X-ray photoelectron spectroscopy (XPS) has been used to investigate the surface characteristics of various novel fluorinated acrylate homopolymers [1,1-dihydroperfluorooctyl acrylate (PFOA), 1,1-dihydroperfluorooctyl methacrylate (PFOMA), 1,1,2,2-tetrahydroperfluorooctyl acrylate (PTAN)] as well as diblock copolymers consisting of both a fluorocarbon block of PFOA and a hydrocarbon block of polystyrene (PS). This technique allows nondestructive depth profiling of the top ∼100 Å of a material, providing both elemental composition and chemical state information. Due to the low surface energy of the fluorinated species, its enhanced presence on the surface is of importance in any potential applications. Angle-dependent XPS surface studies were conducted on polymer thick films to monitor surface segregation of the fluorinated component as a function of depth. Fluorine and the fluorine-containing constituents are surface enriched relative to carbon and oxygen from the acrylate portions of the polymers. This effect also occurs in the diblock copolymers, where the PFOA block prefers the polymer−air interface. Furthermore, this surface segregation is enhanced when the samples are thermally annealed. Also, the quantitative XPS data reveal other subtleties in the overall polymer structures, such as extent of chain branching in PFOA, PFOMA, and the diblock copolymers and the slight variations in average fluorine-containing side chain lengths in PTAN.

Nanoporous polycarbonate (PCTE) nuclear-track-etched membranes were used to effect electric field modulation of the mass transport of cationic, anionic, and neutral species in aqueous buffer. The permeability response to electric fields depended on the molecular charge and electrolyte concentration. Solute and solvent fluxes through nanopores under an electrical potential result from a balance of diffusion, electroosmosis, and ion migration. The Debye length, κ -1 , associated with the electrical double layer within the pores relative to the pore diameter, 2a, plays a critical role in determining electrokinetic transport behavior. The channel walls adsorb anions preferentially to produce a largely immobile negative charge density, leaving a large and mobile cation population to mediate transport in the channel. By adjusting the supporting electrolyte concentration, κa can be tuned such that the electrical double layer is either small in relation to the pore (κa g 1) or more diffuse and spanning the pore (κa < 1). Electroosmotic transport, mediated primarily by buffer cations, dominates when κa < 1. In this case the pores are essentially permselective, and anion electromigration is virtually eliminated. When κa g 1, the sign of the applied potential can be used to select for anion vs cation/neutral molecule transport.

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