Christian Ganser

Atomic force microscopy based manipulation of graphene using dynamic plowing lithography

Tapping mode atomic force microscopy (AFM) is employed for dynamic plowing lithography of exfoliated graphene on silicon dioxide substrates. The shape of the graphene sheet is determined by the movement of the vibrating AFM probe. There are two possibilities for lithography depending on the applied force. At moderate forces, the AFM tip only deforms the graphene and generates local strain of the order of 0.1%. For sufficiently large forces the AFM tip can hook graphene and then pull it, thus cutting the graphene along the direction of the tip motion. Electrical characterization by AFM based electric force microscopy, Kelvin probe force microscopy and conductive AFM allows us to distinguish between the truly separated islands and those still connected to the surrounding graphene.

Adhesion of cellulose fibers in paper

The surface topography of paper fibers is studied using atomic force microscopy (AFM), and thus the surface roughness power spectrum is obtained. Using AFM we have performed indentation experiments and measured the effective elastic modulus and the penetration hardness as a function of humidity. The influence of water capillary adhesion on the fiber-fiber binding strength is studied. Cellulose fibers can absorb a significant amount of water, resulting in swelling and a strong reduction in the elastic modulus and the penetration hardness. This will lead to closer contact between the fibers during the drying process (the capillary bridges pull the fibers into closer contact without storing up a lot of elastic energy at the contacting interface). In order for the contact to remain good in the dry state, plastic flow must occur (in the wet state) so that the dry surface profiles conform to each other (forming a key-and-lock type of contact).

AFM nanoindentation of pulp fibers and thin cellulose films at varying relative humidity

Abstract In papermaking, the formation of bonds between single pulp fibers is influenced by the hardness of the fibers in their wet state. In this work, transversal hardness and modulus of pulp fibers have been studied via atomic force microscopy-based nanoindentation in dependence on relative humidity (RH). Additionally, the change in hardness of cellulose and xylan/cellulose model films was also investigated as a function of swelling in the presence of water and calcium chloride (CaCl2) solution. The hardness of pulp fibers is decreasing slowly from 240 MPa at 5% RH to 90 MPa at 80% RH and exhibits a distinct decrease to 2.7 MPa at the fully wet state. The hardness in water is reduced by a factor of almost 100 compared with the dry state; therefore, a form change is easily possible and facilitates the formation of hydrogen bonds on the fiber surfaces. The investigations on the model films reveal that pure cellulose hardens in the CaCl2 solution, compared with distilled water, whereas xylan on cellulose is becoming softer.

Thin cellulose films as a model system for paper fibre bonds

Thin cellulose films on silicon substrates are used as a model system for paper fiber bonds. The films are formed by spincoating trimethylsilylcellulose on the substrates. The films are regenerated using HCl gas. After swelling in water, two samples can be bonded like a sandwich. It is shown that this model system can be used to measure the bond strength between the two films under controlled conditions. For a detailed characterization the films are studied in terms of roughness with atomic force microscopy (AFM). The hardness of the films is investigated by AFM-based nanoindentation. The chemistry and the thickness of the films is studied by infrared spectroscopy. It is shown that this model system enables the evaluation of different bonding mechanisms discussed in pulp and paper research. Our results clearly indicate that Coulomb interaction is an important bonding mechanism.

Cantilever bending based on humidity-actuated mesoporous silica/silicon bilayers

Summary We use a soft templating approach in combination with evaporation induced self-assembly to prepare mesoporous films containing cylindrical pores with elliptical cross-section on an ordered pore lattice. The film is deposited on silicon-based commercial atomic force microscope (AFM) cantilevers using dip coating. This bilayer cantilever is mounted in a humidity controlled AFM, and its deflection is measured as a function of relative humidity. We also investigate a similar film on bulk silicon substrate using grazing-incidence small-angle X-ray scattering (GISAXS), in order to determine nanostructural parameters of the film as well as the water-sorption-induced deformation of the ordered mesopore lattice. The strain of the mesoporous layer is related to the cantilever deflection using simple bilayer bending theory. We also develop a simple quantitative model for cantilever deflection which only requires cantilever geometry and nanostructural parameters of the porous layer as input parameters.

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.

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.

Application of the page-equation on flat shaped viscose fibre handsheets

The influence of charged species on the relative bonded area (RBA) of flat shaped viscose fibre handsheets was investigated. For this purpose four samples, a reference fibre without any additives as well as three physical modified fibres, were studied. The RBA was determined by the light scattering method and by using the so called Page equation. To use the Page equation, the breaking length of handsheets, the optical bonded area of fibre-fibre joints as well as the bond strength of such fibre joints were determined. The optical bonded area was analysed by polarized light microscopy. Using a micro bond tester, the bond strength of fibre-fibre joints was determined. Both techniques, light scattering and calculation via the Page equation did not deliver absolute values for RBA. Quite contrary, they only give approximate values. However, an increase of relative bonded area due to anionic charges is clearly seen. Additionally, the breaking length is increased by both additives.

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.

Data on synthesis and thermo-mechanical properties of stimuli-responsive rubber materials bearing pendant anthracene groups

The photo-reversible [4πs+4πs] cycloaddition reaction of pendant anthracene moieties represents a convenient strategy to impart wavelength dependent properties into hydrogenated carboxylated nitrile butadiene rubber (HXNBR) networks. The present article provides the 1H NMR data on the reaction kinetics of the side chain functionalization of HXNBR. 2-(Anthracene-9-yl)oxirane with reactive epoxy groups is covalently attached to the polymer side chain of HXNBR via ring opening reaction between the epoxy and the carboxylic groups. Along with the identification, 1H NMR data on the quantification of the attached functional groups are shown in dependence on reaction time and concentration of 2-(anthracene-9-yl)oxirane. Changes in the modification yield are reflected in the mechanical properties and DMA data of photo-responsive elastomers are illustrated in dependence on the number of attached anthracene groups. DMA curves over repeated cycles of UV induced crosslinking (λ>300 nm) and UV induced cleavage (λ=254 nm) are further depicted, demonstrating the photo-reversibility of the thermo-mechanical properties. Interpretation and discussion of the data are provided in “Design and application of photo-reversible elastomer networks by using the [4πs+4πs] cycloaddition reaction of pendant anthracene groups” (Manhart et al., 2016) [1].