Enhanced Cross-Linked Density by Annealing on Fluorinated Polymers Synthesized via Initiated Chemical Vapor Deposition To Prevent Surface Reconstruction

Thin films of 1 H,1 H,2 H,2 H -perfluorodecyl acrylate copolymerized with the cross-linker divinylbenzene (p(PFDA- co -DVB)) were synthesized via initiated chemical vapor deposition (iCVD). The vapor deposited approach is significant as the fluorinated monomer and hydrocarbon cross-linker lack a common solvent. Changes in physical and chemical properties in the copolymers before and after an annealing treatment were characterized by Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), contact angle (CA) measurements with water and mineral oil, X-ray diffraction (XRD), and nanoindentation. Cross-linking results in lower CA hysteresis than measured for the iCVD p(PFDA) homopolymer. Furthermore, annealing improves the extent of reaction of the vinyl bonds on the DVB units incorporated into the film. This further enhancement in the cross-linking density is correlated with an additional reduction in the CA hysteresis observed after annealing of the p(PFDA- co -DVB) films. CA hysteresis values with water as low as 5° were achieved while maintaining advancing angles θ > 130°. For mineral oil, CA hysteresis was found to be lower than 1° with θ > 85°. The cross-linking is concluded to provide a steric barrier to the reconstruction of the surface fluorinated groups when switching between dry and wet conditions and is a primary factor in the observed reduction in CA hysteresis.