Micromechanics of lignocellulosic fibers

Nowadays, our world is dominated by smart technologies, which massively influence our daily life. However, there is a natural and sustainable resource that has improved our way of living for a much longer time. Throughout history, wood has been an important construction material. Separating wood into its smallest fragments lignocellulosic fibers (LCFs) and treating them, leads to products which are connected to our everyday needs: paper, having transferred the written word through time, is a classic in many forms, and paperboard as a reliable packaging material ensures the comfort of online shopping and food delivery. Though the applications differ widely, the base material is always the same. An LCF has a complex hierarchical structure, which consists of several layers. The fiber shape can be imagined as a hollow cylindrical tube with pointed ends. While a tree trunk is massive, single LCFs are delicate. With 1-5 millimeters in length and a diameter of tens of micrometer (like a single hair), handling of single LCFs is not easy. These characteristics result in a lack of available experimental methods which can provide a detailed characterization of the fibers mechanical and structural properties. Since LCFs are the key component of many products, knowledge of their mechanical behavior is essential for improvements. Simply put: If the same mechanical performance can be obtained with less LCFs, less trees need to be cut down. Furthermore, modeling of fiber networks has gained importance and with increasing complexity of the models, the demand for experimental data that accurately represents the fibers behavior is rising. Here, the limits of mechanical techniques (MT) for single LCFs like tensile testing and nanoindentation will be overcome by implementing Brillouin light scattering microspectroscopy (BLSM) as an optical, non-contact method. BLSM is based on the inelastic scattering of light. Laser light is interacting with acoustic phonons, which causes a frequency shift of the Brillouin scattering peaks that can be related to the elastic properties of the LCFs. In a tensile test, it is only possible to access the mechanical properties in the testing direction, which is not sufficient for LCFs because they are anisotropic. BLSM enables the measurement of the full set of elastic constants in all three dimensions. Initially, BLSM will be adapted for LCFs by studying simple cellulosic materials and linking BLSM results to MT findings. In a next step, structural and moisture-induced changes in LCFs will be investigated. The measurement of elastic constants of LCFs with BLSM and their comparison to those known from MT will be essential. Furthermore, the suitability of BLSM data for fiber and fiber network models will be explored. Overall, it is expected that the implementation of BLSM within this project will shine a light on the micromechanical behavior of LCFs and will result in an improved understanding of their performance.

Lignocellulosic fibers are essential in everyday products like tissues, toilet paper, clothing, and packaging. These fibers are incredibly thin and short, making them challenging to study. The project, Micromechanics of Lignocellulosic Fibers, applied a laser-based technique called Brillouin spectroscopy to overcome these challenges. This non-contact method avoids complicated sample preparation and reveals the fibers' full mechanical behavior. Unlike conventional tests, which typically focus on one direction (usually along the fiber's axis), the developed approach captures the fiber's complex, direction-dependent (anisotropic) mechanical properties. The result is a complete description of the fiber's stiffness, which can improve the understanding of the product performance.