TY - THES

T1 - Exploring chemical contrast on cellulosic materials with atomic force microscopy

AU - Czibula, Caterina Marina

N1 - embargoed until null

PY - 2016

Y1 - 2016

N2 - During papermaking and the production of regenerated cellulose fibers, hemicelluloses - such as xylan - are extracted. Like cellulose, the hemicelluloses are polysaccharides, but are – in contrast to the linear cellulose molecule – highly branched. So far, the utilization of the extracted hemicelluloses is limited. However, it has been reported that applying xylan as an additive to paper has a positive influence to the mechanical properties of paper. To understand the nature of this enhancement, the interaction between cellulose and xylan needs to be studied in detail. The goal of this work was to explore whether xylan can be selectively localized on the fiber surfaces. However, paper fibers are an inhomogeneous and complicated hierarchical system. Therefore, in this work, the adsorption of xylan to amorphous cellulose thin films was studied as a first step. Different model films were prepared on quartz-crystal-microbalance substrates and afterwards exposed to a solution of xylan with different filters, ionic strengths, and pH. In the final part of this work, also paper fibers with adsorbed xylan were investigated. Atomic force microscopy (AFM) was employed to characterize the surface topography of films and fibers. To obtain chemical contrast between cellulose and xylan, OH- and CH₃-functionalized AFM tips were used. If the functionalized tip would interact differently with xylan than it does with cellulose, a phase shift in tapping mode AFM would occur, allowing to distinguish xylan from the cellulose background. By also employing so-called AFM force mapping, the spatially resolved adhesion force between functionalized tip and surface can be recorded. The results for the cellulose/xylan thin films showed that the specific surface features are changing with varying ionic strength (0, 1 and 100 mmol NaCl) and pH (7, 8 and 9). These features can be either elongated, spherical, or dot-like. The elongated structures are located along the slope of the hills, but the others prefer to adsorb mostly on top of the film’s hill-like structure. Paper fibers are much rougher than the model thin films. Their surfaces look wrinkled and microfibrils and fibril bundles are visible. Here, xylan seems to adsorb as nearly spherical features with a diameter of about 100 nm and 20 nm – 30 nm in height. For the chemical investigation with OH- and CH₃-functionalized probes, cellulose/xylan films as well as paper fibers showed a contrast in phase as well as in force maps. Since both functionalizations – although chemically different – showed the same contrast, it is likely that there is a topographic influence. In addition, it was observed that a phase or adhesion contrast always corresponded to a change in topography. Although a clear chemical contrast could not be verified between cellulose and xylan, the investigation brought important insight into the established AFM methods and provided a better understanding of probe-sample surface interaction.

AB - During papermaking and the production of regenerated cellulose fibers, hemicelluloses - such as xylan - are extracted. Like cellulose, the hemicelluloses are polysaccharides, but are – in contrast to the linear cellulose molecule – highly branched. So far, the utilization of the extracted hemicelluloses is limited. However, it has been reported that applying xylan as an additive to paper has a positive influence to the mechanical properties of paper. To understand the nature of this enhancement, the interaction between cellulose and xylan needs to be studied in detail. The goal of this work was to explore whether xylan can be selectively localized on the fiber surfaces. However, paper fibers are an inhomogeneous and complicated hierarchical system. Therefore, in this work, the adsorption of xylan to amorphous cellulose thin films was studied as a first step. Different model films were prepared on quartz-crystal-microbalance substrates and afterwards exposed to a solution of xylan with different filters, ionic strengths, and pH. In the final part of this work, also paper fibers with adsorbed xylan were investigated. Atomic force microscopy (AFM) was employed to characterize the surface topography of films and fibers. To obtain chemical contrast between cellulose and xylan, OH- and CH₃-functionalized AFM tips were used. If the functionalized tip would interact differently with xylan than it does with cellulose, a phase shift in tapping mode AFM would occur, allowing to distinguish xylan from the cellulose background. By also employing so-called AFM force mapping, the spatially resolved adhesion force between functionalized tip and surface can be recorded. The results for the cellulose/xylan thin films showed that the specific surface features are changing with varying ionic strength (0, 1 and 100 mmol NaCl) and pH (7, 8 and 9). These features can be either elongated, spherical, or dot-like. The elongated structures are located along the slope of the hills, but the others prefer to adsorb mostly on top of the film’s hill-like structure. Paper fibers are much rougher than the model thin films. Their surfaces look wrinkled and microfibrils and fibril bundles are visible. Here, xylan seems to adsorb as nearly spherical features with a diameter of about 100 nm and 20 nm – 30 nm in height. For the chemical investigation with OH- and CH₃-functionalized probes, cellulose/xylan films as well as paper fibers showed a contrast in phase as well as in force maps. Since both functionalizations – although chemically different – showed the same contrast, it is likely that there is a topographic influence. In addition, it was observed that a phase or adhesion contrast always corresponded to a change in topography. Although a clear chemical contrast could not be verified between cellulose and xylan, the investigation brought important insight into the established AFM methods and provided a better understanding of probe-sample surface interaction.

KW - AFM

KW - cellulose

KW - hemicellulose

KW - adhesion force

KW - force mapping

KW - phase contrast

KW - functionalized probes

KW - AFM

KW - Cellulose

KW - Hemicellulose

KW - Adhäsionskraft

KW - Force mapping

KW - Phasenkontrast

KW - funktionalisierte Spitze

M3 - Diploma Thesis

ER -