Characterization of a new family of nonwettable, nonstick surfaces

Two examples of a new family of water-based, nonstick, hydrophobic polymeric coatings were used to relate wettability to surface composition and adhesion. They were prepared by cross-linking reactive perfluoroalkyl polymeric surfactants, RPPSs, with poly(2-isopropenyl-2-oxazoline). Wettability may be manipulated by varying chemical composition, curing conditions, and degree of cross-linking. Low wettability and nonstick properties result from self-assembly of RPPS-containing chains, surface density and extent of orientation of perfluoroalkyl (R(F)) groups, and subsequent immobilization by conversion of polar-ionic functionality to covalent cross-links. The wettability was measured by obtaining advancing and receding contact angles and tilt angles of water, 0.10 N HCl, 0.10 N NaOH, and hexadecane. Hexadecane advancing angles are sensitive to R(F) surface concentration and orientation. Increased R(F) surface densities and orientation, measured by X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS), respectively, are consistent with higher advancing angles. Increased amounts of cross-linking agent decrease hexadecane advancing and receding angles due to lower RF surface densities and orientations. Surface alkylation increases hexadecane contact angle hysteresis (CA hysteresis) and lowers tilt angles by converting more polar sites to less polar but more hexadecane-wettable sites. CA hysteresis appears to be related to heterogeneities of less than 1 mu m, as determined by time of flight secondary ion mass spectrometry and surface penetration. Scanning force microscopy indicates that nanometer scale surface roughness is unrelated to CA hysteresis. Receding angles of water and aqueous solutions of HCl and NaOH increase with increased cross-link density, reaching a maximum at 1:1 mole ratio of cross-linking functionalities. Residual COOH groups cause contact angles to decrease with increasing pH. Surface alkylation decreases aqueous CA hysteresis by rendering accessible polar sites less polar and thus less water wettable. Increased cross-linking decreases CA hysteresis by decreasing the difference between wettable and nonwettable areas and ease of penetration of liquid into the surface phase. Low intrinsic adhesion is obtained when surfaces exhibit not only high hexadecane and aqueous receding angles but also low CA hysteresis.