3.4. Double-sided heterochromatic characteristics and light transmittance of photothermal membranes
Membranes of different thicknesses will achieve two-sided heterochromatic features and translucency through solvent-induced recrystallization. The loose and porous structure of the electrospun membrane provides a deep penetration network for the photothermal particles, creating a microscopic 3D photothermal absorption space (Fig. 4a and b). The gradient photothermal network creates a two-sided heterochromatic appearance (Fig. 4c, thick membranes (~20 µm, Fig. 4d and Fig. S9)) and allows for excellent photothermal performance on the light side, which greatly improves the fashionability of photothermal materials [25]. Then, with the rapid evaporation of the solvent and the action of PDMS, the fiber film rapidly densifies and tightly crosslinks, binding the photothermal particles inside the membrane, which greatly enhances the internal photothermal factor embedding and the integrated photothermal performance. Also, it creates a gradient distribution between dark and light faces (Fig. 4e).
When the fiber membrane is very thin, the photothermal particles can easily pass through the membrane, so the two-sided heterochromatic feature disappears and is replaced by transparency (Fig. 4f and Fig. S10, thin thickness (~6.5 µm, Fig. 4g, h and Fig. S11 and S12)). The EDS distribution of the transparent cross section is shown in Fig. S13 and S14. The main substance compositions of the photothermal particles CS and MWCNTs are shown in Fig. S15 and S16. Both the double-sided heterochromatic feature and the transparency can improve the shortcoming of the conventional photothermal membrane, which can only be dark in color, and enhance the fashionability of wearable textiles. For the SEM images of APLLA and photothermal membranes (Fig. S17), the EDS distribution (Fig. 4i-k and Fig. S18) show that the adhesive PDMS is uniformly distributed throughout the membrane, providing sufficient adhesive sites. The CS/MWCNTs/APLLA light and dark surfaces show the obvious existence of elemental C, indicating the large distribution of CS particles and MWCNTs particles. Obviously, agglomerated C is also observed in its holes, suggesting that the photothermal particles can penetrate into the interior of the film. The elemental spectrum of the light-colored photothermal film surface is shown in Fig. S19. C and O are the main components, and Si also has big components, displaying the arrangement of the adhesive and the photothermal components.