Direct-ink writing (DIW) is a promising area of 3D printing that allows for greater material versatility and energy efficiency at lower cost. Currently, controlling the extrusion flow and shape remain a major limitation in DIW. As such, knowing which material metrics and rheological properties facilitate direct-ink writing is crucial. To this end, we synthesized bottlebrush block copolymers (BBcP) of polystyrene and polylactide that self-assemble into a lamellar structure. We designed the BBcPs to specifically have a lamellar spacing that enables structural color, ranging from red to blue to opalescent, depending on the polymer concentration. These BBcPs not only serve as a strictly defined model system of study but also allow DIW in different colors from the same material, increasing the efficiency of the DIW process. We characterized the BBcPs using large amplitude oscillatory shear, recovery rheology and steady shear startup alongside optical microscopy to match the flow behavior with its structural color. We observed that the structural color of the BBcPs depends not only on their concentration, but also on the shear rate applied and the solvent used. More specifically, the broadest spectrum of colors was observed when the BBcPs behaved like a yield stress fluid, with the most dramatic change in color being at the yield stress. Furthermore, the structural color of these materials serves as a proxy for their mechanical memory, as such, we leverage BBcPs to understand mechanical memory in polymeric systems. We use recovery rheology to construct memory plots, which we defined as the difference in recoverable strain throughout an oscillation period, at different shear strains. This methodology allows us to observe how the mechanical memory increases as the BBcP yields and is erased once the material has yielded. Using recovery rheology, we can fully define metrics that relate to the structural color, flow behavior, and rheology of BBcPs before and after being extruded in DIW. Thus, this study not only serves as a fundamental basis for understanding the rheological memory and flow behavior of BBcPs with structural color but also help define material metrics and rheology that allows efficient DIW.
