Longitudinal and transverse low frequency viscoelastic characterization of wood pulp fibers at different relative humidity

The viscoelastic behavior of wood pulp fibers plays a fundamental role in the performance of paper and paper products. Wood pulp fibers are hierarchical composites consisting of different cell wall layers and have anisotropic properties. Since accessing the individual fibers is challenging, no measurement technique has been able to characterize the viscoelastic properties in both – the longitudinal and transverse – fiber direction yet. Here, an atomic force microscopy (AFM)-based method is applied to investigate the viscoelastic properties of wood pulp fibers at varying relative humidity (RH) in both fiber directions. Experimental creep tests have been performed to investigate the material's low frequency regime response. A spring-dashpot model has been applied to characterize the viscoelastic behavior. The results indicate surprisingly small differences of the properties between the fiber directions. Transferring the results into a spectral representation explains an opposing trend of the viscosity that is connected to the long-term behavior.