Transverse viscoelastic properties of cellulosic fibers investigated by atomic force microscopy

Pulp fibers have not only anisotropic properties, but also exhibit a very rough surface due to their hierarchical structure. For this reason, an atomic force microscopy (AFM) method was developed to overcome the limitations due to the surface roughness of the pulp fibers and to provide comparable results of local viscoelastic properties in transverse direction at different relative humidity (RH) and in water. The evaluation of the experimental data combines contact mechanics and viscoelastic models which consist of springs and dashpots in series or parallel describing elastic and viscous behavior, respectively. Here, it will be demonstrated that the so-called Generalized Maxwell model of order two (GM2) yields reasonable results for two kinds of single fibers, viscose and pulp fibers, at different RH values and in water. Both fiber types show the same trend. The moisture changes at different RH lead to a steady decrease of the elastic and viscous parameters of the GM2 model with increasing RH. In water, however, all parameters show a jumplike drop by about two orders of magnitude indicating that the viscoelastic behavior in water is different.