Perfluorosulfonic acid ionomer (PFSA, specifically Nafion at EW = 975 g/mol) was visualized at the single molecule level using atomic force microscopy (AFM) in liquid. The diluted commercial Nafion dispersion shows an apparent M(w) = 1430 kg/mol and M(w)/M(n) = 3.81, which is assigned to chain aggregation. PFSA aggregates, imaged on mica and HOPG during adsorption from EtOH-H(2)O solvent at pH(e) 3.0 (below isoelectric point), showed a stable, segmented rod-like conformation. This structure is consistent with earlier NMR, SAXS/SANS, and TEM results that support a stiff helical Nafion conformation with long persistence length, a sharp solvent-polymer interface, and an extension of the sulfonated side chain into solution. Adsorption of Nafion structures on HOPG was observed at even higher pH(e) from EtOH due to screening of the repulsive electrostatic interaction in lower dielectric constant solvent, while the chain adopted an expanded coil conformation. These measurements provided direct evidence of the chain aggregation in EtOH-H(2)O solution and revealed their equilibrium conformations for adsorption on two model surfaces, highly ordered pyrolitic graphite (HOPG) and mica. The commercial Nafion dispersion was autoclaved at 0.10% w/w in nPrOH/H(2)O = 4:1 v/v solvent at 230 °C for 6 h to give a single-chain dispersion with M(w) = 310 kg/mol and M(w)/M(n) = 1.60. The autoclaved chains adopt an electrostatically stabilized compact globule conformation as observed by AFM imaging of the single PFSA molecules after rapid deposition on mica and HOPG at a low surface coverage.

Magnetorheological Finishing (MRF) is a novel process for deterministic figure correction and polishing of optical materials that utilizes a sub-aperture lap created by moving a magnetic field-stiffened magnetorheological (MR) fluid ribbon against an optical surface. MRF has been successfully applied to a wide range of optical materials. A new research platform has been designed and built that is used to generate sub-aperture polishing profiles, i.e., polishing "spots," on optical flats under wellcontrolled conditions. This platform uses the same fluid circulation and conditioning system as the commercial computer numerically controlled MRF machine, thereby allowing fluid performance issues to be investigated. This new machine complements the capabilities of the original MRF research platform that has been in continuous use for over six years. These two machines have been used to generate polishing spots on a variety of optical materials. The spot profiles were measured to calculate material removal rates and the quality of the polished surfaces characterized by measuring the microroughness within the polishing spots. Examples are presented which illustrate how the evaluation of polishing spots was used to develop MR fluids and operating conditions for calcium fluoride, CaF2, and potassium dihydrogen phosphate, KDP.

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ARTICLEsurface distribution of the PFSA was correlated with the surface topography of the substrate. Interaction of Nafion polymer with HOPG was studied in the AFM fluid cell to follow its adsorption onset as the net charge density on the HOPG surface was varied by changes in pH.

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