# Novel Janus Fibrous Membranes with Enhanced Directional Water Vapor Transmission
## 3 Results and Discussion
### Enhanced Directional Water Vapor Transmission Capability  
To explore the water vapor transmission capability of PU-8G/CA-8G Janus fibrous membranes, the directional water vapor transmission performance was characterized and investigated quantitatively by moisture-management tester (MMT). Figure 6a shows the schematic illustration of the water vapor transmission from the hydrophobic PU-8G side to the hydrophilic CA-8G side, as well as from the reverse direction, that is, from the hydrophilic CA-8G side to hydrophobic PU-8G side. In addition, based on our previous research works, we found that once the thickness of the hydrophobic layer was fixed, the directional water transport ability is highly dependent on the thickness of the hydrophilic layer [18; 20; 21]. Herein, the thickness of hydrophobic PU layer was fixed, and the thickness of the hydrophilic CA layer was changed by adjusting its electrospinningning time. Figure 6b presents the water content on both the top and bottom surface of the Janus membranes where water was dropped on the hydrophobic PU and hydrophilic CA side, respectively. It is interesting to note that (i) the water content when water drops from the hydrophobic PU-8G to hydrophilic CA-8G was higher than that in the reverse direction for all as-prepared membranes; (ii) when the CA electrospinningning time is short (not more than 5 min), the water content of the hydrophobic PU-8G was higher than that of the hydrophilic CA-8G. However, when the CA electrospinningning time was lengthened (more than 10 min), the water content of the hydrophilic CA-8G was higher than that of the hydrophobic PU-8G, inversely. The reason for this phenomenon was that when the CA electrospinningning time is short, the capillary force generated by the CA layer was small, so when the water drops on the hydrophobic PU-8G, a large amount of water remains on the surface. When water drops from the hydrophilic CA-8G, the water quickly penetrates the CA layer and reaches the PU layer, so the water content on the PU-8G side was higher. As the CA electrospinningning  
Figure 5: Water contact angles of PU and CA. (**a**) PU membrane prepared by spin-coating; (**b**) electrospun PU membrane; (**c**) CA membrane prepared by spin-coating; (**d**) electrospun CA membrane. time was lengthened, the thickness of the CA layer was increased and the hydrophilic ability was increased, so when water was dropped from the PU-8G, the water content of the PU layer was lowered. However, when water was dropped from the CA-8G, the CA layer's ability to absorb and spread the water increases during the specified test time (120 s) because of the increase in the thickness of the CA layer until the criticality is reached. Therefore, the water content on the CA-8G was higher. iii) When the CA-8G electrospinning time reached 15 min, the water content transport from hydrophobic PU-8G side to hydrophilic CA-8G side reached a minimum value of ((\(1203.7\pm 17.8\))%). This phenomenon was consistent with the one-way transport capacity results, indicating that the combination of PU layer and CA layer reaches the best state and the wicking effect of Janus membrane was the best. Subsequently, the one-way water transport of the Janus membranes from both the hydrophobic to hydrophilic side and from hydrophilic to hydrophobic side were measured. It is obvious that the one-way transport capacity from hydrophobic PU side to hydrophilic CA side of all as-prepared Janus membranes with different CA-8G electrospinning time were higher than those from the reverse direction. When the electrospinning time of CA-8G was 15 min, the one-way transport capacity reached to the maximum value. They were about 296.7% and \(-145.5\)% from the hydrophobic PU-8G side and from the hydrophilic CA-8G, respectively. Meanwhile, they were still higher than the individual PU (\(-25.9\)%, electrospun 5 min) and CA (49.8%, electrospun 15 min) layers.