Parylene

Parylene is the trade name for a variety of chemical vapor deposited poly(p-xylylene) polymers used as moisture and dielectric barriers. Among them, Parylene C is the most popular due to its combination of barrier properties, cost, and other processing advantages.

Parylene is green polymer chemistry. It is self-initiated (no initiator needed) and un-terminated (no termination group needed) with no solvent or catalyst required. The commonly used precursor, [2.2]paracyclophane, yields 100% monomer above 550 °C in vacuum and does not yield any by-products (Gorham Process). There is very little concern that parylene N will be 'over-cracked', meaning [2.2]paracyclophane is converted to p-xylylene cleanly with no side-reactions occurring. However, the same cannot be said for parylene C. The aryl-chlorine bond in dichloro[2.2]paracyclophane readily breaks at 680 °C (standard pyrolysis temperature); and therefore it is desirable to optimize each parylene tool in terms of its pyrolysis temperature using a mass spectrometer.

There are alternative precursors to arrive at the parylene polymers, which possess leaving groups, the most popular using bromine to yield the parylene AF-4 polymer. However, bromine is corrosive towards most metals and metal alloys and Viton O-rings so it is difficult to work with and precautions are needed. More recently, a liquid precursor route was developed yielding parylene N using methoxy leaving group. Although [2.2]paracyclophane is already inexpensive, this precursor is much less expensive and it can delivered reliably using a mass-flow controller (MFC), a huge advantage for process control, which has been lacking for years with the standard Gorham process.

Parylene C and to a lesser extent AF-4, SF, HT (all the same polymer) are used for coating printed circuit boards (PCBs) and medical devices. There are numerous other applications as parylene is an excellent moisture barrier. It is the most bio-accepted coating for stents, defibrillators, pacemakers and other devices permanently implanted into the body.

Parylenes are relatively soft (parylene N 0.5 GPa) except for cross-linked Parylene X (1.0 GPa) and they have poor oxidative resistance (~60-100 °C depending on failure criteria) and UV stability, except for Parylene AF-4. However, Parylene AF-4 is more expensive due to a three-step synthesis of its precursor with low yield and poor deposition efficiency. Their UV stability is so poor that parylene cannot be exposed to regular sunlight without yellowing.

...
Wikipedia