surfaces. The surface tension of the commercially available 5R4E is 49.9 dynes/cm, one of the highest in pure organic liquids. This property is useful in applications where migration of the lubricant into the surrounding environment must be avoided. Applications While originally PPEs were developed for use in extreme environments that were experienced in aerospace applications, they are now used in other applications requiring low volatility and excellent thermo-oxidative and ionizing radiation stability. Such applications include use as diffusion pump fluids; high vacuum fluids; and in formulating jet engine lubricants, high-temperature hydraulic lubricants and greases, and heat transfer fluids. In addition, because of excellent optical properties these fluids have found use in optical devices. Ultra-high-vacuum fluids Vacuum pumps are devices that remove gases from an enclosed space to greatly reduce pressure. Oil diffusion pumps in combination with a fore pump are amongst the most popular. Diffusion pumps use a high boiling liquid of low vapor pressure to create a high-speed jet that strikes the gaseous molecules in the system to be evacuated and direct them into space that is being evacuated by the fore pump. A good diffusion fluid must therefore reflect low vapor pressure, high flash point, high thermal and oxidative stability and chemical resistance. If the diffusion pump is operating in the proximity of ionizing radiation source, good radiation stability is also desired. Data presented in Table 3 demonstrates polyphenyl ether to be superior to other fluids that are commonly used in diffusion pumps. PPEs help achieve the highest vacuum of 4 torr at 25 °C. Such high vacuums are necessary in equipment such as electron microscopes, mass spectrometers and that used for various surface physics studies. Vacuum pumps are also used in the production of electric lamps, vacuum tubes, and cathode ray tubes (CRTs), semiconductor processing, and vacuum engineering. Electronic connector lubricants 5R4E PPE has a surface tension of 49.9 dynes/cm, which is amongst the highest in pure organic liquids. Because of this, this PPE and the other PPEs do not effectively wet metal surfaces. This property is useful when migration of a lubricant from one part of the equipment to another part must be avoided, such as in certain electronic devices. A thin film of polyphenyl ether on a surface is not a thin contiguous film as one would envision, but rather comprises tiny droplets. This PPE property tends to keep the film stationary, or at least