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.