Xylylene Polymers

Abstract

Pinhole‐free coatings of outstanding conformality and thickness uniformity are produced by exposing a substrate to a controlled deposition of pure gaseous monomer through the unique chemistry of p ‐xylylene, which is thermally stable but very reactive toward polymerization with other like molecules, in a process known as vapor deposition polymerization (VDP). A high molecular weight coating of poly( p ‐xylylene) (PPX) is produced. Recent VDP developments are stressed in the article. The convenient generation of the extremely reactive monomer is accomplished by thermal cleavage of its stable dimer, cyclo ‐di‐ p ‐xylylene (DPX), commercially in the Gorham process. The PPXs formed as coatings in the Gorham process are referred to generically as parylenes. Parylene N, Parylene C, and Parylene D are polymer coatings produced by the Gorham process using the dimers DPXN, DPXC, and DPXD, respectively. These dimers are the feedstocks for the coating process. The Parylene C monomer, chloro‐ p ‐xylylene, has become the de facto performance standard owing to its room temperature volatility. VDP processes using means other than pyrolytic cleavage of DPX to generate reactive monomer, none practiced commercially, include photopolymerization and plasma polymerization, as well as imidization to produce polyimide films. Manufacture of commercial DPXN is through two routes: direct pyrolysis of p ‐xylene and the Hofmann elimination route. DPXC and DPXD are prepared from DPXN by chlorinating. Thermodynamic considerations of the parylene process by VDP are covered. The importance of the physical processes of condensation and diffusion in VDP are treated, as is the propagation reaction. VDP produces films of uniform thickness having excellent conformality and purity. Electrical properties merit their use in electronic construction. Barrier, spectral, and optical properties are also useful. Crystallinity determines important properties of mechanical strength at elevated temperatures and solvent resistance, which constitute the principal advantages of PPX materials. Uses include circuit boards, hybrid circuits, semiconductors, capacitors, the bathythermograph, medical uses (as in pacemakers), artifact conservation, and in laser fusion targets. They are also used as barrier coatings, in corrosion control, and as dry lubricants. The dimers present little concern in handling and storage to an experienced operator. In ancillary operations such as cleaning or adhesion promotion, suitable precautions given in manufacturers' Material Safety Data Sheets should be observed.

Pluripotent Stem Cell-Based Therapies in Combination with Substrate for the Treatment of Age-Related Macular Degeneration

“…70,71 This family of polymers consists of poly-p-xylylene, or Parylene-N, which is a linear chain of poly-p-xylylene whose aromatic rings are substituent free (Fig. 2).…”

“…2). 70 Functionalizing the benzene ring with chlorine atoms produces polymer derivatives with varying mechanical, thermal, and electrical properties such as Parylene-C, which possesses 1 chlorine substituent per aromatic ring, and Parylene-D, which possesses 2 (Fig. 2).…”

“…2). 70,72 Initially, the electronics industry capitalized on Parylene-C's low water and gas permeability for producing protective coatings for circuit boards. Also, the chemical vapor deposition during the Parylene production process results in a conformal coat without pinholes.…”

“…Also, the chemical vapor deposition during the Parylene production process results in a conformal coat without pinholes. 70,73 Pinholes arise from bubbles that are trapped during the deposition process, which later burst to create defects in the film, thus rendering any underlying circuitry vulnerable. 74 This conformal coating property expanded Parylene's application to protecting delicate biomedical circuitry implanted into the relatively hostile environment of the body.…”

Thickness uniformity of gas-phase coatings in narrow channels: I. Long channels

“…Coating of high thickness uniformity can be deposited from a substance whose components have very low and approximately equal values of SC. The example of such a process is pyrolytic polymerization of poly-pxylylene (PPX) [10]. The SC of the paraxylylene (PX) monomer is estimated as low as 10 −4 -10 −5 [11], therefore PPX deposited from the gas phase is the most promising polymer to cover the surfaces of very complex topography with coating of high thickness uniformity.…”