CVD of Poly(α,α′‐dimethyl‐p‐xylylene) and Poly(α,α,α′,α′‐tetramethyl‐p‐xylylene)‐co‐poly(p‐xylylene) from Alkoxide Precursors II: Thermal Oxidative Stability

Abstract

The Achilles heel of the parylene polymers is their oxidative stability at room temperature with UV exposure and at elevated temperature >115 8C in air. This weakness is attributed to the aliphatic carbon-carbon single bond in the main-chain backbone. Fluorinating this chemistry helps the oxidation substantially, but the precursor then becomes nearly prohibitive to use due to cost of the precursor and the processing cost. Another option is to explore the methylation of this benzyl position. Poly(a,a 0 -dimethyl-p-xylylene) and poly(a,a,a 0 ,a 0 -tetramethyl-p-xylylene)-co-poly( p-xylylene) are deposited on the parylene platform via alkoxy precursors. Their oxidation as a function of temperature to 300 8C; and time at 150 8C is explored via variable angle spectroscopic ellipsometry (VASE) and infrared spectroscopy (IRS). Results show very little difference between the two polymers as a function of oxidation temperature, and better results compared to what was previously observed with the poly( pxylylene) homopolymer, however the oxidation of poly(a,a 0 -dimethyl-p-xylylene) at 150 8C as a function of time is less positive with 16% thickness loss over 1000 h with the post-deposition annealed sample.

Functionalization of Poly(para‐xylylene)s—Opportunities and Challenges as Coating Material

“…The dimethyl homopolymer exhibits a good thermal stability up to 420 °C, but this was only deduced from thickness measurements and it was also emphasized that PPX should be annealed close to their degradation temperature after the deposition to provide higher densities and induce the outgassing of impurities. [ 82,83 ] Nevertheless, the results indicated a higher oxidative stability for the homopolymer as well as for the copolymer.…”

“…[ 85 ] Senkevich showed that poly(α,α′‐dimethyl‐ p ‐xylylene) could be made out of dimethoxy precursors. [ 82,83 ] The formed methoxy by‐product did not really disturb the polymerization, because of its low molecular weight, it just “fly‐by” the deposition chamber. The synthesis of the tetramethylated homopolymer was not successful and was explained with the steric hindrance but a copolymer with PPX could be achieved.…”