# Novel Janus Fibrous Membranes with Enhanced Directional Water Vapor Transmission
## 3 Results and Discussion  
### Fabrication and Morphology of Polyurethane (PU)/Cellulose Acetate (CA) Janus Fibrous Membrane  
In this work, poly[4,4'-methylenebis (phenylisocyanate)-alt-1,4-butanediol/di(propylene glycol)/plycaprolactone) (PU) and cellulose acetate (CA) were used as the raw polymers to fabricate the hydrophobic/hydrophilic Janus fibrous membrane. The Janus membrane was fabricated by so-called \"layer-by-layer\" electrospinning. Accordingly, Figure 1 provides a schematic diagram of the preparation process. PU was dissolved in a solvent mixture of \(N\),\(N\)-dimethylformamide (DMF)/tetrahydrofuran (THF) and was electrospun and collected on an aluminum (AI) foil covered on a rotating drum. The obtained PU fibrous membrane is hydrophobic. After that, the second CA fibrous layer showing hydrophilic properties was fabricated by dissolving CA in the mixture of acetone/dichloromethane (AC/DCM). The hydrophilic CA fibrous layer was deposited on the top of the PU fibrous layer in sequence by seamless coupling with the PU fibrous layer. Finally, when peeled off from the AI substrate, the hydrophobic/hydrophilic Janus fibrous membrane was successfully fabricated. Electrospinning is widely recognized as a simple top-down method of fabricating materials with different morphologies such as \"spheres,\" \"knot-in-fiber structure,\" and fibers with diameters from tens of nanometers to submicrometers by stretching a viscous conductive fluid into a thin jet via electrostatic force [30]. Among them, the electrospun solutions and electrospun needle nozzles are regarded as the main factors affecting the morphology of the electrospun materials. Therefore, we explored the effects of different concentrations of electrospun solutions and inner diameters of the electrospun needle nozzles on the morphology of nanofibrous materials [31]. Figure 2a-f presents the scanning electron microscope (SEM) images of PU layers which were electrospun with the PU/(DMF/THF 9:1) concentrations of 5 wt %, 7.5 wt %, 10 wt %, 12.5 wt %, 15 wt %, and 17.5 wt %, respectively, and the electrospun needle type 8G (inner diameter-0.8 mm). It is obvious that there are PU spheres at the low concentration of 5 wt %. With the increase of the solution concentration, the amount of PU spheresdecreased. Meanwhile, the PU fibrous structures formed. When the concentration reached to 10 wt %, PU fibrous membrane on which some fibers adhere to each other (pink dotted-circle, in Figure 2c) are obtained. By continuously increasing the concentrations of the electrospun solutions, PU fibrous membrane with uniform fiber distribution are obtained (Figure 2d-f). To prepare the Janus fibrous membrane, the second CA fibrous layer was fabricated with CA/(Ac/DMF) solution and deposited on the top of the PU layer. Figure 2g-i shows the morphologies of CA with the concentrations of 7.5 wt %, 10 wt %, and 12.5 wt %, respectively. It can be seen that at the concentration of 12.5 wt %, CA fibrous membrane can be obtained. Figure 1: Schematic diagram of the fabrication process of poly[4,4’-methylenebis (phenylisocyanate)-alt-1,4-butanediol/di(propylene glycol)/plycaprolactone (PU)/cellulose acetate (CA) Janus fibrous membrane. The PU fibrous membrane which is hydrophobic was obtained as the first layer by electrospinning. Then another layer, CA fibrous membrane which exhibits hydrophilic properties, was electrospun onto it and by seamless coupling with the upper PU fibrous layer. When the composited two layers were peeled off from the Al foil substrate, the Janus membrane with wettability contrast was formed. Figure 2: Scanning electron microscope (SEM) images of PU and CA fibrous membrane. PU/(DMF:THF = 9:1) = (**a**) 5 wt %; (**b**) 7.5 wt %; (**c**) 10 wt %; (**d**) 12.5 wt %; (**e**) 15 wt %; (**f**) 17.5 wt %. CA/(Ac:DMF) = (**g**) 7.5 wt %; (**h**) 10 wt %; (**i**) 12.5 wt %.