Passive radiative cooling technology has great potential for the effective cooling of terrestrial objects by simultaneously reflecting sunlight and radiating heat into the cold outer space. Due to the intrinsic properties of polymethyl methacrylate (PMMA), many studies have used its materials to prepare high-performance radiative cooling structures. However, the use of PMMA for the efficient preparation of radiative-cooled structures remains a problem. Recently, Yang Liu, a Ph.D. student in Prof. Zheng Yi's research group at Northeastern University, proposed a simple, economical and scalable method based on multi-stage solvent displacement to fabricate layered gradient porous PMMA ultrathin films with efficient passive radiative cooling performance. Here, the "ouzo effect" acts as a driving force for the formation of micro/nanopores during solvent displacement, and the size of the pores can be controlled according to different solution ratios. The ultra-high sunlight reflectance of 0.99 and the mid-infrared thermal emissivity of 0.97 enable the PMMA ultrafilm to achieve significant radiative cooling temperature (peak and mean values of 8.2 °C and 4.6 °C, respectively) and cooling power (mean 90 W/m2). In addition, the multi-stage solvent displacement method offers advantages in fabricating porous structures, further providing a cost-effective, environmentally friendly, and sustainable manufacturing path for high-performance radiative cooling applications. The work was recently published in the Chemical Engineering Journal.
Cooling accounts for 20% of global electricity consumption and is especially necessary in hot weather. However, the use of a large number of air conditioning systems will significantly increase global energy consumption, thereby accelerating global warming and climate change. Passive daytime radiative cooling (PDRC) is a promising alternative to traditional cooling that can help slow global warming. PDRC is mainly achieved by spectrally selective materials that simultaneously reflect sunlight (0.3-2.5 μm) and radiate infrared thermal radiation into cold outer space (3K) through an atmospheric window between 8 and 13 μm. Therefore, high sunlight reflectivity (Rsolar, λ∼0.3–2.5 μm) and high infrared thermal emissivity (εIR, λ∼8–13 μm) are the key points for the design of efficient PDRC materials. Recently, a large number of PDRC structures and materials with high Rsolar and high εIR have been developed, including multilayer photonic structures, particle distribution structures, porous structures, metamaterials, biomaterials, and biomimetic materials. However, most of the reported PDRC structures are difficult to achieve an optimal balance between structural design and processing cost, which will hinder the large-scale commercial application of efficient PDRC materials. Therefore, there is a great need for a simple and cost-effective PDRC manufacturing method for the practical application of radiative cooling technology. Here, we fabricated PMMA ultrathin films with hierarchical gradient micro/nanopores with internal structure by multi-stage solvent substitution method. The prepared ultra-clear PMMA film has excellent sunlight reflectance of 0.99 and infrared thermal emissivity of 0.97 in the atmospheric transparent window. The average temperature difference and peak temperature difference of PMMA ultrafilm can be 4.6°C and 8.2°C, respectively. In addition, the multi-stage solvent displacement method is conducive to the large-scale fabrication of porous radiative cooling materials.
As shown in Figure 1, we prepared fractional gradient porous polymers using a multi-stage solvent displacement method, starting with precursor solutions of PMMA (polymer), acetone (solvent), and ethanol (non-solvent). The precursor solution is then poured onto a smooth substrate to achieve the desired thickness. Depending on the area and thickness of the PMMA film, the solution is left to sit for tens of minutes to prevent irregular deformation of the sample due to the buoyancy of the water during the subsequent immersion process. The deposited PMMA film gradually creates gradient micro/nanopores within the membrane and is eventually solidified during immersion in deionized water. Porous PMMA ultrafilms are obtained by peeling the film off the surface of the substrate.
Figure 1. Schematic diagram of the radiative cooling mechanism of hierarchical gradient porous PMMA ultrathin films and their manufacturing process. (a) Porous PMMA ultra-thin films with high Rsolar and ɛir are able to achieve cooling temperatures below ambient temperature when exposed to sunlight. (b) Schematic diagram of the multi-stage solvent replacement process.
Figure 2 illustrates the porous structure of a layered gradient in a PMMA ultrafilm, where the pore size gradually decreases from the bottom layer to the top layer. When the bottom layer of the PMMA film is directly facing the sun, this structure can achieve high Rsolar and εIR at the same time. The bottom aperture is widely distributed and concentrated around ~1.0 μm, which can effectively scatter sunlight. At the same time, nanopores with a size of less than 1.0 μm will further enhance Rsolar, which enhances the scattering of shorter and visible wavelengths. Figure 3 shows the hemispherical spectral reflectance of the porous PMMA ultrathin film, which exhibits an ultra-high sunlight reflectance Rsolar = 0.99. At the same time, the PMMA film has an ultra-high thermal emissivity of 0.97 in the transparent window of the atmosphere (8-13 μm), thus ensuring that infrared radiation can be radiatively cooled through the atmospheric window to the cold outer space.
Figure 2. SEM image of PMMA ultra-thin film. (a) Bottom, (b) cross-section, and (c) top. (d-f) Pore size distribution along the decreasing cross-sectional gradient of PMMA ultrafilms.
Figure 3. (a) Hemispherical spectral reflectance of 1 mm thick transparent PMMA films and PMMA ultrafilms. (b) High solar reflectivity Rsolar and (c) thermal emissivity εIR of PMMA ultrathin films at different angles of incidence.
To investigate the effect of the addition of ethanol (a non-solvent for PMMA) to the precursor solution on the formation of micro/nanoporous PMMA films, we compared different ethanol inputs (0 g, 5 g, 10 g, and 15 g). The precursor solution prior to the addition of ethanol consisted of 5 g of PMMA and 10 g of acetone. Figure 4a-d clearly shows that the samples (PMMAe5g, PMMAe10g, and PMMAe15g) exhibit a distinctly white appearance after water immersion, while this is not the case in the samples (PMMAe0g). In addition, Figure 5e-h shows SEM images of the underlying layer of PMMAe0g, PMMAe5g, PMMAe10g, and PMMAe15g, respectively. The average pore size of these PMMA samples increased with the increase in the ethanol ratio in the PMMA-acetone-ethanol precursor solution, but no significant micro/nanopore formation (burr-like structure) was seen in the PMMAe0g. Therefore, the sunlight reflectance of PMMAe0g will be greatly affected, because the loss of pores will seriously affect the scattering performance of incident light.
Figure 4. (a-d) optical images and (e-h) SEM images of the bottom surface of PMMA e0g, PMMA e5g, PMMA e10g, and PMMA e15g.
Figure 5 illustrates the mechanism of micro/nanopore formation in PMMA ultrathin films for multi-stage solvent displacement processes. Here, the "ouzo effect" acts as a driving force for solvent displacement to form micro/nanopores. Ouzo is an alcoholic beverage in Greece. Before eating, it is customary to pour a clear liquid into a glass with water, allowing it to quickly transform into an opaque milky white solution, which is the result of trans-spontaneous aggregation (emulsification) of solutes insoluble in water. The ouzo effect has been expanded to describe a universal physical phenomenon in which metastable colloidal dispersions spontaneously arise when water (as a non-solvent) is mixed with a water-miscible solvent containing hydrophobic solutes. Hydrophobic PMMA particles are dissolved in acetone solvent. Since PMMA is insoluble in ethanol and ethanol and acetone dissolve each other, the concentration of acetone is reduced by adding ethanol to the precursor solution. Finally, the concentrations of both acetone and ethanol drop dramatically during water immersion. Local supersaturation of hydrophobic PMMA molecules leads to spontaneous nucleation in the form of small structures over time, resulting in the formation of abundant micro/nanopores in PMMA ultrafilms. Here, ethanol acts as a buffer during pore formation. Due to the slow dissolution of PMMA particles in acetone, it is difficult for acetone to quickly escape from the PMMA-acetone mixture without the addition of ethanol during water immersion, resulting in the formation of micropores/nanopores without deformation of the overall PMMA film structure. This is the reason why there are no micro/nanopores inside the PMMAe0g. However, the early addition of ethanol can reduce the binding force between PMMA and acetone, which helps acetone to quickly detach from PMMA after immersion in water. Over time, this favors the formation of subsequent pores in the PMMA ultrathin film. Therefore, ethanol is added as a buffer during pore formation. At the same time, the distribution of micro/nanopore delamination gradients in PMMA ultrafilms is mainly attributed to the adhesion between the PMMA ultrafilms and the support substrate. During the initial phase of PMMA curing during water immersion, the underside of the PMMA-acetone-ethanol precursor solution establishes good contact with the substrate and exhibits strong adhesion, mainly due to the intrinsic viscosity of PMMA. As a result, the bottom layer of the PMMA ultrafilm is subjected to a strong external traction from the fixed substrate, which prevents the pores of the lower layer from becoming smaller due to solvent displacement or reduced due to permeation of the surrounding PMMA solution. However, the effect of external traction from the support substrate on the upper layer of the PMMA film gradually diminishes. As the solidification of PMMA progresses, the pores formed in the upper layer may not be fully developed and may gradually shrink in size or be filled by the surrounding PMMA solution. Therefore, the decreasing external traction force from bottom to top contributes to the formation of graded gradient micro/nanopores in PMMA films throughout the curing process of PMMA.
Figure 5. Schematic diagram of micro/nanopore formation in PMMA ultrathin films driven by the "ouzo effect" during solvent displacement.
At the same time, in order to verify the actual outdoor cooling effect of the PMMA ultra-thin film, we measured its temperature drop and cooling power (Figure 6). The PMMA ultra-thin film achieves an average temperature of 4.6 °C below ambient and an average radiative cooling power of 90 W/m2. At the same time, the PMMA ultra-thin film can achieve a peak temperature drop of 8.2 °C at 780 W/m2 sunlight intensity. All this shows that PMMA ultra-thin films have excellent radiative cooling ability.
Figure 6. Outdoor radiative cooling performance of PMMA ultra-thin films. (a) Schematic diagram of outdoor real-time measurement setup. (b) solar intensity, (c) wind speed and relative humidity during the experiment. (d) Temperature data of ambient air, wood boards, and PMMA ultrafilms during the experiment. (e) Radiative cooling power of PMMA ultra-thin films.
Paper Links:
Hatps://doi.org/10.1016/J.Cage.2024.150657
Source: Frontiers of Polymer Science