The facile surface modification of poly(p-xylylene) ultrathin films

The parylene polymers studied here are vacuum deposited by a common but unique method that is self-initiating and unterminating. The lack of chain termination allows the chain endings of the polymers to react with atmospheric oxygen and 50 wt.% aqueous ammonium sulfide. However, the latter reaction in particular can only take place at the polymerʹs surface due to the excellent barrier properties of the parylenes, which leaves the surface of parylene hydroxyl terminated and consequently hydrophilic. High parylene polymer typically cannot be easily modified since few chain ends exist at its surface; however, this is in contrast to ultrathin oligomeric parylene films. The surface chemistry of the modified and annealed ultrathin parylene films was undertaken with X-ray photoelectron spectroscopy and contact angle goniometry. Results show that as-deposited ultrathin parylene films (15–32 Å) react with atmospheric oxygen (from XPS) due to a rather large O1s peak and asymmetry in the C1s spectrum due to CO bonding. The ultrathin film also exhibits a precipitous drop in its contact angle after exposure to 50 wt.% ammonium sulfide. However, after post-deposition annealing at 200 °C, little oxygen is observed by XPS and none after 250 °C. Further, films annealed at 350 °C in high vacuum show little reaction with aqueous ammonium sulfide much like the thick parylene films >2000 Å giving evidence that annealing forms a higher molecular weight polymer. The work here has implications for the gentle surface modification of parylene and most other polymers, both for functionalizing their surfaces and leading to improved adhesion to other dielectrics and metals.