3.3. Analysis of the effect of nanowrinkled structures on photothermal performance
A novel solvent-induced recrystallization procedure is proposed for the first time in this study, which enables the high specific surface area and functional modification of electrospun membranes in a very short period of time (less than 6 min). It simplifies the previously cumbersome four-step process: morphology modification, mechanical reinforcement, adhesive generation and adherence and allows obtaining high-performance photothermal membranes. Moreover, this method can help photothermal membranes realize special nano-wrinkled structure. As shown in Fig. 3a, compared with the porous structure, the nano wrinkled membrane will provide more attachment sites for the photothermal particles, which is beneficial to the photothermal performance.
In this study, acetone exist as a modifier for hierachical porous morphology, and PDMS can achieve good dispersion and rapid volatilization only by tetrahydrofuran [23, 24]. Thus, we studied various combinations of the two solvents. Tetrahydrofuran does not have a solubilizing effect on PLLA, so only tetrahydrofuran as a solvent cannot modify the fiber morphology and causes dense accumulation between the fiber layers, resulting in a significant agglomeration of photothermal particles on the surface (Method 2, Fig. 3b and S5a). Since PDMS is insoluble in acetone, when treated only with acetone, PDMS adheres to PLLA before the solvent evaporates, resulting in the inability to form wrinkled structures (Method 5, Fig. 3c and S5b). For acetone treatment, drying and then treating with tetrahydrofuran containing CS/ MWCNTs, PLLA was insoluble in tetrahydrofuran, making it difficult to form a distinct wrinkled structure (Method 1, Fig. 3d and S6a). For treatment with tetrahydrofuran containing CS/MWCNTs, drying and then acetone treatment, PDMS was deposited before acetone evaporation and could not form wrinkles (Method 3, Fig. 3e and S6b). For acetone treatment and then treatment with tetrahydrofuran containing CS/MWCNTs, PLLA was insoluble in tetrahydrofuran and it was difficult to form a distinct wrinkled structure (Method 4, Fig. 3f and S6c). The tetrahydrofuran treatment and subsequent treatment with acetone harvested more desirable morphology and photothermal characteristics, as shown in Fig. 3e and S7 (Method in this study). We also evaluate the photothermal performance of the membranes under different morphology and specific surface area. Heat distribution plots of the membranes before (Fig. S8) and after (insert images in Fig. 3b-g) 5 min of 1 sunlight irradiation and the maximum temperature are plotted in Fig. 3h. Methods 1, 2, and 4 have poor photothermal performance, while methods 3 and 5 both have superior photothermal properties, but their vanishing or very small wrinkled structures limit the potential upper limit of the photothermal film and need to be further optimized. The method in this study shows best photothermal performance. BET data show that the film containing the photothermal particles has only a slight decrease in specific surface area compared to APLLA, they all exhibit an exceptionally high specific surface area (Fig. 3i and Table S2). The nano-wrinkled surface greatly increased the specific surface area about 164 times, from 0.3057 m2/g (macroscopic) to 50.00 m2/g (macroscopic).