Phenyl ether polymers are a class of polymers that contain a phenoxy or a thiophenoxy group as the repeating group in ether linkages. Commercial phenyl ether polymers belong to two chemical classes: polyphenyl ethers (PPEs) and polyphenylene oxides (PPOs). The phenoxy groups in the former class of polymers do not contain any substituents whereas those in the latter class contain 2 to 4 alkyl groups on the phenyl ring. The structure of an oxygen-containing PPE is provided in Figure 1 and that of a 2, 6-xylenol derived PPO is shown in Figure 2. Either class can have the oxygen atoms attached at various positions around the rings.
Structure and synthesis
The proper name for a phenyl ether polymer is poly(phenyl ether) or polyphenyl polyether, but the name polyphenyl ether is widely accepted. Polyphenyl ethers (PPEs) are obtained by repeated application of the Ullmann Ether Synthesis: reaction of an alkali-metal phenate with a halogenated benzene catalyzed by copper. PPEs of up to 6 phenyl rings, both oxy and thio ethers, are commercially available. See Table 1. They are characterized by indicating the substitution pattern of each ring, followed by the number of phenyl rings and the number of ether linkages. Thus, the structure in Figure 1 with n equal to 1 is identified as pmp5P4E, indicating para, meta, para substitution of the three middle rings, a total of 5 rings, and 4 ether linkages. Meta substitution of the aryl rings in these materials is most common and often desired. Longer chain analogues with up to 10 benzene rings are also known. The simplest member of the phenyl ether family is diphenyl ether (DPE), also called diphenyl oxide, the structure of which is provided in Figure 4. Low molecular weight polyphenyl ethers and thioethers are used in a variety of applications, and include high-vacuum devices, optics, electronics, and in high-temperature and radiation-resistant fluids and greases. Figure 5 shows the structure of the sulfur analogue of 3-R polyphenyl ether shown in Figure 3.
Physical properties Typical physical properties of polyphenyl ethers are provided in Table 2. Physical properties of a particular PPE depend upon the number of aromatic rings, their substitution pattern, and whether it is an ether or a thioether. In the case of products of mixed structures, properties are hard to predict from only the structural features; hence, they must be determined via measurement. The important attributes of PPEs include their thermal and oxidative stability and stability in the presence of ionizing radiation. PPEs have the disadvantage of having somewhat high pour points. For example, PPEs that contain two and three benzene rings are actually solids at room temperatures. The melting points of the ordinarily solid PPEs are lowered if they contain more m-phenylene rings, alkyl groups, or are mixtures of isomers. PPEs that contain only o- and p-substituted rings have the highest melting points.
Thermo-oxidative stability PPEs have excellent high temperature properties and good oxidation stability. With respect to volatilities, p-derivatives have the lowest volatilities, and the o-derivatives have the highest volatilities. The opposite is true for flash points and fire points. Spontaneous ignition temperatures of polyphenyl ethers lie between 550 and 595 °C (1,022 and 1,103 °F), alkyl substitution reduces this value by ~50 °C (122 °F). PPEs are compatible with most metals and elastomers that are commonly used in high-temperature applications. They typically swell common seal materials. Oxidation stability of un-substituted PPEs is quite good, partly because they lack easily oxidizable carbon-hydrogen bonds. Thermal decomposition temperature, as measured by the isoteniscope procedure, is between 440 and 465 °C (824 and 869 °F).
Radiation stability Ionizing radiation affects all organic compounds, causing a change in their properties because radiation disrupts covalent bonds that are most prevalent in organic compounds. One result of ionization is that the organic molecules disproportionate to form smaller hydrocarbon molecules as well as larger hydrocarbons molecules. This is reflected by increased evaporation loss, lowering of the flash and fire points, and increased viscosity. Other chemical reactions caused by radiation include oxidation and isomerization. The former leads to increased acidity, corrosivity, and coke formation; the latter causes a change in viscosity and volatility. PPEs have extremely high radiation resistance. Of all classes of synthetic lubricants (with the possible exception of perfluoropolyethers) the polyphenyl ethers are the most radiation resistant. Excellent radiation stability of PPEs can be ascribed to the limited number of ionizable carbon-carbon and carbon-hydrogen bonds. In one study, the performance of PPE under the influence of 1×1011 ergs/gram of radiation at 99 °C (210 °F) was compared with synthetic ester, synthetic hydrocarbon, and silicone fluids. PPE showed a viscosity increase of only 35%, while all other fluids showed a viscosity increase of 1700% and gelled. Further tests have shown PPEs to be resistant to gamma and associated neutron radiation dosages of 1×1010 erg/g at temperatures up to 315 °C (599 °F).
Surface tension PPEs have high surface tension; hence these fluids have a lower tendency to wet metal 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.
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