Effect of Fractional Fluorination on the Properties of ATRP Surface-Initiated Poly(hydroxyethyl methacrylate) Films

Acylation of a surface-initiated poly(hydroxyethyl methacrylate) (PHEMA) film with a perfluoroalkyl acid chloride (C7F15COCl) results in a highly blocking film with fluorocarbon side groups that orient at the air−film interface to produce an ultralow-energy surface. The kinetics of the acylation step was monitored with reflectance-absorption infrared spectroscopy to reveal that the rate of fluorination depends on the chain length and solution concentration of the perfluorinated acid chloride as well as the solvent used. By control of the time of exposure to C7F15COCl, films with fractional conversion of the hydroxyl groups to fluorinated esters between 0 and 0.8 were produced. Increasing fractional conversion from 0 to ∼0.2 has a dramatic effect on surface wettability and electrochemical barrier properties as a densely packed, oriented fluorocarbon surface region is formed that greatly reduces water and ion transport. Additional fluorination has no effect on wettability but gradually increases film resistance while lowering film capacitance. These results suggest that structuring of fluorocarbon groups at the outermost surface of the film has a major impact on film properties and establish the minimum content of fluorocarbon groups required to achieve the structured interface via this grafting process.