Tailoring Adhesion and Wetting Properties of Cellulose Fibres and Model Surfaces Using Layer-by-Layer Technology
Sammanfattning: The versatile layer-by-layer (LbL) technique, for consecutive adsorption of polyelectrolytes and charged nanoparticles onto a substrate, was used to modify cellulose fibres and model surfaces for improved mechanical and wetting properties. In addition to being used to modify cellulose substrates, the LbL technique was also used to create cellulose surfaces suitable for high resolution adhesion measurements. LbL assembly of cellulose nanofibrils and polyethylenimine was used to prepare cellulose model surfaces on polydimethylsiloxane hemispheres which allowed for the first known Johnson-Kendall-Roberts (JKR) adhesion measurements between cellulose and smooth, well-defined model surfaces of cellulose, lignin and glucomannan. The work of adhesion on loading and the adhesion hysteresis were comparable for all three systems which suggest that adhesion between wood constituents is similar. The LbL technique was also used to decrease the hydrophilicity of paper, while improving the dry strength, by coating cellulose fibres with a polylallylamine hydrochloride (PAH) and polyacrylic acid (PAA) LbL film, followed by adsorption of anionic wax particles. Paper sheets made from the modified fibres were highly hydrophobic with a contact angle of 150°, while retaining, and in some cases improving, the tensile index of the paper. It was also observed that PAH/PAA modified sheets without the addition of wax became hydrophobic when heat treated. The mechanism behind the increased hydrophobicity was studied by the interface sensitive technique, vibrational sum frequency spectroscopy, which indicated that the increased hydrophobicity is a result of the reorientation of polymer chains to expose more hydrophobic CH2 and CH groups at the polymer-air interface. Paper sheets prepared from LbL-modified bleached softwood fibres using PAH and the biopolymer hyaluronic acid (HA) exhibited a 6.5% strain at break and a tensile index which was increased 3-fold compared to unmodified fibres. The wet adhesive properties of the PAH/HA system were studied by colloidal probe atomic force microscopy and correlated to film growth and viscoelastic behavior. The presence of background salt was a crucial parameter for achieving high adhesion but time in contact and LbL film thickness also strongly affected the adhesion. Finally, the wet adhesive properties of carboxymethylcellulose (CMC), which had been irreversibly adsorbed to regenerated cellulose, and polyvinylamine (PVAm) were evaluated by means of 90° peel tests. Strong wet adhesion was achieved for dried rewetted samples without any obvious chemical crosslinking, which was attributed to interdigitation and complex formation in PVAm-CMC films. This system also gave significant wet adhesion for non-dried systems at water contents around 45%.
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