Initiated chemical vapor deposition of fluoropolymer coatings for the surface modification of complex geometries

Abstract

Initiated chemical vapor deposition (iCVD) is a one-step, soventless process that can be used to produce polymeric thin films. The iCVD technique has been used to polymerize a wide variety of vinyl monomers such as glycidyl methacrylate (adhesive) and 2-hydroxyethyl methacrylate (hydrophilic). The proposed polymerization mechanism is the classical free radical polymerization mechanism of vinyl monomers. Monomer and initiator gases are fed into a vacuum chamber where resistively heated wires are used to thermally decompose the initiator molecules into free radicals. The free radicals then attack the vinyl bonds of the monomer molecules. Propagation occurs on the surface of a cooled substrate. This thesis presents an in-depth mechanistic study of the iCVD deposition of low surface energy poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA) thin films. PPFDA films have many uses due to their hydrophobic and oleophobic properties. Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy of the iCVD PPFDA films showed complete retention of the fluorine moieties. Deposition rates as high as 375 nm/min were achieved. It was found that the deposition rate and molecular weight increases with decreasing substrate temperature and increasing monomer partial pressure.

(cont.) Quartz crystal microbalance measurements showed that these effects correlated with an increased monomer concentration at the surface. Dimensionless analysis was used to scale up this iCVD polymerization to a custom modified roll-to-roll reactor. The roll-to-roll process allows for the fast production of realistic size samples. The use of liquid solvents in membrane coating processes often creates a blanket coating in which the pores are clogged due to surface tension problems and wettability. These problems do not exist for solventless processes such as iCVD. This thesis presents the use of the iCVD technique to functionalize electrospun fiber mats and polymeric capillary pore membranes in order to make water-repellent, self-cleaning membranes. X-ray photoelectron microscopy data confirmed the presence of the PPFDA coating on the topside and the backside of the membranes and electron microprobe analysis confirmed the presence of the coating along the pore wall. It was found that the iCVD process can be used to functionalize membranes with very high aspect ratio (-80:1) pores.