3.2. The physical properties of photothermal membranes
The light absorption characteristics of the photothermal membranes can be obtained by UV-vis-NIR spectroscopy (Fig. 2a-c). APLLA, CS/MWCNTs/APLLA light-colored surfaces and dark-colored surfaces all exhibit lower reflection and higher absorption at shorter wavelengths of UV. However, as the UV wavelength increases, the reflections of APLLA and light-colored surfaces increase steeply, while the dark surface remains constant or even decreases slowly. In the near-UV band, dark-colored surface shows excellent absorption (86.71%), while the light-colored surface is slightly inferior to that of the dark-colored surface, but still presents a high absorption (80.07%). In the vis-NIR band, the absorption of both light and dark surfaces decreases slowly, but the absorption of the dark surface is always larger than that of the light surface, and the absorption of APLLA rises slowly and is much smaller than that of CS/MWCNTs/APLLA. In the vis-NIR band, the reflections of the dark surfaces are always maintained at very low values. The light side and APLLA decrease, and the reflection of the light side is much lower than that of APLLA. In conclusion, the dark side of CS/MWCNTs/APLLA shows excellent absorption in the full band, and the light side is a little inferior to the dark side, but the absorption is excellent and much higher than that of APLLA, which demonstrates the excellent photothermal performance. The reflectance, transmittance and absorptance satisfy the following equation [19]:
\(R+T+A=1\) (1)
The XRD patterns of the dark surface show more peaks than those of the APLLA and light surfaces, appearing at 2 Theta of 108 and 118 (Fig. 2d). This is because the dark surface has a higher concentration of CS and MWCNTs. This also indicates that the light surfaces have a very low content of CS and MWCNTs, which is almost close to pure APLLA. The CS/MWCNTs/APLLA TGA curve is almost identical compared to APLLA, and the presence of CS and MWCNTs causes the material to decompose more rapidly during thermal degradation (Fig. 2e). The DSC data (Fig. 2f) exhibits the variation of the heat flow with temperature versus the sapphire (see Fig. S3 for those with baseline). The specific heats of APLLA and CS/MWCNTs/APLLA can be obtained by Temperature - ∆Heat Flow curves and comparative calculation (Fig. 2g). cp is the specific heat, m is mass, and p is heat flow. The specific heat of sapphire has a standard, so the specific heat of APLLA and CS/MWCNTs/APLLA can be calculated, as shown in Table S1. The loose structure of the electrospun membrane also leads to weak connections between fibrils and poor strength [20]. The strain-stress curves (Fig. 2h and Supplementary Note S1) show that the strength of the treated photothermal film increases dramatically compared to the pristine PLLA, with Young’s modulus exceeding four times that of PLLA. The water contact angle of APLLA was 127.65°, showing high hydrophobicity, and the contact angle slightly increased to about 137°. The contact angles are almost the same for the darker and lighter surfaces after adding photo-thermal particles (Fig. S4). The material exhibits superior hydrophobicity, which enhances the application of wearable membranes in variable weather [21, 22]. These physical characteristics further demonstrate that the double-sided heterochromatic photothermal film has a gradient concentration distribution, which explains the formation of a double-sided heterochromatic structure and shows the absorption features of UV-vis-NIR spectra.