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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 (Gorham Process). There is very little concern that parylene N will be 'over-cracked', meaning ()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()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.〔J.J. Senkevich “Non-Halogen Liquid Precursor Route to Parylene” Chem. Vapor Dep. 17(4-6) 76-9 (2011). DOI: 10.1002/cvde.201104304〕 Although ()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)〔C. Chiang, A. S. Mack, C. Pan, Y.-L. Ling, D. B. Fraser ''Mat. Res. Soc. Symp. Proc.'' vol. 381, 123 (1995).〕 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. Nearly all the parylenes are insoluble at room temperature except for the alkylated parylenes, one of which is parylene E and the alkylated-ethynyl parylenes.〔J.J. Senkevich, “t-butylethynyl-parylene and phenylethynyl-parylene” Chem. Vapor Dep. 19, 1-5 (2013). DOI: 10.1002/cvde.201307071〕 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%. ==Parylene N== Parylene N is a polymer manufactured (chemical vapor deposited) from the ''p''-xylylene intermediate. The ''p''-xylylene intermediate is commonly derived from ()paracyclophane. The latter compound can be synthesized from ''p''-xylene involving several steps involving bromination, amination and Hofmann elimination. Parylene N is an un-substituted molecule. Heating ()paracyclophane under low pressure (0.01 – 1.0 Torr) conditions and cracking it at 450-700 °C gives rise to the ''p''-xylylene intermediate, which polymerizes when physisorbed on a surface. The ''p''-xylylene intermediate has two quantum mechanical states, the benzoid state (triplet state) and the quinoid state (singlet state). The triplet state is effectively the initiator and the singlet state is effectively the monomer. The triplet state can be de-activated when in contact with transition metals or metal oxides including Cu/CuOx. Many of the parylenes exhibit this selectivity based on quantum mechanical deactivation of the triplet state, including parylene X. However, like any selective process there is a 'selectivity' window based on mostly deposition pressure and deposition temperature for the parylene polymers. What is more, the intermediate, ''p''-xylylene has a low reactivity and therefore a small 'sticking coefficient' and as a result parylene N produces a highly conformal thin film or coating. The deposition of parylene N is a function of a two-step process. First, physisorption needs to take place, which is a function of deposition pressure and temperature. The physisorption has inverse Arrhenius kinetics, in other words it is stronger at lower temperatures than higher temperatures. All the parylenes have a critical temperature called the threshold temperature above which practically no deposition is observed. The closer the deposition temperature is to the threshold temperature the weaker the physisorption. Once physisorption occurs, the ''p''-xylylene intermediate needs to react with itself (2nd step) for polymerization to occur. For parylene N, its threshold temperature is 40 °C. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「parylene」の詳細全文を読む スポンサード リンク
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