Poly(L-Lysine) Adsorption to Fluoropolymer Films
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POLY(L-LYSINE) ADSORPTION TO FLUOROPOLYMER FILMS
MOLLY S. SHOICHET AND THOMAS J. McCARTHY University of Massachusetts, Department of Polymer Science and Engineering, Amherst, MA 01003. ABSTRACT The adsorption of poly(l-lysine) (PLL) from aqueous solution to poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) is investigated. Adsorption is controlled by choice of solvent (and PLL solution conformation) or by FEP surface chemistry. PLL adsorbs from pH 11 buffer solution to FEP: PLL adsorption is concomitant with the liberation of water molecules from the FEP-water interface resulting in a decreased interfacial free energy between organic and aqueous phases. PLL does not adsorb from an aqueous pH 7 solution to FEP unless the FEP surface is carboxylic acid-functionalized (FEPCO2H): an ionic interaction between FEP-carboxylate and PLL-ammonium enhances adsorption. The surface modifications are charaterized by attenuated total reflectance infrared spectroscopy (ATR IR), ultraviolet-visible spectrophotometry (UVvis), contact angle, and quantitatively by x-ray photoelectron spectroscopy (XPS). The adsorbed amount of PLL to FEP (from pH 11) is proportional to molecular weight whereas the adsorbed amount of PLL to FEP-CO2H (pH 7) is independent of molecular weight and likely proportional to surface charge density. The modified film samples (relative to the unmodified film samples) show improved characteristics in terms of wettability, chemical reactivity, adhesion, cell growth. INTRODUCTION Polymer surface modification can be accomplished by several techniques including plasma irradiation [1], chemical reaction, or adsorption. We have reported several chemical modifications of polymer film samples where both the depth and chemical functionality on the film were controlled. For example poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) was reduced by reaction with sodium naphthalide to controlled reaction depth and subsequently hydroborated and oxidized to alcohol functionality [2]. The adsorption of a polymer in solution, poly(styrene) or poly(propylene sulfide), to a solid substrate, metal or metal oxide, was studied [3]. These two fields of interest are combined to study the adsorption of a polymer in solution, poly(1-lysine) (PLL) to a chemically modified polymer film, carboxylic acid-functionalized poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP-CO2H). Our objective
Mat. Res. Soc. Symp. Proc. Vol. 218. 01991 Materials Research Society
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is to control the adsorption of PLL to FEP by chemical surface modification or by choice of solvent. FEP is a chemically inert, hydrophobic polymer; adsorption of PLL from an aqueous solution dramatically changes the surface properties of the solid FEP film in terms of hydrophilicity, chemical reactivity, and adhesive nature. Poly(1-lysine), a poly(amino acid) with interesting and well-characterized solution properties, adopts one of three solution conformations according to temperature and pH of the aqueous solution: x-helix, P3-sheet, or random coil/disordered. PLL is
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