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).