John Klier, Co-Chair, Chemical Engineering
Jessica D. Schiffman, Co-Chair, ChemicalEngineering
High wear environments such as microelectronics, aviation, and biomedicaldevices require extremely durablepolymer coatings. For example, the FDA reported in 2015 that coatingsdelaminated from medical devices during use, and the free coatingblocked patients' blood flow. Chemically grafting the polymersin the coating to the substratecan increase coating durability with covalent bonding.Current covalent linkingtechnologies graft coatingsto substrates by either photo-initiated radical grafting of polymers orcontrolled radical polymerization. The photo-initiated radical grafting technique is simple, but the combination ofsubstrate, monomer, and solvent can decreaseefficiency; the technique also results in low grafting density.Controlled radical polymerization techniques create tethered polymers with high grafting density but requirerigorously inert conditions. To meet these challenges, I have studied a novel class of monomers capable of polymerizingand grafting from polymer substrates under practical manufacturing conditions.
Methylidene malonates are a class of 1,1 disubstituted alkenes capable ofanionic polymerization to high polymersin the presence of air and water. Due to their highly reactive structure,methylidene malonates can be initiated by common nucleophiles such as carboxylate salts, secondary amines,and hydroxide anions.We hypothesized this reactivity would enable facile polymerization and subsequent graftingto substrates, including soft polymericsubstrates with common nucleophiles embedded into the polymer backbone. Thepolymerization, grafting, andtransport of methylidene malonates on polymer substrates were studied as afunction of environment and substratechemistry. Poly(ethylene-co-acrylic acid) (pEAA) was chosen as a modelsubstrate because the polymercontains mostly ethylene groups, a traditionally difficult to functionalizepolymer, and a small amount of acrylicacid groups (<20%), which can be transformed into nucleophilic initiatorsvia simple base treatment. Additionally,the base treated polymers are thermoplastic elastomers, the same class ofmaterials used in catheters. Characterizingthe surface chemistry and physics of tethered polymers is non-trivial onpolymeric surfaces. Polymerization and grafting were primarily monitored using attenuated total reflectance FourierTransform Infrared Spectroscopy (ATR FTIR), a surface sensitive technique. Environmental studiesdetermined that the polymerization occurred in the presence of air and water,which usually quenches anionic polymerization and controlled radical polymerization. Studies of initiatorcontent showed polymerization occurred in all cases (e.g. even with a poly(ethylene) control), but grafting onlyoccurred when bound initiator was present. The acrylic acid content studiesalso revealed that non-base treatedpEAA is a substrate with imbedded chain transfer agents.Chain transfer agents can terminate the growth of onepolymer chain but are then transformed into an initiating species for new chain growth. Thus, non-base treated pEAAwas also capable of grafting methylidene malonate polymers. The surface acrylic acid content andreactivity of methylidene malonates affect the transport of the monomer and polymer during polymerization;heterogenous polymerization was seen across all surfaces, and the patterns of grafting followed surprising trends. By understanding the reaction conditions and subsequent graftingheterogeneity, more durablecoatings can be created with methylidene malonates.