## 3 Results and Discussion  
### Morphology  
Figure 3 shows the surface morphology of the pristine electrospun PLA nanofibers before and after alcoholic treatment. The pristine PLA nanofibers were randomly oriented in the SEM image and the average diameters were \(2.01\pm 0.33\)\(\upmu\)m. It can be seen that the morphology of the PLA nanofibers does not change significantly after being treated with the alcoholic vapor with a low molecular weight. Figure 4 shows the images of the PLA/shellac/PLA sandwich-structural membrane before and after the treatment with the alcoholic vapor with a low molecular weight. In the experiment, different solution ratios (1 mL/1 mL/1 mL, 1 mL/2 mL/1 mL, 2 mL/1 mL/2 mL) were used. In Figure 4, one can see that, after the treatment, the PLA fiber layers were tightly stuck together, and the shellac nanofiber layer were partially dissolved after the alcoholic vapor treatment. In contrast, the morphology of the PLA nanofibers did not change much after the treatment. For the shellac layer, however, the nanofibers dissolved and slowly penetrated mutually in between the gap until a gap-filling coating was formed on the surface of the PLA membrane. This can explain the reduction in thickness of the composite film after the alcoholic treatment. Figure 5 shows that the PLA/shellac/PLA sandwich-structural membrane changes from opaque to transparent after the treatment of alcoholic vapor with a low molecular weight. Figure 2: The experimental apparatus and the transparency change of the membrane before and after the treatment with the alcoholic vapor. Figure 4: SEM images of the PLA/shellac/PLA sandwich-structural membrane ((**a**,**b**) 1 mL/1 mL/1 mL, the left image is \(\times\)700 and the inset image is \(\times\)10.0 k; (**c**,**d**) 1 mL/2 mL/1 mL, the left image is \(\times\)1.00 k and the inset image is \(\times\)10.0 k; (**e**,**f**) 2 mL/1 mL/2 mL, the left image is \(\times\)1.00 k and the inset image is \(\times\)10.0 k) before being treated with the alcoholic vapor with a low molecular weight. Figure 3: Scanning electron microscope (SEM) images of PLA nanofiber membrane before and after treated with the alcoholic vapor with a low molecular weight. ((**a**) The left image is \(\times\)2.00 k and the inset image is \(\times\)25.0 k; (**b**) the left image is \(\times\)2.00 k and the inset image is \(\times\)13.0 k). The shellac resin is not a single compound but consists of an intimate mixture of several polar and non-polar components, e.g., five hydroxyl groups including vicinal hydroxyl group, one carboxyl group, in free state, three ester groups, one double bond and one partly masked aldehyde group, and the probable linkages ester, acylal, acetal and ether in an average molecule [15]. The resinous characteristics are believed to be due to the association of components through the hydrogen bonds, which also make shellac react with various monohydric alcohols _viz_, ethyl alcohol, butyl alcohol, allyl alcohol and dihydric alcohols to form ethers [15]. This is quite different from the PLA in terms of the solubilities in different solvents. The chemical structures of three main components of shellac are shown in Figure 6[17] and the chemical structure of the main component of poly (lactic acid) is shown in Figure 7[18]. Figure 5: The PLA/shellac/PLA sandwich-structural membrane changes from opaque to transparent after treated with the alcoholic vapor with a low molecular weight. (The PLA/shellac/PLA sandwich-structural membrane was on the pure aluminum foil). Figure 6: The chemical structures of the main components of shellac. The color change is associated with the mutual solubility of the solute and the solvent, which makes up the mechanism of the sensor for the sandwich structure to react with the alcoholic vapor.