Surface modification of poly(tetrafluoroethylene-co-hexafluoropropylene) by adsorption of functional polymers

Surfaces containing controllable densities of hydrophilic functional groups (carboxylic acids and amines) were prepared by the adsorption of functional polymers from aqueous solutions to fluoropolymer film samples. The adsorption behaviors of poly(acrylic acid) (PAA), poly(allylamine hydrochloride) (PAH), and polyethylenimine (PEI) at the poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP)water interface were studied and compared with the behavior of poly(L-lysine) (PLL), a polymer that has been reported to adsorb at the FEP/water interface. Insight into how polymer structure and adsorption variables (pH, time, ionic strength) contribute to "hydrophobic interactions" (minimization of interfacial free energy) was gained. Wettability of the functionalized FEP film samples was assessed by advancing and receding contact angles (theta (A)/theta (R)), and the adsorbed amounts were determined using X-ray photoelectron spectroscopy. Adsorbed amounts are highest at pH values at which the polymers are not charged; this is predominantly because of lower solubility under these conditions. For the three polyamines studied, the maximum adsorbed amount decreases in the order PLL > PAH > PEI, which is the opposite order of their solubilities. Of the two structurally similar polymers, PAA (-CO2H) promotes greater water wettability than PAH (-NH2). Added salt increases the adsorbed amount dramatically by screening repulsive interactions between ionic groups at pH values when these weak acids and bases are charged. Adsorption of a single functional polymer, PLL, PAA, PAH, or PEI, did not result in significant improvement of wettability-water contact angles (theta (A)/theta (R)) changed from 117 degrees /93 degrees (FEP) to 104 degrees /16 degrees (FEP-PLL), 103 degrees /21 degrees (FEP-PAH), 113 degrees /72 degrees (FEP-PEI), and 99 degrees /22 degrees (FEP-PAA). Sequential adsorption of two functional polymers, however, resulted in highly wettable surfaces: 39 degrees /15 degrees (FEP-PLL-PAA), 66 degrees /17 degrees (FEP-PAH-PAA), 74 degrees /11 degrees (FEP-PEI-PAA), and 61 degrees /17 degrees (FEP-PAA-PAH). Peel tests indicate that the adhesive properties of the fluropolymer are improved after adsorption of functional polymers, especially after adsorption of two functional polymers.