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, paracyclophane, yields 100% monomer above 550 °C in vacuum and does not yield any by-products. That said there are alternative precursors to arrive at the parylene polymers that possess leaving groups as opposed to the cyclophane precursor, 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.
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 (~115 °C) 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.
Nearly all the parylenes are insoluble at room temperature except for the alkylated parylenes, one of which is parylene E. This lack of solubility has made it difficult to re-work printed circuit boards coated with parylene.
Copolymers and nanocomposites (SiO2/parylene C) of parylene have been deposited at near-room temperature previously; and with strongly electron withdrawing comonomers, parylene can be used as an initiator to initiate polymerizations, such as with N-phenyl maleimide. Using the parylene C/SiO2 nanocomposites, parylene C could be used as a sacrificial layer to make nanoporous silica thin films with a porosity of >90%.
Read more about Parylene: Parylene N, Common Halogenated Parylenes, Reactive Parylenes, Parylenes As Molecular Layers, Adhesion, History, Characteristics and Advantages, Typical Applications