We have previously shown that the pH and ionic strength response of weak, polybasic brushes is dependent on the location of the counterion within the Hofmeister series and the relative hydrophobicity of the polymer. Recent measurements of a hydrophobic poly(2-diisopropylamino) ethyl methacrylate (PDPA) brush on Platypus reflectometer show that a collapsed conformation independent of anion identity is formed at low ionic strength, 0.1 mM. At higher ionic strengths, up to 500 mM, extended conformations are observed in potassium acetate solutions, while potassium thiocyanate solutions collapse the brush. Numerical self consistent field (nSCF) calculations allowed previous hypotheses regarding the accumulation and hydration strength of the counterions to be tested. Addition of a single Flory-Huggins interaction parameter (χ) analogous to the anion hydrophilicity was sufficient o replicate the measured brush behaviour.

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Last edited: Friday September 10, 2010

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