Abstract:

Objective Unidirectional liquid transfer materials are widely used for developing moisture-wicking textiles that maintain a dry, comfortable microclimate for the wearer. In this study, superabsorbent polymers (SAP) is incorporated into unidirectional liquid transport structures by introducing a functional storage layer positioned above the hydrophilic layer to rapidly capture sweat transported from the skin so that the retained moisture can subsequently evaporate under body heat or ambient conditions. This design enables efficient sweat removal even at high perspiration rates and in humid environments.

Method The prepared unidirectional liquid transport nanofiber membrane with enhanced moisture-wicking comprised three layers, a hydrophobic polyurethane (PU) layer adjacent to the skin, a hydrophilic and moisture-wicking polyacrylonitrile/polyethyleneimine(PAN/PEI) layer in the middle, and a super-absorbent PAN/SAP layer as the outermost layer. These three nanofiber membranes were assembled layer-by-layer by electrospinning. The micro-morphology, water absorption rate, air permeability, unidirectional liquid transport performance, and mechanical properties of the samples were also characterized.

Results In terms of water absorption rate, equilibrium moisture content, air permeability, and moisture permeability, the PAN/SAP nanofiber membrane achieved optimal overall performance at an SAP mass fraction of 3%. Under this condition, the water absorption rate and equilibrium moisture content were 986.5% and 90.8%, respectively. A Janus bilayer membrane composed of a PU hydrophobic layer and a PAN/PEI hydrophilic layer was then prepared to evaluate the influence of hydrophobic layer thickness on unidirectional moisture transport. When the PU thickness was 20 μm, the difference in hydrostatic pressure between the hydrophilic and hydrophobic sides was maximal, yielding the best unidirectional transport performance. Characterization and mechanistic analysis of the sandwich-structure nanofiber membrane indicated that its liquid absorption capacity increased by 143.8% and 129.4%, respectively, compared with the Janus bilayer membrane without a storage layer and with untreated cotton fabric. Furthermore, this study revealed that the spontaneous migration of liquid droplets from the hydrophobic layer to the hydrophilic layer originates from the difference in surface energy. The hydrophobic side possesses higher surface energy, and according to the principle of Gibbs free energy minimization, droplets tend to move toward the hydrophilic region with lower surface energy. During vertical transport, the droplets were jointly influenced by hydrostatic pressure and capillary force. The hydrophilic layer generated a positive Laplace pressure (wetting force), which drove droplet spreading and penetration, whereas the hydrophobic layer produced a negative Laplace pressure (intrusion force), preventing downward permeation. When droplets moved from the hydrophobic layer into the hydrophilic layer, the wetting force promoted horizontal spreading and gradual downward transport. Conversely, during upward transport against gravity, the porous fibrous membrane generated a capillary force that overcomes gravity. Moreover, the Laplace pressure was found to increase progressively from the hydrophobic layer, through the hydrophilic layer, to the water-storage layer, forming a gradient capillary force that drives upward droplet transport. Meanwhile, the intrusion force from the underlying hydrophobic layer effectively prevents backward leakage, thereby achieving unidirectional liquid transport without backflow.

Conclusion A sandwich-structured nanofibrous membrane was successfully constructed by electrospinning, comprising a PU hydrophobic layer, a PAN/PEI hydrophilic layer, and a PAN/SAP storage layer. This multi-layer structure not only exhibits excellent unidirectional moisture transfer but also demonstrates a significant enhancement in liquid adsorption, representing an increase of 143.8% and 129.4% compared with the bilayer Janus nanofiber membrane without a storage layer and pure cotton fabric, respectively, highlighting its great potential for application in moisture-wicking textiles. Mechanistic analysis reveals that droplet migration arises from surface energy differences and a progressive Laplace pressure gradient. This design offers a promising strategy for developing next-generation moisture-management fabrics, with predicted applications in sportswear and personal thermal management.

Key words: unidirectional liquid transport, hygroscopic fiber, electrospinning, composite nanofiber membrane, sandwich structure, sports sweat absorption