Polyethylene (C2H4)n

Publisher Summary This chapter focuses on the optical properties of polyethylene (C 2 H 4 ) n . In the far-infrared region of the electromagnetic spectrum, polyethylene is one of the most widely used materials for the fabrication of optical elements such as windows and lenses. This is not only because of its transparency but also its availability, low cost, machinability, and relative stability. The optical properties of polyethylene, and indeed of many other polymers in the far infrared, are specimen dependent and vary significantly depending on the past history of the specimen. In the spectral region, it is not unusual to find differences of up to a factor of two between the absorption spectra of specimens with different histories. Some of the factors that affect this include the particular manufacturing process used and the density and degree of crystallinity achieved in the final material. The presence in the material of impurities such as antioxidants, agents to improve flow during the manufacturing process, heavy-metal salts or oxides as residues of the catalysts used in polymerization, and microdroplets of water also vary the optical properties of polyethylene. Similarly, exposure of the material to environmental factors such as high temperatures, ultraviolet radiation, mechanical stress, and high-energy irradiation significantly change the far-infrared optical constants of this material. It is, therefore difficult to arrive at a unique set of intrinsic optical constants for polyethylene in the far-infrared. There are several measurements of the transmission properties of polyethylene, but there is little quantitative information available on the spectral variation of both optical constants over this region.