Cross-linking and ultrathin grafted gradation of fluorinated polymers synthesized via initiated chemical vapor deposition to prevent surface reconstruction.

Poly(fluoroalkyl acrylate)s with long perfluorooctyl pendant groups have been found to lead to the release of biopersistent perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS). Those with no more than six perfluorinated carbons in pedant groups do not cause such problems. They, however, give poor dynamic water repellency due to extensive reorganization of surface fluorinated groups when exposed to the water interface. In this work, thin films exhibiting improved dynamic water repellency, as evidenced by water contact angle (WCA) measurements, were synthesized via substrate-independent initiated chemical vapor deposition (iCVD) from 1H,1H,2H,2H-perfluorooctyl acrylate (C6PFA) and divinylbenzene (DVB) using two methods: copolymerization and ultrathin grafted gradation. The copolymerization between C6PFA and the cross-linker, DVB, was confirmed by Fourier transform infrared (FTIR) spectroscopy. The cross-linking is concluded to hinder the reorganization of surface fluorinated groups. The grafted gradation, consisting of an ultrathin pC6PFA top layer and a pDVB base layer, was characterized by angle-resolved X-ray photoelectron spectroscopy (ARXPS) measurements, which indicated that the top layer of pC6PFA is <2.5 nm thick to achieve the best dynamic water repellency. The outmost surface of this structure is fully covered by fluorinated groups, giving hydrophobicity. Concurrently, thanks to the interlayer grafting and the ultrathinness of the top layer, the fluorinated groups' tendency to migrate away from water interface is sterically blocked by the highly cross-linked pDVB base layer. The proposed approaches effectively reduced WCA hysteresis of C6PFA-based thin film to as low as 26.9° while maintaining sufficient hydrophobicity (advanced WCA of 119.6°). Due to the conformal and substrate-independent nature of iCVD technique, the films could be used to coat textured surfaces to generate superhydrophobicity.