at the interface, and comprehensively presents the relationship between the dynamic process and particle adsorption for the first time. The relationship provides a fundamental understanding of the dynamic process of particle adsorption and offers important insights into how dynamic processes quantitatively control particle adsorption, which is difficult to explain with conventional thermodynamic theories. This dissertation is expected to provide an effective strategy to control dispersion of nanocomposite materials based on the interfacial physisorption principle, as well as pave the way to find novel applications with broad impact in both academia and industry.