In Situ Visualization of the Hierarchical Anisotropy of 3D Printed Lyotropic Liquid Crystals

Natural materials have unique and complex structures at different length scales; however, mimicking their structures is not a trivial task. Inspired by the natural structure of mineralized collagen fibrils in bone, which hierarchical architecture define their flexibility, strength, and toughness; polymer-ceramic composites can be created with an ordered and anisotropic nanostructure.¹ Self-assembly is a powerful and efficient strategy to create ordered arrangements of polymeric subunits at different length scales before the mineralization process. Lyotropic liquid crystals of block copolymers have been proved to be materials with effective self-assembly in a wide range of nanostructures by carefully adjusting their composition. Hexagonally packed cylinders or bilayers in a lamellar stack offer different environments for mineralization that create a composite material with high anisotropy at the nanoscale. The anisotropy of the polymeric matrix can be extended to the micro and macroscale using extrusion-based 3D printing, which is a promising method to manufacture hierarchical materials. The shear and extensional flow inside the 3D printing nozzle modify the orientation of the self-assembled cylinders and bilayers which results in anisotropic 3D printed filaments. However, knowledge to maximize the anisotropy and understand the source of the defects during and directly after printing is yet to be achieved. A better understanding of the processes occurring during the 3D printing of the polymeric matrix is necessary to set a reliable protocol to fabricate materials with a controlled anisotropy and nanostructure.<br/>In this work, a study of the alignment of lyotropic liquid crystals during 3D printing was performed using small-angle X-ray scattering (SAXS) as the main method. The alignment of the self-assembled cylinders and lamellae during the extrusion in the 3D printing nozzle was mapped using a micro-focused X-ray beam, which raster scanned the microfluidic channels. The 2D scattering pattern was collected and analyzed at each scanning point, giving information on the nanostructure and anisotropy with a spatial resolution of 40 µm. The study of the anisotropy during real flow conditions revealed abrupt changes in the orientation and morphological transitions associated with the shear rates experienced during the extrusion.² The hexagonal phase was orientated perpendicular to the flow in areas with low shear rate and high extensional flow. Complementary rheological measurements showed strain overshoot, which is believed to be a consequence of reoriented and ruptured structures with a well-defined multidomain structure. The lamellar phase showed a reversible structural transition from ordered lamellae to multilamellar vesicles, observed in the scattering signal at low shear rates. The anisotropy and self-assembled structure of the 3D printed filament was visualized using <i>in situ</i> 3D printing, scanning SAXS, and birefringence microscopy.³ The use of larger nozzles (550 μm) resulted in a more anisotropic and homogeneous nanostructure, with self-assembled cylinders and parallel lamellae aligned in the direction of extrusion. The lack of controlled atmosphere in the 3D printer led to a phase transition caused by the evaporation of the solvent as well as the appearance of new domains with different self-assembled structures.<br/>These results identify the structural changes and key mechanisms of reorientation during flow. They also highlight the importance of a controlled environment and a careful characterization of the manufacturing process <i>in situ</i>, to guarantee de desired degree of anisotropy and well-defined structures.<br/>We acknowledge the Paul Scherrer Institut, Villigen, Switzerland for provision of synchrotron radiation beamtime at beamline cSAXS of the SLS.<br/>1- A.K. Rajasekharan et al., Small, 2017, 13 1700550<br/>2- A. Rodriguez-Palomo et al., Small, 2021, 17, 2006229<br/>3- A. Rodriguez-Palomo et al., Additive Manufacturing, 2021, 47, 102289