3. Results and discussion
3.1. Nano-wrinkled photothermal membranes by solvent induced recrystallization
The electrospun fibrous membranes with solvent-induced recrystallization have microscopic wrinkles (Fig. 1a and b). The wrinkle morphology is similar to that of human white blood cells (Fig. 1c), and this surface wrinkled structure greatly enhances the specific surface area and thus the light absorption ability of the film. Unlike white cell membranes, the size of the wrinkles in our photothermal membranes is much smaller and reaches the nanometer scale. Nano-wrinkles have superior surface properties but are more difficult to achieve, which highlights the advantages of solvent-induced recrystallization for the preparation of nano-wrinkles. The process of solvent-induced recrystallization is as follows: The ultrathin photothermal membrane firstly is prepared by the electrospinning technique, afterwards, the photothermal particles CS/MWCNTs and the adhesive PDMS are sufficiently dispersed in the porous membrane by a short ultrasonication (20 s), and then the optimized photothermal membrane is obtained by acetone treatment for 5 min. Later, after the evaporation of the organic solvent, the photothermal membranes are gained, as shown in Fig. 1d.
The whole solvent-induced recrystallization process can be divided into two steps, the first step is the recrystallization of PDMS with the solubility of PDMS in tetrahydrofuran, which aims to enhance the strength of the fiber membrane and make the photo-thermal particles tightly attached; the second part is the recrystallization of PLLA with the fact that PLLA is slightly soluble in acetone, which forms a hierachical morphology on the surface of the fibers after volatilization. fiber membrane before solvent-induced recrystallization is shown in Fig. e-g and Fig. S1, and there is no wrinkle structure. After solvent treatment, the photothermal particles form a wrapping relationship with PLLA fibers and acquire the photothermal capability (Fig. 1h). The photothermal performance and the schematic diagram of the principle are shown in Fig. 1i, j and Fig. S2.