## 3 Results and Discussion
### Alcoholic Vapor-Sensing Characterization  
In this part, we aligned the laser to the surface of the photoresistor and then injected 100 mL of the alcohol reagent into a specific transparent apparatus to form an alcoholic vapor atmosphere at 25 \({}^{\circ}\)C. The sample was then placed in this apparatus and the membrane was placed with 5 cm in distance directly above the photoresistor. By measuring the change of the resistance of the photoresistor, the transparency of the PLA/shellac/PLA sandwich-structure membrane in alcoholic vapor with a low molecular weight was discussed. The properties after the alcoholic treatment are listed in Table 1[31]. As mentioned above, the experiment was carried out in a glove box. The photoresistor was connected to a multimeter, which was connected to the computer to record the data of resistance change in real-time. Figure 10 shows the response of the PLA/shellac/PLA (1 mL/1 mL/1 mL) sandwich-structural membrane to the alcoholic vapor with a low molecular weight, including the methanol and ethanol over time at 25 \({}^{\circ}\)C. The \(y\)-axis is the normalized resistance change:  
\[\frac{\Delta R}{R_{0}}\times 100(\%)=\frac{\left[R_{0}-(R-R_{PR})\right]}{R_{0}} \times 100(\%) \tag{6}\]  
here \(R\) stands for the resistance of the photoresistor when the laser passes through the membrane; \(R_{0}\) denotes the resistance of the photoresistor in the absence of laser light; \(R_{PR}\) is the resistance of the photoresistor when it is directly illuminated by the laser without any absorption. As shown in Figure 5, when the sandwich structure is exposed to the methanol and the ethanol respectively, the PLA/shellac/PLA sandwich-structural membrane gradually becomes transparent, then the laser light intensity passing through the membrane increases, the resistance of the photoresistor will decrease, thus the resistance change rate will increase accordingly. Finally the resistance change rate will be saturated for a long period of time, e.g., 300 s.  
\begin{table} \begin{tabular}{c c c c} \hline \hline
**Solvents** & **Molar Mass (g/mol)** & **Molar Volume (cm3/mol)** & \(\delta_{h}\)*(MPa)\({}^{\frac{1}{2}}\) \\ \hline Methanol & 32.04 & 40.7 & 22.3 \\ Ethanol & 46.07 & 58.5 & 19.4 \\ \hline \hline \end{tabular}  (\(\delta\)*\(\delta\)*\(\delta\)*\(\delta\)*\(\delta\)*  
Compared to the ethanol, the methanol vapor showed faster and larger change rates of resistance. Among the alcoholic vapors, the methanol vapor exhibits a higher value in the normalized resistance change than the ethanol vapor. It is critical for the sandwich structural membrane to distinguish the methanol, ethanol and other alcohols. It was found that the response time is one of the selective methods. Especially, for a 10% of change rate of reduction of the resistance, the response time is 5 s for methanol, and \(\sim\)25 s for ethanol. The response time depends on the expansion of the shellac in the sandwich structure to the alcohol, e.g., explosion surface area. One can enlarge the expansion area in terms of the porous structure of the PLA on the surface of shellac, the reduction of thickness of the PLA to reduce the contact distance of alcohol to the shellac, as well as enhancement of the capillary force of the porous PLA. As shown in Table 1[31], the molar volume of methanol was 40.7 cm\({}^{3}\)/mol, which is much lower than ethanol. In addition, the solubility of the polar part of methanol is larger than the ethanol. That can make the shellac form hydrogen bonds with the methanol much easier and faster than the ethanol. Therefore, the reaction rate of the methanol vapor with the membrane is much faster than the ethanol vapor and the composite membrane has a much smaller response time for the methanol vapor than the ethanol. Thus, the methanol vapor could make the membrane more transparent than the ethanol vapor.