1. Introduction
With the advancement of net-zero carbon emission targets, the effective utilization of renewable resources has attracted widespread attention [1, 2]. Photothermal materials can capture near-infrared light dissipated in the atmosphere and convert it into thermal energy to be utilized, which is highly promising for wearable outdoor devices, disease-assisted therapy, seawater evaporation, catalysis and so on [3-6]. Flexible electrospun-based photothermal membranes (such as Poly (L-lactic acid) (PLLA) and nylon) can offer high comfort, large specific surface area, and wide applicability, so have great potential in the field of wearable photothermal applications [7]; however, durability, excellent photothermal conversion and simplicity of fabrication of flexible photothermal membranes are still key challenges.
The photothermal performance of membranes is directly related to surface micromorphology. Functional micro/nanostructures can exchange or promote the engineering characteristics of materials [8]. Organisms in nature often evolve special microstructures to better adapt to their environment [9]. The wrinkles of the cerebral cortex enhance intelligence [10]; the wrinkles of the guts enhance nutrient absorption [11]; and the wrinkled membranes of leukocytes ensure a higher specific surface area and enhance deformability [12, 13]. Inspired by this, scientists have tried to explore thin membranes with wrinkled microstructures through various methods to enhance the specific surface area and absorption capacity, however, the current imitation of wrinkled sizes still remains at the micrometer scale [9]. The usual methods for obtaining wrinkle microstructures, such as mechanical force [9], photopolymerization [14], infrared induction [15], and post-polymerization [16], have been able to achieve the desired wrinkle morphology. 3D printing allows for the generation of controllable nanowrinkles [17], but its high preparation cost limits the preparation of large-area nanowrinkles. Mechanical force and photopolymerization, etc. enable fast batch preparation, but it is difficult to form nanoscale wrinkles. Post-polymerization enables low-cost preparation of nanowrinkles, but the long cross-linking reaction time and instability limit the enhancement. The realization of low-cost, large-area and durable nanowrinkle membranes remains a challenge. Also, ultra-thin electrospun membranes are often easily damaged [18]. Therefore, an effective treatment procedure based on electrospun membranes is urgently needed to develop the durability of the material while realizing high specific surface area photothermal films.
Here, a solvent-induced recrystallization approach is developed firstly to obtain high-strength CS/MWCNTs/PLLA photothermal membranes with nanowrinkled structures. The adhesive polydimethylsiloxane (PDMS) dissolved in the solvent will residue on the fiber membrane after the solvent evaporates, which not only tightly connects the photo-thermal particles, but also forms an adhesive network to facilitate the intensive cross-linking between the fibrils, so significantly enhancing the membrane strength. The membranes also show two-sided heterochromatic features or transparency because of the gradient photothermal particle concentration, which considerably extends the fashionability and customizability. Remarkably, the membrane also has excellent response sensitivity and can heat up immediately after exposure to light. The ultrathin nanowrinkled photothermal membrane by solvent-induced recrystallization enables rapid batch preparation of enhanced functional composite membranes, showing broad prospects in snow clothing, Winter Olympics garments, and so on.