Nanomechanical characterisation methods for cellulosic fibres

Paper is a material that has been used for thousands of years. It is made from natural and renewable raw materials. It consists of randomly distributed fibres that form a whole sheet of paper through their connections. Because paper is also recyclable and can be produced in large quantities, it is a promising substrate material for various applications such as sensor technologies or microfluidics. However, the technical application of paper is impeded by the loss of mechanical stability when paper comes into contact with moisture. Thus, the loss of mechanical properties must be understood under the influence of relative humidity and the wet strength must be improved. In this work, advanced atomic force microscopy-based methods are introduced to characterise the mechanical behaviour of cellulosic fibres under humid conditions. Furthermore, hydrophobic terpolymers are introduced as wet strength polymers and investigated as function of the relative humidity to show the enhancement of the wet strength of the cellulosic fibres. At first, the mechanical properties of individual, freely suspended cotton linter fibres were investigated as a function of the relative humidity. Individual fibres were bent stepwise by static atomic force microscopic bending tests along the longitudinal axis with a colloidal probe. This created a detailed picture of the mechanical behaviour of the fibre as a function of the relative humidity. The data were evaluated in combination with confocal laser scanning microscopy and scanning electron microscopy. This enabled insights into the dependency of swelling, bending ability, contact stress, and stress-strain diagrams and the macroscopic fibril orientation on the fibre surface. In addition, the intrinsic mechanical properties of cellulosic fibres were determined by force- volume mapping with atomic force microscopy. Here, the mechanical properties of unprocessed cotton fibres were compared with processed cotton linter fibres in the dry and wet state. By measuring the local elastic modulus as function of the fibre depth, the wall structure of the fibres could be assumed. The wall structure was also observed by dying the fibres with fluorescence-active protein markers using confocal fluorescence laser scanning microscopy. This allowed the investigation of the papermaking process and the removal of the upper layers. In addition, the influence of fibre wetting on the predicted wall structure was investigated. Subsequently, the influence of the relative humidity on the mechanical properties of cotton linter and eucalyptus fibre bundles as well as single fibres was examined. In addition, theeffectiveness in improving the wet strength of a promising polymer coating (terpolymer P(S- co-MABP-coPyMa)) was determined. The polymer coating distribution was revealed with scanning electron microscopy and fluorescence microscopy. Contact angle measurements confirmed the hydrophobic character. With the help of Raman spectroscopy, it could be demonstrated that the amount of water that was absorbed differed between the coated and uncoated fibres. Static and quasi-static atomic force microscopy measurements revealed differences in the mechanical properties between the cotton linter and eucalyptus fibres and the polymer-coated fibres in different states of relative humidity. In a last step, the stability of fibre-fibre joints was investigated via a static atomic force microscopy-based method. Through static force-distance curves and a high spring constant cantilever the fibre-fibre joints could be displaced and the maximum applied force was determined. Additionally, the relative humidity was varied to investigate the behaviour of the joints under these humid conditions. The maximum applied force on the fibre-fibre joint was related to the bonded area and the angle between the two fibres to identify the more characteristic parameter for fibre joint strength. Furthermore, a technology promising terpolymer coating P(DMAA-co-MABP-co-RhodBMA) was investigated to prevent softening of the joints with increasing RH. The polymer coatings were tested on a H2O and an IPA solution basis and exhibited both an increase in fibre-fibre joint strength under dry and humid conditions