Thin Film Formation by Surface-Initiated ARGET Polymerization and UV-Hydrosilylation Methods

Abstract

Modification strategies involving polymer or molecular coatings have the ability to tailor the interfacial properties of surfaces by control of the structure, thickness, and composition of these adlayers. This work utilized activators regenerated by electron transfer (ARGET) polymerization to generate polymers, particularly those with zwitterionic groups, from silicon surfaces. An achieved goal was the development of experimental conditions that provide reliable nanometer-level control of the grafted polymer film’s thickness. Using this approach, fouling resistant polymers including poly(sulfobetaine methacrylate) and other zwitterionic systems were grafted as thin films from silicon substrates. For these superhydrophilic coatings, a method for analyzing the surface energy components of these films was developed by integrating the Cassie, Fowkes, and Young equations. Kinetic terms were also incorporated to describe relationships between reaction conditions and the compositions of copolymer surfaces comprising a zwitterionic monomer of interest and an associated less-hydrophilic co-monomer. Methods for modifying silicon surfaces with molecular films were also investigated for producing fouling-resistant zwitterionic coatings. These approaches relied on silanization, “click” chemistry, and substitution reactions performed on surfaces. For grafting films to silicon, UV-induced attachment of alkene-containing compounds with H-terminated silicon was used as an alternative to methods that form organosiloxane films. This approach provided surface attachment via robust Si-C bonds and was investigated to address long-term film stability issues associated with organosiloxanes. The UV-induced hydrosilylation to attach 4-vinylbenyl chloride (VBC) provided Si-C bound films useful for ARGET polymerization initiation and nucleophilic substitution reactions. Of note was the development of novel, one-step attachment procedures utilizing either UV or thermal hydrosilylation to produce azide-containing molecular films on silicon. The utility of these azide surfaces for “click” chemistry was demonstrated via copper(I)-catalyzed azide alkyne cycloaddition (CuAAC) using various terminal alkynes and via copper-free “click” chemistry using a ring-strained alkyne molecule. The various films were characterized by methods including ellipsometry, wetting, FT-IR, profilometry, SEM, and other techniques. Zwitterionic and other hydrophilic coatings were examined by fluorescence microscopy for quantifying the ability of these films to prevent the non-specific adsorption of fluorescently-labeled albumin.