Alignment of polymer-free cyclodextrin fibers: interplay of solvent environment, process parameters, and jet dynamics

This work presents a systematic investigation of the alignment of single-component, polymer-free electrospun fibers produced separately from hydroxypropyl-β-cyclodextrin (HP-β-CD) and HP-γ-CD, with particular focus on how the solvent environment governs jet dynamics. Aqueous and DMF-based solutions were compared, together with the key electrospinning parameters-applied voltage, flow rate, drum speed and tip-to-collector distance-for their combined effects on fiber alignment and morphology. Solvent selection modulates two coupled parameters-viscosity and electrical conductivity-identified here as the dominant, though not the only, controls on jet velocity and thickness, with other solvent properties changing in parallel. As resolved directly by high-speed imaging, aqueous solutions with higher conductivity and lower viscosity produced thin, faster jets (1.37 and 1.64 m s^-1 for HP-β-CD and HP-γ-CD, respectively) that resisted alignment, whereas DMF-based solutions yielded thicker, slower and more stable jets (0.91 and 0.86 m s^-1). Only this slower-jet regime yielded aligned fibers: under the conditions imaged here, alignment was obtained exclusively in the DMF systems, where the ratio of drum surface velocity to axial jet velocity exceeded 5, whereas the aqueous systems did not exceed 4.5 and never rose above an alignment index of 0.41. Operating in this regime yielded alignment indices of 0.82 (HP-β-CD) and 0.93 (HP-γ-CD) on a 0-1 scale. Tensile testing along the fiber direction showed the aligned DMF-spun mats to be stronger than their randomly oriented aqueous counterparts, while XRD showed both systems to be predominantly amorphous irrespective of fiber orientation.