We investigate the potential of Brillouin Light Scattering (BLS) Microspectroscopy for fast non-invasive all-optical assessment of the mechanical properties of viscose fibers and bleached softwood pulp. Using an optimized Brillouin spectrometer, we demonstrate fast spatial mapping of the complex longitudinal modulus over extended areas ([ 100 lm). Our results reveal that while the softwood pulp has a relatively uniform moduli, the viscous fibers have significant spatial heterogeneous in the moduli. Specifically, the viscose fibers exhibited a regular pattern of increasing and decreasing modulus normal to the fiber axis. The potential influence of a locally changing refractive index is investigated by holographic phase microscopy and ruled out. We discuss our results in light of the anisotropic mechanical properties of the fibers and are able to estimate the relative difference between the modulus along the fiber axis and that perpendicular to it. Results are presented alongside reference measurements of the quasi-static mechanical properties transverse to the fiber axes obtained using AFM-nanoindentation which reveal a similar trend, hinting at the potential usefulness of BLS for mechanical characterization applications. However, more detailed investigations are called for to uncover all the factors influencing the measured high-frequency BLS modulus and its significance in relation to physical properties of the fiber that may be of practical interest.

The influence of strain during drying is known to have a large effect on its mechanical properties like tensile strength, Young’s modulus and hygroexpansion. In this study we investigate free and restrained dried paper and investigate the relation between the paper microstructure and its mechanical properties. The first part of the work investigates the development of the mechanical properties as well as paper internal stresses and strains (elastic-, inelastic- and hygrostrain) at different moisture contents during the drying process. Emphasis is put on the changes of hygrostrains and the paper hygroexpansion coefficient during drying. One main finding was that in constrained drying the drying stresses are considerably below the yield stress and, as a consequence, the deformations are mainly inelastic (creep) and only marginally elastic. In the second part we are analyzing the microstructure of free and restrained dried sheets by X-ray microtomography ($$\mu$$ μ -CT). From the $$\mu$$ μ -CT analysis relevant network parameters such as number of contact points, fiber bond area, free fiber length and fiber curl were extracted. Also a novel method to investigate the interface angle of the fibers in the vicinity of the bond sites was established. The results showed that the major difference from the drying procedure manifests itself in the out-of-plane curl of the fibers. Statistically number of contact points, bond area and in-plane curl were not different whereas in-plane curl and free fiber length were different. The interface angle appears to be slightly affected by the drying procedure but is overall very low.

The degree of molecular contact, i.e. the contact area on the nanometer scale, between paper fibers is crucial for the van-der-Waals and hydrogen bond adhesion between the fibers and thus for the fiber-fiber bond strength. We apply Förster resonance energy transfer (FRET) to investigate the degree of contact in the distance range of 1-10 nm between pulp fiber bonds and between thin films. The FRET system with DCCH and FTSC as fluorescence dyes has been validated for spectrophotometry and for local imaging with widefield microscopy, using pHema thin films. Bonding between thin films can be detected with this Electronic supplementary material The online version of this article (