Twist-Free Enhancement of Strength and Modulus in Electrospun Yarns via Liquid-Assisted Capillary Densification

Electrospun yarns often fall short of the strength and stiffness of their constituent nanofibers because of loose packing and inter-fiber slip. We report a simple, twist-free route to close this gap by liquid-assisted rolling: yarns are briefly wetted (water or ethanol) and subjected to gentle rolling action (mechanical strokes perpendicular and parallel to the yarn axis), then dried under controlled conditions so that meniscus forces compact the assembly into tightly bound bundles. The treatment yields large gains in tensile strength and modulus, and as yarn diameter decreases the properties of liquid-treated yarns approach single-fiber limits, indicating more efficient load transfer. Dry-rolling controls produce negligible changes compared to as-spun yarns, confirming that capillarity-driven consolidation, rather than mechanical pressing, dominates the improvement. Water consistently outperforms ethanol, reflecting its larger elastocapillary driving term gamma*(1 + cos theta) on PAN and thus stronger capillary compaction; a short post-treatment anneal near Tg further increases stiffness with a corresponding reduction in ductility. To rationalize these trends, we quantify microstructure via SEM-derived alignment and packing density and show that these complementary descriptors jointly explain variability in mechanical response. A compact constitutive framework, grounded in distributed fiber recruitment and adhesion/frictional contact, captures the observed strengthening-ductility trade-off across processing routes. The results establish capillarity-driven consolidation as a scalable pathway to engineer processing-structure-property relationships in hierarchical polymer fiber assemblies and provide practical guidance for upgrading electrospun yarns, alone or as precursors to twisted and composite architectures.