Polymer nanocomposites (PNCs) have received significant attention due to their enhanced material properties compared to neat polymers, making them indispensable in numerous industrial applications. The interactions between structure and properties in PNCs, particularly during non-equilibrium processing, are crucial for optimizing their performance in advanced manufacturing techniques such as 3D printing. Fused filament fabrication (FFF) is an emerging fabrication method for prototyping, tooling, and fabrication of unique parts in fields such as aerospace, robotics, and biomedical applications. While FFF offers several advantages over traditional manufacturing techniques, its potential for large-scale production is limited by structural imperfections. These imperfections, including residual stresses within printed parts, can result in issues like warping and layer delamination, ultimately reducing the mechanical performance of the final product. The objective of this study is to obtain a more complete understanding of the processes involved in the structure formation during FFF of polymer composites by combining time-resolved WAXS, which is sensitive to ordered structures on Angstrom length scales, with time-resolved coherent small angle scattering (c-SAXS), which provides information about structural processes on nano- to micrometer length scales. Understanding these unique elements is essential for analyzing the effects of individual processing steps on the final structures, properties, and performance of the materials. Isotactic polypropylene (iPP) was selected as a model material for this work due to its semicrystalline nature, which enables effective monitoring of structural evolution, along with its strong mechanical properties and excellent resistance to moisture, heat, and chemicals. Additionally, iPP-based composites have been widely used in high-performance applications. Specifically, this work focuses on a model system of monodisperse spherical silica nanoparticles (Si-NPs) dispersed within an iPP matrix. The NPs were synthesized by the Stober method and surface functionalized to facilitate dispersion in the iPP matrix. A twin-screw extruder was used to fabricate filaments with a diameter of 1.75 mm for 3D printing using commercial FFF printheads. Si-NPs with sizes of approximately 40 nm were incorporated to create different loadings, ranging from 0.5% to 5% by volume. The impact of shear profiles, influenced by nozzle sizes and print speeds, and temperature profiles, determined by the gradient of printbed and nozzle temperatures, on the crystallization of the extruded filaments were systematically examined. Structure formation during the 3D printing process is largely determined by spatially heterogeneous shear and temperature profiles, along with external stimuli. Spatially resolved measurements with micrometer-sized X-ray beams allow for the investigation of spatial heterogeneities, such as high-shear regions near the top surface or at filament interfaces. Besides the most common alpha phase, preliminary results reveal the presence of the iPP β-phase in the polymer composite matrix, primarily at the edges of the filament. The shear-induced formation of the β-phase during the FFF process couldenhance the toughness of polymer nanocomposites, thereby offering promising possibilities for improving the performance of 3D printed parts. In addition to time- and spatially resolved crystallization kinetics, simultaneous time-resolved coherent X-ray scattering (c-SAXS/XPCS) permits following structural relaxation processes, driven by residual stresses, even minutes after the molecular structure (as probed by WAXS) became static. Combining the results from simultaneous, time-resolved WAXS and c-SAXS enables the creation of scattering-based ‘imaging’ of the extruded filaments in terms of structure and mesoscale dynamics. This study demonstrates how combining different X-ray scattering techniques within the same in-situ experiment provides a more comprehensive understanding of the various processes that determine the structure-property relationships and their dependence on material and processing parameters in polymer FFF.
