Orientation of Liquid Crystalline Polymers after Filament Extrusion and after Passing through a 3D Printer Nozzle

Abstract

Three-dimensional (3D) Printing with liquid crystalline polymers (LCP) enables the production of high-strength parts, whereby the mechanical properties can be tailored to a large extent by simply varying the printing parameters. Within this work, the orientation behavior of a thermotropic liquid crystalline polymer was studied for different draw ratios after filament extrusion by using wide-angle X-ray scattering. Moreover, the orientational order was quantified within a 3D printer nozzle and after exiting the nozzle. To study thermal and rheological effects, the nozzle temperature, the nozzle diameter, the building chamber temperature as well as the shear rate were varied in a range that is relevant for typical fused filament fabrication (FFF) processes. The extruded, drawn filaments exhibit a high orientational order in extrusion direction. However, by passing through the short 3D printer nozzle, the high initial orientation is lost and the melt is partly disoriented. This finding is contrary to the conventional assumption in literature, which assumes that the shear and elongational flow within the 3D printer nozzle led to a uniform aligning of the liquid crystalline polymer domains. The reason for this opposite observation is primarily attributed to the flow phenomenon of so-called director tumbling, which is known to occur in thermotropic and lyotropic LCP. Moreover, it is conceivable that the comparatively long residence time within the printer nozzle enables thermal relaxation to a certain extent. These phenomena are supposed to apply particularly for the upper nozzle region where the flow channel diameter is large, giving rise to a long residence time as well as a low shear rate.