# Novel Janus Fibrous Membranes with Enhanced Directional Water Vapor Transmission
## 3 Results and Discussion
### Enhanced Directional Water Vapor Transmission Capability
The enhanced amount of water vapor transmission from the hydrophobic PU side to the hydrophilic CA side than that in reverse direction can be attributed to the asymmetric wettability of the Janus  
Figure 6: (**a**) Schematic illustration of water vapor transmission. (**b**) Moisture-management testing (MMT) results of hydrophobic PU/hydrophilic CA Janus fibrous membrane with different hydrophilic CA-8G electrospinning times. The water contents tested from the hydrophobic side to hydrophilic side and the reverse direction were taken into consideration. (**c**) The measurements of the one way transport capacity of all as-prepared Janus fibrous membranes. (**d**) The mechanism of the enhanced water vapor transmission. membrane itself. The mechanism is illustrated in Figure 6d. When water vapor is attached to the hydrophobic PU-8G layer, the droplets exhibited a spherical or a hemispherical shape ((I) in Figure 6d), as water exists in Wenzel state or Wenzel-Cassie state and is subjected to the hydrophobic force (HP) because of the hydrophobicity of the surface. The tiny droplets grew larger with increasing time. When the gravity of water droplet overcomes the HP, it attached to the hydrophilic CA-8G layer, and the hydrophilic capillary force (CF) caused tiny water droplets to get absorbed and spread in the hydrophilic layer. Meanwhile, fresh hydrophobic surfaces were provided to capture more water droplets. Once the thickness of hydrophilic CA-8G increased ((II) in Figure 6d), the mass of the fibrous membrane increased and the pores between the fibers decreased, making it more difficult for the water vapor to pass through. In addition, it's necessary to mention that when the hydrophilic CA was electrospun in a very short time, the CA layer was thin. It can be understood that the hydrophilic micro/nanopopes form abundant capillaries can rapidly absorb water and allow the directional transmission. If the hydrophilic CA layer was very thin, there was no enough CF to \"drag\" tiny water droplet from the hydrophobic PU layer efficiently and thus cannot guarantee the best one-way transport capability. Therefore, both the water vapor transmission and one-way transmission capacity decreased. Compared to (I) in Figure 6d, once tiny water droplet attached on the hydrophilic CA-8G layer ((III) in Figure 6d), they spread or were adsorbed much more quickly resulting from the CF, and the HF prevented its further penetration or absorption, which led to less amount of water vapor transmission at the same measure time. The one-way transmission capacity (%) of this work has reached the GB-4 standard in GB/T 21655.2-2009 (Textiles-Evaluation of absorption and quick-drying--Part:2 Method for moisture management tests), which is very closed to the GB-5 standard. The GB/T 21655.2-2009 standard is a Chinese national standard, divided into five standards in a scale of 1-5. When the one-way transport capacity value is between 200 and 300, it corresponds to the level 4 standard. While when the one-way transport capacity value is \(>\)300, it reaches the level 5 standard. And the greater the standard number, the better the one-way transport capacity of the membrane. In addition, we have also conducted research on related work in recent years and found that other works have also achieved excellent results. The details of the investigation show that there is a slight difference between our work and the works previously reported. The structure and properties of the material are the main factors contributing to the difference in the results. Moreover, there are variable testing methods and evaluation standards in reported works showed in Table 1. Qualitatively, the hydrophobic/hydrophilic Janus fibrous membranes, as well as the membranes which have been reported in previous works, perform well in the case of their testing standards.