Origin and Manipulation of the Orientation of Colloidal 2D Nanoplatelets in Electrospun Fibers

Stable jet electrospinning (SJES) enables the incorporation of fluorescent colloidal 2D nanoplatelets (NPLs) into polymer fibers for potential optical applications and the manipulation of the orientation of the nanoplatelets. Three polymers with excellent optoelectronic properties are used as matrices, namely poly(methyl methacrylate) (PMMA), cyclic olefin copolymer (COC) and copolycarbonate. The electrospinning parameters are optimized for the production of highly aligned microfibers by stable jet electrospinning. Polarization-resolved fluorescence microscopy is used to determine the orientation of the NPLs within the SJES fibers. For the PMMA-fibers spun from a 35 wt% solution a preferential vertical alignment of the NPL is found. We systematically investigate the origin of the NPL orientation using a series of polymer solutions with well controlled rheology. During the stable-jet electrospinning process, a die-swell effect is observed when a highly concentrated viscoelastic polymer solution with a strong elastic component leaves the cannula. The resulting extensional flow causes the anisotropic NPLs to rotate and realign themselves through hydrodynamic forces. Conversely, for concentrations below the overlap concentration the viscosity drops sharply and shear‑thinning disappears, indicating a negligible elastic contribution. The COC and copolycarbonate systems—limited to sub‑overlap concentrations—exhibited only random NPL orientations. The vertical alignment of the NPLs in the fiber is therefore based on the viscoelastic properties of the non-Newtonian fluid and can only be achieved under strict and specific conditions.