Functional Layer-by-Layer films and aerogels of cellulose nanofibrils
Abstract: This thesis deals with the preparation of functional Layer-by-Layer (LbL) films of cellulose nanofibrils (CNFs) and polyelectrolytes. LbL films ranging in thickness from 10 nm to 5 mm were deposited onto both solid surfaces and porous nanocellulose aerogels in order to prepare functional surfaces and materials.The LbL deposition technique relies on the consecutive deposition from aqueous solution of macromolecules, in many cases polyelectrolytes, nano-particles, carbon nanotubes (CNTs) and biological molecules such as peptides and proteins, containing complementary charged groups to form complex thin films with a thickness from a few nanometers to a few micrometers. Unless otherwise stated, the LbL films described in the present thesis were assembled either by dipping a solid support into the dispersions/solutions or by filtering the dispersions through the porous support. In Paper I, freestanding layer-by-layer (LbL) films of anionic CNFs and a branched cationic polyelectrolyte, polyethyleneimine (PEI), were prepared and characterized in terms of their structural and mechanical properties. The consecutive build-up of PEI and CNFs on a hydroxylated and trifunctional organosilane-coated silicon substrate was monitored with X-ray photoelectron spectroscopy, a quartz crystal microbalance with dissipation and a dual polarization interferometry technique. Modification of the supporting substrate with hydrophobic molecules to form a thin precursor layer made it possible to peel the films from the substrate and perform tensile tests using a dynamic mechanical analysis method.In Paper II, a bio-mimetic surface modification approach was applied to prepare of adhesive LbL films of CNFs and PEI. Dopamine molecules bearing catechol groups were attached covalently to the fibril surface using carbodiimide-chemistry. The fibrils were cross-linked through the catechol groups to form organometallic complexes. The colloidal, interfacial and adhesive properties of the modified CNF differed considerably from those of their non-modified analogs. The degree of chemical modification had a significant influence on the LbL film deposition as well as on the colloidal properties of the CNF in aqueous dispersion. Metal-ion-induced complexation of the catechol groups with Fe3+ led to a strong adhesion of the thin films prepared with modified CNF to hydrophobic surfaces such as polystyrene.In Paper III, we report a robust and rapid LbL film deposition method for the preparation of a supercapacitor. The porous electrodes of single-wall CNTs and PEI were prepared by the filter-deposition technique onto a wet-resilient CNF aerogel. It has been shown that the cross-linked aerogels have an elasticity that eliminates the structural deformation over the filtration process. The charge density of the fibrils increased significantly due to the chemical structure of the cross-linker (BTCA). We demonstrate that the enhanced compressive strength, super elasticity in water and high charge density create a novel porous electrode structure for electrical charge storage devices.In papers IV and V, the light-weight, flexible and conductive thin papers and aerogels of cellulose nanofibrils were used for the preparation of energy storage devices. It has been shown that the strong interaction of CNF with electronically active hydrophilic nanomaterials such as functionalized carbon nanotubes makes it possible to prepare efficient energy storage devices. The unique properties of CNF-based materials such as high mechanical stiffness, elasticity in water and high porosity eliminate the common mechanical issues upon cycling of supercapacitors and improve the overall device performance.
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