Microbeam SAXS Reveals Flow-Induced Nanostructural Heterogeneity and Tilted Domains in 3D-Printed Block Copolymers.

The nozzle geometries associated with extrusion-based 3D printing impart significant spatial heterogeneities in both the type (shear vs extension) and rate of the flows applied during extrusion. For polymeric and nanostructured inks, these heterogeneous flow histories influence the underlying structure at the nano- to microscale. We previously demonstrated that HOT-DIW 3D printing can orient the nanostructure of a cylinder-forming block copolymer along a programmed print path, enabling tunable mechanical anisotropy. To probe the subfilament heterogeneities in nanostructure that arise in these oriented materials from the spatially varied flow histories, we apply scanning microbeam small-angle X-ray scattering (SM-SAXS). From this data, we identify a flow-rate-dependent emergence of off-axis tilted domains, which develop upon thermal annealing. We propose that application of shear followed by extension during 3D printing imparts asymmetric stretching of bridging chain conformations. Relaxation of this trapped stress upon thermal annealing imparts a torque on the shortened polystyrene domains which drives the formation of long-range ordered cylinders at an angle offset to the print direction. We show that the distribution of tilts within 3D-printed filaments is consistent with the variability in printed filament stiffness. Ultimately, these flow-structure insights highlight the need to understand how "real" mixed flows of complex fluids such as block copolymer melts drive the assembly, structure, and properties of printed materials.