Preparation and properties of cellulose nanocrystal based structurally colored radiative cooling composite films
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摘要: 辐射制冷通过向外太空发射热量来降低自身温度，是一种绿色、低碳和可持续的降温策略。大多数辐射制冷材料外观颜色单调，多为白色或透明状，而传统着色剂的加入会使材料吸收热量，降低辐射制冷性能。通过自组装法制备具有可调结构色的纤维素纳米晶体/聚乙二醇(CNC/PEG)复合辐射制冷薄膜。并将复合薄膜与多孔结构的醋酸纤维素(CA)膜结合，得到结构色辐射制冷双层复合膜。结果表明：纤维素纳米晶复合薄膜具有鲜艳的结构色，具有明显的双折射现象。随着PEG含量的增加，复合薄膜结构的螺距增大，颜色由蓝绿色转变为红色。CNC/PEG结构色复合薄膜在可见光波段最高反射率可达68.5%，在大气窗口波段的发射率高达93%，具有3.4 ℃左右的环境降温效果。CNC/PEG-CA双层复合膜在可见光波段最高反射率高达91.8%，在大气窗口波段的发射率可达32.2%。与复合薄膜相比，双层复合膜的降温性能更好，与环境温度相比，具有14.3 ℃左右温差。在户外测试中，与环境温度相比，复合薄膜可达到2 ℃左右的降温效果，双层复合膜可达到6 ℃左右的降温效果。Abstract: Radiative cooling lowers its own temperature by emitting heat into outer space and is a green, low-carbon and sustainable cooling strategy. Most radiative cooling materials have monotonous color appearance, mostly white or transparent, and the addition of traditional colorants will cause the materials to absorb heat and reduce the radiative cooling performance. Cellulose nanocrystal/polyethylene glycol (CNC/PEG) composite radiative cooling films with tunable structural colors were prepared by self-assembly method. And the composite films were combined with cellulose acetate (CA) films with porous structure to obtain structure-colored radiation-cooled bilayer composite films. The results show that the cellulose nanocrystalline composite films have bright structural colors with obvious birefringence phenomenon. With the increase of PEG content, the pitch of the composite film structure increases, and the color changes from blue-green to red. the CNC/PEG structural color composite film has the highest reflectance up to 68.5% in the visible light band, and the emissivity in the atmospheric window band is up to 93%, with an ambient cooling effect of about 3.4 ℃. the CNC/PEG-CA bilayer composite film has the highest reflectance in the visible light band up to 91.8%, and the emissivity in the atmospheric window band is up to 32.2%. Compared with the composite film, the bilayer composite film has better cooling performance, with a temperature difference of about 14.3 ℃ compared with the ambient temperature.In outdoor tests, compared with the ambient temperature, the composite film can achieve a cooling effect of about 2 ℃, and the bilayer composite film can achieve a cooling effect of about 6 ℃.
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图 1 (a)纤维素纳米晶体(CNC)的TEM图像;(b)CNC的粒径图像;(c)CNC的电位图像
Figure 1. (a) TEM image of Cellulose nanocrystal (CNC); (b) Particle size image of CNC; (c) Potential image of CNC
图 2 (a) 纤维素纳米晶体/聚乙二醇(CNC/PEG)-10%、(b) CNC/PEG-20%、(c) CNC/PEG-30%、(d) CNC/PEG-40%的光学图像；(e) CNC/PEG 的紫外-可见反射光谱图；(f) 复合薄膜辐射制冷示意图
Figure 2. Digital photographs of (a) Cellulose nanocrystal/polyethylene glycol (CNC/PEG)-10%, (b) CNC/PEG-20%, (c) CNC/PEG-30%, (d) CNC/PEG-40%; (e)UV-vis reflection spectra of CNC/PEG; (f) Composite film radiative cooling schematic
图 3 不同PEG含量的复合薄膜横截面电镜图像
Figure 3. Cross-sectional electron microscope images of composite films with different PEG contents
图 4 不同PEG含量的复合薄膜的偏光显微镜示意图
Figure 4. Polarized light microscopy schematic of composite films with different PEG contents
图 5 不同PEG含量的复合薄膜:(a)发射率曲线;(b)在可见光范围内的反射率曲线
Figure 5. Composite film with different PEG contents: (a) Emissivity profile; (b) Reflectivity profile in the visible light range
图 6 不同PEG含量的双层复合制冷膜:(a)发射率曲线;(b)在可见光范围内的反射率曲线
Figure 6. Bilayer composite film with different PEG contents: (a) Emissivity profile; (b) Reflectivity profile in the visible light range
图 7 (a)室内氙灯模拟装置图；(b)自组装温度测量装置图；(c)CNC/PEG-10%和空气 ;(d)CNC/PEG-30%和空气温度对比图
Figure 7. (a) Photos of indoor xenon lamp simulation device; (b) Self assembling temperature measuring device; Temperature comparison of (c) CNC/PEG-10% and air; (d) CNC/PEG-30% and air
图 8 ((a)-(c))醋酸纤维素膜、滤纸和A4纸红外热成像图;(d)醋酸纤维素膜表面电镜图;(e)CA与空气温度对比图
Figure 8. ((a)-(c)) Infrared thermograms of CA film, filter paper and A4 paper; (d) SEM of CA film surface; (e) Temperature comparison of CA and air
图 9 ((a)-(c))CNC/PEG-20%、CNC/PEG-20%-CA与带有蓝色涂料的CA薄膜红外热成像图;(d)带蓝色涂料的CA和空气；(e)CNC/PEG-20%-CA和空气；(f)CNC/PEG-10%-CA和空气；(g)CNC/PEG-30%-CA和空气温度对比图
Figure 9. ((a)-(c)) Infrared thermograms of CNC/PEG-20%, CNC/PEG-20%-CA and and CA films with blue coatings; Temperature comparison of (d) CA with blue painting and air; (e) CNC/PEG-20%-CA and air; (f) CNC/PEG-10%-CA and air; (g) CNC/PEG-30%-CA and air
图 10 (a,b)测试CNC/PEG-30%-CA、CNC/PEG-30%和空气的温差变化的户外装置图；(c)CNC/PEG-30%-CA、CNC/PEG-30%与空气的温差图
Figure 10. (a,b) Diagram of an outdoor installation for testing the change in temperature difference between CNC/PEG-30%-CA, CNC/PEG-30% and air; (c) Temperature comparison of CNC/PEG-30%-CA、CNC/PEG-30% and air
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