Optical engineering of polymer materials and composites for simultaneous color and thermal management
A field of research that has gained a significant amount of attention in recent years involves the development of sustainable products with tailored optical, mechanical, and thermal properties. Material degradation in the environment is due to photo-oxidation and exposure to UV radiation as well as microorganisms. Owing to its morphology and high crystallinity, polyethylene (PE), and especially the ultrahigh molecular weight (UHMW) variety, exhibits stability to microbial growth and is considered non-biodegradable under natural environmental conditions. This paper examines engineering approaches involving semi-crystalline UHMWPE films. These films demonstrate high mechanical and thermal functionalities and, in contrast with other conductive materials such as metals or carbon, are lightweight and have low cost. The authors discuss the fabrication and characterization of a family of organic-inorganic UHMWPE composite films and examine various methods to shape their functionalities. They examine targeted addition of compounds and selective absorption and phosphorescence to tailor the optical properties of composites across a broad (visible and IR) spectral range. They demonstrate examples of dark or brightly colored composites that exhibit low temperatures under direct illumination by sunlight. They point out that the infrared transparency of polyethylene composites may be modified by inclusion of silica, aluminum oxide, graphite, and tin oxide. These composites may also be utilized in the production of fibers for use in "wearable electronics" or smart textiles. The authors also indicate that the composites may have applications in IR "coloring," heat spreading, and passive cooling via radiation, which can be used to reduce thermal damage. The effect of whitening of the color of the composites due to the addition of TiO2 or other UV attenuators, to offset the photo-discoloration of the colorants, is addressed by adjusting the size of the particles and by reducing the scattering of the visible light. Finally, the authors point out that the inclusion of the colorants in PE films does not mask the NIR signature of the composite, which is utilized in the recycling process. Overall, while additional research is needed, this paper provides some valuable and exciting insights for researchers interested in fabricating materials that require tailored spectral, mechanical, and thermal properties.