TY - BOOK

T1 - Viscoelastic and plastic characterization of cellulosic materials on the submicrometer scale

AU - Czibula, Caterina Marina

N1 - no embargo

PY - 2020

Y1 - 2020

N2 - Cellulosic fibers are extensively used in paper and textile industries, but structure-property relations on the fiber scale are quite complicated and not yet fully understood. Changes in moisture content of single fibers have a strong impact on the physical properties and performance of paper. Therefore, it is of considerable interest to characterize the time dependency of the mechanical properties of single pulp fibers. In particular, their viscoelastic behavior has not been thoroughly studied in literature so far. One reason are experimental difficulties. Most industrial cellulosic fibers are processed wood fibers, which consist of several cell wall layers. The S2 layer, where the cellulose microfibrils arrange in an angle close to the longitudinal axis, is the thickest. Therefore, this layer dominates the mechanical behavior of the fibers, especially in longitudinal direction. Here, studies on the influence of relative humidity (RH) and the viscoelastic behavior of this layer are still missing. Furthermore, wood pulp fibers also exhibit a very rough surface due to their severe shrinkage in the production process. For this reason, an atomic force microscopy (AFM) method was developed to reduce the impact of fiber roughness on the results. Therefore, an experimental protocol needed to be established to reduce the roughness of the pulp fibers. The evaluation of the experimental data combines contact mechanics in the form of the Johnson-Kendall-Roberts (JKR) model and viscoelastic models, which consist of springs and dashpots in series or parallel describing elastic and viscous behavior, respectively. It will be demonstrated that the so-called Generalized Maxwell model (GM) yields reasonable results for single viscose and pulp fibers at different RH values and in water in transverse direction. The experimental curves have been fitted with a GM2 model. With increasing RH, the elastic and the viscous parameters are decreasing, whereas the relaxation times stay constant. In water, the GM3 model with an additional Maxwell element – introducing a third relaxation time – is used to fit the data properly. Compared to the results at different RH, the values of the elastic and viscous parameters show a pronounced drop in values of a few orders of magnitude. Furthermore, microtome cuts of the S2 layer are chemically and morphologically characterized. The viscoelastic properties are studied at different RH, and transverse and longitudinal direction are compared. Here, the GM3 model was applied to fit the experimental data for the S2 layer in longitudinal direction accurately. Surprisingly, little difference has been found between the longitudinal and the transverse direction. Additionally, the suitability of the experimental results for material modeling is presented. Experimental AFM indentation data can be used for the viscoelastic characterization of the matrix material in a minimum-input material model of the pulp fiber. The influence of RH is further investigated by bending experiments of cellulose thin films. A cantilever coated with a cellulose film on one side will bend upwards due to the swelling of the cellulose film if the RH is increased. This effect is visible on a short-time scale. However, on a long-time scale, the behavior gets more complicated and is also reproducible with an uncoated cantilever, leaving open questions. Besides cellulose-water interaction, the morphology changes due to phase separation of different cellulose blend film ratios are studied and the frictional behavior of these films is investigated. Furthermore, the applicability of friction force microscopy (FFM) to obtain semi-quantitatively mechanical parameters of cellulose blend films is demonstrated. It is revealed that it is possible by blending to tune the frictional, morphological, and mechanical properties of such films.

AB - Cellulosic fibers are extensively used in paper and textile industries, but structure-property relations on the fiber scale are quite complicated and not yet fully understood. Changes in moisture content of single fibers have a strong impact on the physical properties and performance of paper. Therefore, it is of considerable interest to characterize the time dependency of the mechanical properties of single pulp fibers. In particular, their viscoelastic behavior has not been thoroughly studied in literature so far. One reason are experimental difficulties. Most industrial cellulosic fibers are processed wood fibers, which consist of several cell wall layers. The S2 layer, where the cellulose microfibrils arrange in an angle close to the longitudinal axis, is the thickest. Therefore, this layer dominates the mechanical behavior of the fibers, especially in longitudinal direction. Here, studies on the influence of relative humidity (RH) and the viscoelastic behavior of this layer are still missing. Furthermore, wood pulp fibers also exhibit a very rough surface due to their severe shrinkage in the production process. For this reason, an atomic force microscopy (AFM) method was developed to reduce the impact of fiber roughness on the results. Therefore, an experimental protocol needed to be established to reduce the roughness of the pulp fibers. The evaluation of the experimental data combines contact mechanics in the form of the Johnson-Kendall-Roberts (JKR) model and viscoelastic models, which consist of springs and dashpots in series or parallel describing elastic and viscous behavior, respectively. It will be demonstrated that the so-called Generalized Maxwell model (GM) yields reasonable results for single viscose and pulp fibers at different RH values and in water in transverse direction. The experimental curves have been fitted with a GM2 model. With increasing RH, the elastic and the viscous parameters are decreasing, whereas the relaxation times stay constant. In water, the GM3 model with an additional Maxwell element – introducing a third relaxation time – is used to fit the data properly. Compared to the results at different RH, the values of the elastic and viscous parameters show a pronounced drop in values of a few orders of magnitude. Furthermore, microtome cuts of the S2 layer are chemically and morphologically characterized. The viscoelastic properties are studied at different RH, and transverse and longitudinal direction are compared. Here, the GM3 model was applied to fit the experimental data for the S2 layer in longitudinal direction accurately. Surprisingly, little difference has been found between the longitudinal and the transverse direction. Additionally, the suitability of the experimental results for material modeling is presented. Experimental AFM indentation data can be used for the viscoelastic characterization of the matrix material in a minimum-input material model of the pulp fiber. The influence of RH is further investigated by bending experiments of cellulose thin films. A cantilever coated with a cellulose film on one side will bend upwards due to the swelling of the cellulose film if the RH is increased. This effect is visible on a short-time scale. However, on a long-time scale, the behavior gets more complicated and is also reproducible with an uncoated cantilever, leaving open questions. Besides cellulose-water interaction, the morphology changes due to phase separation of different cellulose blend film ratios are studied and the frictional behavior of these films is investigated. Furthermore, the applicability of friction force microscopy (FFM) to obtain semi-quantitatively mechanical parameters of cellulose blend films is demonstrated. It is revealed that it is possible by blending to tune the frictional, morphological, and mechanical properties of such films.

KW - Viskoelastizität

KW - Cellulose-Materialien

KW - Mikromechanische Charakterisierung

KW - Rasterkraftmikroskopie

KW - Nanoindentation

KW - Viscoelasticity

KW - cellulosic materials

KW - micromechanical characterization

KW - atomic force microscopy

KW - nanoindentation

M3 - Doctoral Thesis

ER -