Study of the flow-induced structure and anisotropy in lyotropic liquid crystals for hierarchical composites

Licentiate thesis, 2020

Controlling the micro and nanostructure of materials is highly beneficial in order to tailor their physical properties. Extrusion-based 3D printing is a promising tool to produce hierarchical structures with controlled architecture. Combining additive manufacturing and self-assembled materials, complex structures with high anisotropy can be created. Lyotropic liquid crystals offer a wide variety of structures and compositions, in which hexagonal and
lamellar phases are very interesting options. Far from the idealistic concepts of 3D printing and extrusion, the variability of the different systems, physical properties of the inks and environmental conditions play a fundamental role in the appearance of imperfections, undesired nanostructures and the limitation in the maximum effective alignment achieved. To understand the mechanisms that induce alignment in liquid crystalline phases and produce secondary effects and imperfections, a combination of different methods was utilized. Using small-angle X-ray scattering as the main characterization tool, the nanostructure of the liquid crystals as well as the anisotropy was measured. The use of imaging techniques adds an extra dimension which brings a broader view of the self-assembled structure. Microfluidic channels were used to study the mechanisms of alignment in the confined space offered by the nozzle walls and the high pressures applied in the printing process. The confined flow in the printing nozzle has different properties and constraints compared to the open flow that the extruded filament encounters in the printing platform, which was studied by in-situ 3D printing in the X-ray beam. By complementary rheological characterization, a more detailed analysis understanding of the flow behaviour was achieved and birefringence microscopy opened up the possibilities of a time-resolved study of the anisotropy in the filament. The results demonstrated the role of the shear stress in liquid crystals in confined flow, highlighting both the effect it has on the anisotropy as well as on morphological transitions in the self-assembled structures. The performed experiments also reflect on the possible causes of misalignment such as stress release and try to find the optimal parameters in the nozzle design which lead to the best alignment in terms of homogeneity in the strand and maximizing the orientation. Finally, the results also show the importance of time and environmental conditions during 3D printing, which may affect the final structure and orientation prior the fixation of the nanostructure.