Tailored layer-by-layer films of nanofibrillated cellulose

Abstract: The preparation of multifunctional films, coatings and membranes from natural, sustainable and low-cost raw materials has attracted considerable interest during the last decade. In this respect, cellulose-based products possess a great promise for research and industry due both to the availability of large amounts of material in nature and to the preparation of new classes of nano-sized and well-characterized building-blocks of cellulose from trees or annual plants. This thesis deals with the preparation of nanofibrillated cellulose (NFC)-based layer-by-layer (LbL) films. The LbL deposition technique relies on the consecutive deposition of charged materials, in many cases polyelectrolytes, nano-particles and biological macromolecules, containing complementary charged groups from aqueous solution to form complex thin films with a thickness from nanometers to a few micrometers. The negatively charged carboxymethylated cellulose nanofibrils with a diameter of 3-5 nm and a length of 2-3 μm have been combined with a positively charged and hyperbranched polyelectrolyte, polyethyleneimine (PEI), to build-up multilayer films on solid substrates. In the first study, self-supporting layer-by-layer (LbL) films of anionic carboxymethylated cellulose nanofibrils (NFC) and a cationic branched polyelectrolyte, polyethyleneimine (PEI), have been prepared and characterized in terms of their structural and mechanical properties. The consecutive build-up of PEI and NFC on a hydroxylated and trifunctional organosilanecoated silicon substrate was monitored with X-ray photoelectron spectroscopy (XPS), quartz crystal microbalance with dissipation (QCM-D) and dual polarization interferometry (DPI) techniques. The hydrophobic functionalization of the supporting substrate with trichlorosilanes made it possible to easily peel off the films from the substrate and perform tensile tests using a dynamic mechanical analysis (DMA) method. The need to overcome some critical features of the cellulose-based materials such as weak wet-stability and high hydrophilicity was the basis of a study on nanofibril functionalization. In this respect, a biomimetic approach inspired by marine mussel threads was adopted by chemical modification of the nanofibrils with dopamine (DOPA) molecules, as shown in the second study. As expected, the colloidal, interfacial and adhesion properties of the modified NFC differed considerably from those of their non-modified analogs. The degree of chemical modification had a significant effect on the LbL film deposition as well as on the colloidal properties of the NFC in aqueous dispersions. Moreover, metal-ion-induced complexation of the catecholic groups with Fe3+ led to a strong adhesion of the thin films prepared with modified NFC to hydrophobic surfaces such as polyethylene.