Aromatic Polymer Brushes for Next Generation Surfaces using Surface Initiated Substituent Effect Chain Growth Condensation Polymerization
Open AccessSurface modification using polymers plays an important role in almost every facetof daily life from protective coatings to biological coatings to advanced coatings on electronic devices. One advanced application of polymer films is modifying surfaces on the nanometer scale using polymers that are directly attached to the substrate. Growing these polymers directly from the surface has proven to be one of the most desired ways to gain important functionality and unique topography not possible from traditional films. The ideal structure for these films is a polymer brush where the polymer chains are packed tightly enough on the surface that they extend away from the surface introducing increased short- and long-range order. Surface initiated polymerization using living radical polymerization techniques has risen as one of the dominant methods of growing polymer brushes from surfaces. A developing area of interest in the field is growing rigid rod or semiflexible polymers from substrates to access their unique physical properties. However, these structures cannot be prepared using traditional radical polymerization techniques. The desired properties of rigid rod and semi-flexible polymers include conjugation in the polymer backbone, biocompatibility, high tensile strength and modulus, improved thermal integrity, enhanced chemical stability, biodegradability, secondary bonding forces, and a high degree of alignment. Rigid rod and semiflexible polymers are traditionally prepared using the stepgrowth polymerization technique. This polymerization mechanism is not compatible with surface-initiated polymerization, however, due to the random addition of monomer and step change in molecular weight during the polymerization. Much effort has focused on converting step-growth polymerizations into living chain-growth polymerizations to expand the application of these polymers into different areas, including the preparation of polymer brushes. The primary polymerization techniques that that have been successfully converted are ring opening polymerization (ROP), catalyst transfer chain growth condensation (CGC) polymerization, and substituent effect CGC polymerization. While the focus of this dissertation is on the substituent effect CGC polymerization method, the unique chemistry and application of each of these methods is reviewed in chapter two. The synthesis of aromatic polyamides is one such technique that has been converted from a step-growth polymerization to a chain-growth polymerization using substituent effect CGC polymerization. Aromatic polyamides are extensively used in high performance applications due to their excellent thermal properties, chemical stability, and high mechanical strength. Reverse osmosis (RO) membranes for desalination of seawater have become one of the biggest applications for aromatic polyamide films. Building from previous work in our group, studies were conducted to find monomer structures that allowed for the preparation of aromatic polyamide brushes that mimic the functionalities of current RO membranes and improve on key weaknesses of current technology, such as fouling and coating stability. To achieve this, monomers with a protecting side chain, to allow for hydrogen bonding along the backbone after a post-polymerization deprotection reaction, and a polyethylene glycol (PEG) side chain, to improve fouling characteristics were synthesized. The solution and surface-initiated polymerization kinetics of both monomers were studied, along with the properties of polymer brushes made using each system. Aromatic polyethers are another class of polymer that has been converted to substituent effect CGC polymerization. Similar to aromatic polyamides, these polymers have a wide range of uses due to their desirable properties including excellent thermal resistance and chemical stability, and a high degree of mechanical strength. One growing area of use for aromatic polyethers is as ion exchange membranes. In an effort to improve on the current membranes, the modification of filler materials with polyethers is desired to provide better miscibility. As such, a monomer containing a trifluoro functionalized side chain were synthesized and the solution and surface-initiated polymerization of this monomer studied. Using the information gained, aromatic polyether brushes were then synthesized on both flat wafers and high surface area surfaces and their properties investigated.
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