Polyphenyl Ethers

Dr. David S. Stone and Manuel E. Joaquim

Polyphenyl ethers, a group of molecules with a long history of performance in extreme conditions, are now being used as high-performance optical materials.

Polyphenyl ethers, or PPEs, are a class of molecules new to optical applications, but with a long history of stability and robustness in other fields.

These short-chain cyclic compounds are linked by oxygen atoms. A characteristic formulation has two to eight aromatic groups, or rings, allowing PPEs to retain their chemical and performance stability against ionizing radiation, thermal excursions, chemical contamination and pressure changes. Their optical clarity, high refractive index and ability to be formulated as fluids, gels or solids have enabled them to meet rigorous performance demands of signal processing in advanced photonic systems.

The unusual combination of optical and thermal properties in polyphenyl ethers (PPE) is derived from their molecular structure, as demonstrated by this five-ring PPE molecule.

The toughness of PPEs is a combination of stability and flexibility, qualities that are related to molecular structure. Resonance energy of the aromatic rings makes them resistant to high temperatures, radiation and oxidation, while their oxygen linkages provide a local point of rotation that allows the molecules to flex. The molecular structure also gives the material a high index of refraction. A subgroup called thioethers, in which sulfur replaces the oxygen linkages, has similar properties.

PPEs first found commercial application as high-temperature lubricants and corrosion blockers in the engine turbines of the SR-71 spy aircraft, where operating temperatures of 316 °C (600 °F) would oxidize or decompose other hydrocarbon molecules. At the other extreme, they can remain in liquid form in temperatures below 0 °C, where rigid molecules would pack tightly and become a solid. Their consistent performance across wide temperature ranges opened up such applications as lubricants on space satellites and as fluids in vacuum-diffusion pumps — where they were selected for very low vapor pressures, in the range of 4 x 10^—10 t.

On the pins of electronic connectors, where they are used as lubricants and corrosion blockers, their lifetime is between 40 and 50 years. Applied to gold, tin/lead and other electronic metals, PPEs virtually eliminate metal wear and prevent fretting or galvanic corrosion by capturing or blocking corrosive particles on the connector surface or in the atmosphere.

PPEs have unusually high surface tension, in the range of 50 dynes per centimeter. The benefit is that they stay in position when placed against a flat substrate, unlike conventional fluids such as long-chain hydrocarbons or silicones. Also extraordinarily resistant to ionizing radiation, they are used as lubricants in nuclear power plants.

Fluid PPEs have a long history of application in unusual or extreme situations and can be formulated as gels, coatings or resins.

Fluid PPEs are being used on a small scale in commercial production as optical coupling media, and gels and coatings are available for sampling and testing. Synthesis of PPEs for optical and nonoptical applications is far from trivial, but experience from other fields is useful in synthesizing optical-grade materials.

The optical clarity of PPEs resembles that of other optical polymers: propagating wavelengths between approximately 400 and 1700 nm and tending toward opaqueness in the UV and mid-IR portions of the spectrum.

Optical polymers generally have refractive indices between 1.3 and 1.7. Refractive index is important when matching materials across whose boundaries light must propagate: The closer the match of the two materials’ indices, the less light is lost at their conjunction. PPEs have refractive indices from 1.6 to 1.7, near and even above those of other optical polymers.

This makes them suitable for matching high-index glasses such as LaBSF (refractive index 1.68), optical plastics such as polyimide (1.66) and photonic semiconductors (2.0 and above). Also, PPEs are the only fluid-phase optical polymers that combine high refractive indices with overall robustness for thermal, oxidative and chemical stability; low vapor pressure; compatibility with other polymers; and nontoxicity.

High refractive index

As the ambient temperature (T) changes, the refractive index (n) of optical polymers also changes. The thermo-optic coefficient of refractive index (dn/dT) for optical polymers is typically from 1 to 3 x 10^—4 per degree Celsius, and is roughly one to two orders of magnitude greater than the coefficient for most optical glasses. Changes in index, both linear and reversible, are used beneficially in devices such as optical switches, attenuators and filters to enable the processing of optical signals.

Along with a high refractive index, PPEs also have high thermo-optic coefficients, generally from 4 to 5 x 10^—4 per degree Celsius. This permits higher thermo-optic phase modulation over a given path length and temperature range than would be possible with optical materials with lower coefficients.

As formulated for nonoptical applications, PPEs are a low-viscosity colorless or yellowish fluid. A five-ring PPE has a viscosity of 360 centipoise (cP) at 100 °F; a four-ring PPE at that temperature has a viscosity of 70 cP. Water, by comparison, has a viscosity of 1 cP, and light oils are around 100 cP.

Fluid PPEs used in current optical applications are contained mechanically within the optical device by seals or other enclosures. However, emerging optical PPE gels, coatings and resins will appear in a form that will not migrate from a device’s optical interface.

The thermal and oxidative stability of PPEs is much higher than that of other optical polymers. As measured by an isoteniscope, a five-ring PPE has a thermal stability of 453 °C, and a four-ring, 441 °C. By comparison, most conventional hydrocarbon polymers begin to degrade in the 100 to 200 °C range. Silicone polymers are somewhat more stable, but they, too, must be analyzed closely for stability behavior above 200 °C. The low-temperature service limit for these materials is the point at which a sudden nonlinear change occurs in thermal expansion, analogous to a glass transition. Various PPEs exhibit glass transition points in the range from 25 to below —40 °C.

Manufacturability

Thermal stability is also a significant consideration during manufacture, particularly when an optical polymer is briefly exposed to the temperatures of solder reflow ovens or, less frequently, of wave soldering in optoelectronics manufacturing. Temperatures in these environments are trending upward. For decades, the solder used almost universally in electronics manufacturing has been an alloy of tin and lead, with a melting point around 183 °C and requiring reflow temperatures of around 225 °C.

In the past few years, however, environmental concerns and marketing strategies have gradually phased out lead-based solder. This movement began in Japan, spread to Europe and is only beginning to cause manufacturing changes in the US.

A frequent replacement for lead is a tin-silver-copper alloy with reflow temperatures up to 260 °C, roughly 35 °C above the temperatures required to reflow conventional tin-lead. As optical components become more highly integrated through optical board-level assembly, their internal optical polymer materials will be pushed to survive these higher surge temperatures.

The stability of PPEs at temperatures far above reflow heat means that they can be incorporated into devices that will undergo soldering without significant concerns about evaporation, oxidation or decomposition. Yellowing is the most common symptom of failure for conventional polymers under these conditions.

As with other polymer chemistries, one can select PPE formulations that are the most favorable mechanically for a given application. The fluid, gel and solid versions permit control of the tensile modulus over a wide range, from a pure fluid (zero modulus) to a hard coating (high modulus). This allows the selection of a consistency that minimizes stress buildup in an assembly — a key parameter for ensuring thermal and mechanical reliability of a design.

Optical applications

The volumetric coefficient of thermal expansion of PPEs is in line with that of other optical polymers, which range from 20 ppm/°C for hard plastics up to 1000 ppm/°C for the softer gels and fluids. These considerations help avoid mismatch of coefficients at material interfaces, where thermal changes can create mechanical stress. Selection of a particular formulation may provide the solution for a specific design problem — for example, using a PPE in gel form to reduce mechanical stress at an interface while maintaining optical properties.

Their extreme resistance to ionizing radiation gives PPEs an advantage in the manufacture of solar cells and solid-state UV/blue emitters. Solar cells are exposed to the full range of the solar spectrum, including the UV. Incorporating a coating or layer of PPE can permit index matching to the cell and thereby increase the cell’s photoconversion efficiency.

Outdoor flat panel displays, like solar cells, are subjected to direct sunlight. A layer of PPE between the display emitter array and the high-index cover glass can improve image contrast in ambient light because they have higher indices of refraction than silicones and hydrocarbons. Also, their superior UV-hardness renders them less susceptible to delamination and yellowing with age.

Index-matching capabilities make PPEs useful in high-performance backlit flat panel color displays.

Projection display systems for next-generation digital theater are a rough environment for hydrocarbon, fluorocarbon and silicone lens coatings. Before the high-intensity light beam projects onto a screen, it travels through a sequence of image manipulation optics. The light flux includes the shortest wavelengths in the visible spectrum and can even be marginally ionizing. The intense radiation flux at longer visible wavelengths heats even high-quality glass lenses and prisms to elevated temperatures. A fluid PPE used as a lens face coolant can transfer more heat and do it more quietly than forced-air cooling. It can also keep dust particles out of the image field and improve the optical efficiency between elements.

The exotic high-index glasses or semiconductors from which many optical signal processing components are fabricated for telecommunications require media with high indices of refraction for index matching of adjoining materials. PPE coatings in fluids, gels or in hard resin form are useful — particularly resin coatings, which provide the advantage of serving as radiation-hardened antireflection coatings at air interfaces.

Meet the authors

David S. Stone and Manuel E. Joaquim are the founders and principals of SantoLight JV, a unit of Findett Corp. in St. Charles, Mo.