## 3 Results and Discussion
### Morphology
The solubility of material in a solvent has been investigated for decades, and the well-recognized theory is based on the energy of mixing \(\Delta\)E\({}_{mix}\), which can be expressed in terms of the following equations [19],  
\[\Delta\)E\({}_{mix}=\phi_{1}\phi_{2}V_{m}(\delta_{1}-\delta_{2})^{2} \tag{1}\]  
\[\text{C.E.D.}=\frac{\Delta E_{up}}{V_{m}}=\delta^{2} \tag{2}\]  
where \(\phi_{1}\), \(\phi_{2}\)--volume fractions of the components, \(V_{m}\)--average molar volume, \(\delta_{1}\), \(\delta_{2}\)--solubility of the components, C.E.D.--cohesive energy density, \(\Delta E_{up}\)--energy of evaporation. In general, the solubility \(\delta\) can be further expressed as the summation of three components, i.e., [19],  
\[\delta^{2}=\delta_{d}^{2}+\delta_{p}^{2}+\delta_{h}^{2} \tag{3}\]  
where \(\delta_{d}\)--solubility due to the dispersion force, \(\delta_{p}\)--solubility due to the dipole forces, \(\delta_{h}\)--solubility due to the hydrogen bonds. Correspondingly, the three factors can be combined into an enthalpy correction parameter \(\chi_{H}\), i.e., [19],  
\[\chi_{H}=\frac{V_{m}}{RT}\bigg{[}(\delta_{d1}-\delta_{d2})^{2}+\left(\delta_{ p1}-\delta_{p2}\right)^{2}+(\delta_{h1}-\delta_{h2})^{2}\bigg{]} \tag{4}\]  
here, subscripts 1 and 2 represent the solvent and the polymer, respectively. The smaller the \(\chi_{H}\), the larger the solubility of polymer solute and the solvent. Now the solubility behaviors of ethanol to PLA and shellac can be illustrated, respectively. The solubility parameter of ethanol to PLA is 78.1 (\(\delta_{d}^{2}+\delta_{p}^{2}\)) since the PLA has no hydrogen bond existed in its chemical structure, while to shellac is 168 (\(\delta_{d}^{2}+\delta_{p}^{2}+\delta_{h}^{2}\)) since there are plenty of hydrogen bonds existed in the chemical structure of shellac [16]. In addition, the molar attraction constants for PLA and shellac are quite different. The molar attraction constant \(\Sigma\)F can be expressed as [20; 21; 22],  
\[\sum\text{F}=\delta V_{m}=V_{m}(\text{C.E.D.})^{2} \tag{5}\]  
The \(\Sigma\)F for PLA is 799 cal\({}^{1/2}\)-c.c.\({}^{1/2}\), while for shellac is over 2000 cal\({}^{1/2}\)-c.c.\({}^{1/2}\) on average [20; 21; 22]. In fact, the PLA is not dissolved in ethanol due to the polarity of PLA, while the ethanol is a weak polar solvent, which is different from DMF, a strong polar solvent. Since the ethanol has good solubility to the shellac, thus the shellac can be used to sense the ethanol quantity in terms of the solubility of ethanol to shellac. The resin properties of shellac are believed to be due to the hydrogen bonding of the components. The resin components of shellac, soft resin, and hard resin all contain hydroxyl acids, and their polar groups are present at the interface of the molecules [15]. Therefore, when the shellac encounters the alcoholic vapor with a low molecular weight, it can form hydrogen bonds with the alcoholic vapor and dissolution occurs. The alcoholic vapor with a low molecular weight contacts with the shellac membrane through the pores of the PLA membrane. The strong interaction of hydrogen bonds between the polar groups makes the PLA/shellac/PLA sandwich-structural membrane very  
Figure 4:0 The chemical structure of the main component of poly (lactic acid). sensitive to the alcoholic vapor with a low molecular weight. When the composite nanofiber membrane is treated with the alcoholic vapor with a low molecular weight, the shellac nanofiber membrane layer dissolves and slowly passes through the gap between the PLA fiber membranes until a gap-filling coating is formed on the surface of the membrane, which reduces the light scattering of the membrane and increases the transparency of the membrane. The change of transparency can be used to detect the alcoholic vapor.