Tuning polymer solution elasticity for electrospinning: effects of molecular weight distribution and elastic doping

Electrospun nanofibers are used in a wide range of applications, from filtration to biomedical scaffolds, yet their successful formation relies on carefully tuned polymer solution properties. Among these, extensional elasticity plays a central role in stabilizing the fluid jet against capillary breakup. While increasing solution viscosity can modestly delay thinning, it is the presence of entangled high–molecular-weight chains that provides the cohesive elastic stresses needed to sustain a continuous filament until solidification. In this work, the influence of molecular weight distribution (MWD), matrix viscosity, and long-chain content on extensional rheology and electrospinnability was investigated using blends of poly(ethylene glycol) (PEG) and poly(ethylene oxide) (PEO). Shear viscosity and filament thinning dynamics were characterized using rotational rheometry and a custom-built capillary breakup extensional rheometer (CaBER). Initial comparisons confirmed that low molecular weight PEG, despite its high shear viscosity, failed to produce fibers due to the absence of entanglements and extensional elasticity, whereas entangled high molecular weight PEO readily formed uniform nanofibers. MWD was systematically broadened at constant weight-average molecular weight by blending PEO samples of different molecular weights, which increased CaBER-measured relaxation times and led to fibers with larger diameters. Subsequent experiments with Boger-type fluids revealed that shear viscosity alone did not correlate with electrospinning behavior, while relaxation time measured by CaBER remained predictive. Increasing the background matrix viscosity further enhanced extensional elasticity by slowing chain relaxation and filament thinning. Finally, a practical demonstration of elastic doping showed that adding a trace amount of long-chain PEO to an otherwise unspinnable PEG matrix enabled fiber formation and achieved a fivefold increase in polymer throughput. Together, these results establish extensional relaxation time as a robust predictor of electrospinnability and demonstrate that the decoupling of matrix viscosity and chain entanglement offers a versatile strategy for designing high-yield nanofiber formulations.