Characterization of alkyl grafted polyurethane block copolymers by variable takeoff angle X-ray photoelectron spectroscopy

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Abstract
Variable takeoff angle x-ray photoelectron spectroscopy was used to determine the surface composition of five polyurethane block copolymers. The high-resolution C1s spectra from all five polyurethane samples had peaks at binding energies of 285.0, 286.5, and 289.5 eV, which are consistent with the presence of hydrocarbon, ether, and urethane carbon species. Both the measured elemental compositions at the low takeoff angles (deepest sampling depths) and the calculated elemental compositions for depths > 15 Å generally showed good agreement with the expected bulk compositions. The outer 15 Å of the surface of all five samples was depleted in the nitrogen-containing hard segment. For the base polyurethane sample in this series, an enrichment of the poly(tetramethylene oxide) soft segment and the presence of an organic silicone impurity were detected. Doubling the chain length of both the hard and soft segments resulted in a further decrease of the hard segment concentration in the outer surface of the sample. The grafting of C2 and C18 alkyl groups onto 10 or 20% of the ure-thane linkages resulted in an increase in the percentage of aliphatic carbon species present on the surface. In the case of the C18-alkylated material, the presence of alkyl chains at the surface was inferred using contact angle measurements.