Properties and interfacial behaviour of cellulose nanofibrils

Abstract

The properties and interfacial behaviour of cellulose nanofibrils were studied using cellulose model surfaces and surface methods such as quartz crystal microbalance with dissipation (QCM-D) and atomic force microscopy (AFM). The overall aim of the work was to characterise cellulose nanofibril materials, and to study the interactions of the nanofibrils with water, polymers and enzymes in order to understand the behaviour of nanofibrils in future nanotechnology applications. Nanofibrils were prepared from delignified pulp fibres using an enzymatic pre-treatment combined with mechanical shearing, followed by disintegration in a homogenizer. The procedure resulted in high-aspect-ratio nano-scale fibrils with a width of about 5 nm and a length up to several micrometers. The high-aspect-ratio nanofibrils were found to form an entangled network which under aqueous conditions forms high-water-content gel-like structures. Cellulose nanofibril model films were prepared by spin-coating aqueous nanofibril dispersions on silica substrates. These model surfaces were further utilised to study the interactions of the nanofibrils with water, enzymes and polymers. The interactions of the nanofibrils were shown to be similar to the interactions of cellulosic fibres, but the gel-like structure and large specific surface area of the nanofibrils affected many interfacial phenomena of nanofibrils, such as swelling, surface forces, enzymatic degradability, and the adsorption of polymers on nanofibril surfaces. Polymer adsorption on cellulose nanofibril surfaces was found to be highly dependent on the nature of nanofibril/polymer interaction. Adsorption of a neutral polysaccharide caused a dispersing effect of the nanofibrils and water uptake in the film, whereas adsorption of a cationic polyelectrolyte caused water removal and densification of the film. Choosing the right type of polymer interaction is essential for further applications where polymers and nanofibrils are mixed together. The potential of cellulose nanofibrils as a strength reinforcing agent was demonstrated by using nanofibrils together with a cationic polyelectrolyte to enhance paper strength. The paper strength experiments together with fundamental interaction studies showed that the use of nanofibrils as a strength reinforcement agent together with a cationic polyelectrolyte leads to significant improvements in paper strength properties.