Interfacial forces in nanocellulose based composite materials

Abstract

In this work, fundamental interactions in nanocellulose systems and their utility in fabricating novel and advanced cellulose-based materials was studied. More specifically two major aims were investigated. The first concentrates on cationic NFC and its morphology, its fundamental behavior with relation to charge and solvent, and its interactions with other nanocellulose particles. In the second, the approach focuses on the interfacial interactionbetween NFC and polymers (polysaccharides) to assess the importance of their mutual interplay on the mechanical performance of the nanocomposite films. Atomic force microscopy(AFM) in different imaging conditions (liquid and air, and different force detection modes) and the quartz crystal microbalance with dissipation (QCM-D), and tensile tests were combined to carry out this research.

The dimensions of cationic (cat) NFC were determined through AFM imaging. Individual nanofribils were found to be smaller in liquid than in air, which pointed at the importance of characterizing nanocellulosic material in situ, before drying. The influence of electrolyte concentration and pH on the swelling of cat NFC films was studied using the QCM-Dtechnique. Cationically modified fibrils were amphoteric in nature; therefore, the swelling of the film was associated with pH conditions. Films with higher water content were observed at lower pH. Cationic NFC incorporated with anionic CNCs was also examined in terms of layerbuild-up. The significance of a dense and well-constructed first layer for successful multilayers formation was shown. Layer-by-layer (LbL) films were further measured by applying thecolloidal probe AFM technique which pointed to the importance of charge and the interpenetration of the layers in the behavior of the films. Interfacial forces between NFC and the polysaccharide or polymer were studied by applying colloidal probe AFM and QCM-D. The importance of the aqueous lubrication of CMC-g-PEG between the NFC segments for the stability and film formation were assessed by studying lateral responces between the interacting particles. The adsorption of CMC-g-PEG markedly increased stability, repulsions and the lubrication in liquid. The reduction of the friction coefficient compared to the bare NFC film was observed both in liquid and in air. This lubricating effect was evaluated in termsof the mechanical properties of films composed of NFC/CMC-g-PEG. An NFC based nanocomposite with a high fraction of reinforcement was prepared by ionic complexation of anionically charged NFC and cationic block copolymers micelles. This prevented macroscopic phase separation and gave rise to composites with an alternating nanoscale hard/softarchitecture. Due to the nanoscopic control over fracture, energy dissipation a significantly larger strain and toughness was achieved.