Caterina Czibula

Modifying cellulose fibers by adsorption/precipitation of xylan

Xylan was precipitated on bleached and unbleached softwood kraft pulps (SKPB and SKPUB), a bleached sulfite pulp, and viscose fibers to investigate the influence on physical properties of handsheets as well as to gain more detailed information of the distribution of adsorbed xylan on cellulosic surfaces. The adsorption step was carried out at constant conditions. The values of the parameters were chosen after a series of different adsorption steps (as presented elsewhere) also in regard as a possible industrial application. Afterwards, suspension and physical strength properties of unrefined and refined samples were determined. Refining resulted in a stronger increase in the beating degree of the pulps with additional xylan than the reference samples. The water retention value was not significantly changed. Comparing the tensile index of the handsheets versus the beating degree a remarkable increase in the strength properties was not observed. The investigation of the surface of the SKPB fibers via ATR suggests that precipitated xylan is heterogeneously and nonuniformly distributed. On cellulose model films, particles could be detected via atomic force microscopy. Phase images recorded with OH-functionalized tips suggested that these particles consist of xylan. An inhomogeneous distribution of xylan could be a reason that there is no measurable influence on the strength properties of the handsheets.

How xylan effects the breaking load of individual fiber–fiber joints and the single fiber tensile strength

Bleached and unrefined softwood kraft fibers were treated with aqueous solutions of xylans and potassium hydroxide (KOH) to vary the amount of xylan present on the fiber surface and within the fiber wall. This was done in order to directly measure the influence of xylan on the joint strength of individual fiber–fiber joints as well as the tensile strength of individual fibers to determine the elastic modulus. The results were compared with two unbleached and unrefined softwood kraft pulps (Kappa 42 and 54). Additional xylan had a statistically significant effect on the mean values of the breaking load of individual fiber crossings. Moreover, the breaking load was strongly affected by the cooking time during pulping. However, no correlation between the optical bonded area and the joint strength was observed. The modulus of elasticity of individual fibers was not influenced by additional xylan or alkaline extraction. The elastic modulus was influenced more by chemical pulping and bleaching. The results indicate that additional xylan on the fiber surface has a strength-enhancing effect on the joint strength individual fiber crossings which also depends on the distribution of xylan. On the other hand, the elasticity of a fiber seems to be more influenced by the morphology and the composition of the fiber walls, especially in the secondary S2 wall.

Distributed Bragg reflectors: Morphology of cellulose acetate and polystyrene multilayers

The optical quality and photonic properties of all-polymer distributed Bragg reflectors are related to the morphology of the layers and the optical responses of the materials. We introduce the X-ray reflectivity method to determine the thickness, the interface- and surface-roughness of cellulose acetate and polystyrene layers which are two polymers often used in the domain of spin casted multilayer systems. Atomic force microscopy and spectroscopic ellipsom-etry were used as complementary techniques for investigating the surface roughness and the film thickness. The shrinkage and the change of interface roughness of the polymers were investigated up to temperatures of 200 °C Up to 170 °C the interface roughness stays constant at about 1 nm while it increases up to 2nm at 200 °C. The thickness of the polystyrene layer remains constant up to 170 °C, well above its glass transition temperature Tg. For cellulose acetate a monothonic decrease is observed with increasing temperature. It could be shown, that the change in the optical response of a thermally treated distributed Bragg reflector is related to the change of the layer thickness of cellulose acetate. Spectra of (PS CA)20PS distributed Bragg reflectors (DBR) are in a good agreement with calculated spectra with parameters optained from of the X-ray reflectivity measurements.

Topography effects in AFM force mapping experiments on xylan-decorated cellulose thin films

Abstract Xylan-coated cellulose thin films has been investigated by means of atomic force microscopy (AFM) and force mapping experiments. The birch xylan deposition on the film was performed under control by means of a multiple parameter surface plasmon resonance spectroscopy (MP-SPR) under dynamic conditions. The coated films were submitted to AFM in phase imaging mode to force mapping with modified AFM tips (sensitive to hydrophilic OH and hydrophobic CH3 groups) in order to characterize and localize the xylan on the surfaces. At the first glance, a clear difference in the adhesion force between xylan-coated areas and cellulose has been observed. However, these different adhesion forces originate from topography effects, which prevent an unambiguous identification and subsequent localization of the xylan on the cellulosic surfaces.