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.