Solvent evaporation induces stiffening in caber and electrospinning of volatile polymer solutions

Nanoscale polymer fibers have gained increasing attention over the past two decades due to their applications in filtration, sensing, and biomedical devices. These fibers are commonly produced by electrospinning, where a polymer solution is stretched in extension and rapidly solidifies through solvent evaporation. Since flow behavior plays a key role in determining fiber morphology, extensional rheology techniques such as Capillary Breakup Extensional Rheometry (CaBER) are often used to characterize polymer solutions. Like electrospinning, CaBER involves the formation and thinning of a filament exposed to ambient air. In this study, the influence of solvent evaporation on both electrospinning and CaBER measurements is investigated under controlled ambient vapor pressures (25% to 90%) using aqueous pullulan solutions. Electrospinning results show that lower vapor pressure accelerates solvent evaporation, leading to early jet solidification and the formation of thicker fibers. In parallel, CaBER measurements reveal longer relaxation times at low vapor pressure, suggesting evaporation-induced stiffening. To better understand this effect, a model was developed to calculate solvent evaporation at the filament surface and the resulting radial redistribution of solvent within the filament by diffusion. Using experimental filament radius data as input, the polymer concentration profile across the filament cross-section was computed over time. From these local concentration fields, the corresponding apparent bulk viscosity was calculated using a concentration-dependent viscosity relationship. Simulation results show that at low vapor pressure, polymer concentration increases near the surface due to solvent loss, which in turn leads to a significant rise in the apparent viscosity as the filament thins. The convergence of results from electrospinning, CaBER, and modeling highlights the strong influence of solvent evaporation on extensional flow behavior. These findings emphasize the need for careful environmental control and evaporation-aware modeling when characterizing volatile polymer solutions.