Defence of doctoral thesis: Linnea Björn – Synchrotron Imaging of Synthetic and Lignocellulose-based Packaging Materials

The widespread success of polymer-based packaging materials is associated with having melt processability, which allows to create products with complex shapes at a low cost. However, the resulting polymer morphology is influenced by a combination of factors, including material properties, processing conditions, and environmental factors such as humidity. These structural variations directly impact the mechanical performance of the packaging material and consequently, understanding the correlation between material, processing parameters, and resulting morphology remains an important challenge. Furthermore, to expand the use of renewable cellulosic materials, intrinsic limitations in cellulose that impede melt processing must be overcome. This can be achieved by chemically modifying the cellulose, however chemical modifications impact the morphology formed during processing. 
 
This thesis applies advanced X-ray based imaging techniques to investigate polymer based packaging materials, aiming to correlate their hierarchical structures with material performance. The thesis covers a diverse range of polymer materials, from conventional synthetic polymers to lignocellulose-based papers and chemically modified cellulose. In injection-molded polyethylene, scanning small and wide-angle X-ray scattering (SAXS/WAXS) combined with computational simulations was used to reveal a complex multilayered morphology with oriented structures near the sample edges. These organized structures were strongly influenced by material composition as well as local shear and cooling rates during the injection process and could be linked to enhanced mechanical strength (Papers I and II). In chemically modified cellulose, dialcohol cellulose is explored as a new path toward sustainable packaging, as it can be melt-processed both by itself and in composites with synthetic polymers. Scanning SAXS and WAXS showed that both degree of modification and processing conditions influenced the cellulose orientation, and that using humidity control during processing helps maintain favorable crystalline structures during processing (Papers III and IV). In non-modified cellulose fibers for drinking straw application, SAXS and WAXS demonstrated how liquid exposure and changes in relative humidity induced multiscale structural changes crucial for understanding the real-world performance of commercially available pulp materials (Paper V). Finally, the potential of using near-edge X-ray absorption fine-structure spectroscopy (NEXAFS) coupled with scanning transmission X-ray microscopy (STXM) was explored as a chemical imaging tool for fiber-based materials. Sample preparation strategies were shown to critically influence the quality of the measurement, influencing sample homogeneity, spectral quality and sensitivity to radiation damage (Paper VI). The combined findings of this work advance structural and chemical characterization of both fossil- and lignocellulose-based polymers, contributing to a deeper understanding of structure–property relationships critical for tailoring material performance in packaging applications.