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Concentration dependent effects of urea binding to poly(N-isopropylacrylamide) brushes: a combined experimental and numerical study
Micciulla, Samantha; Michalowsky, Julian; Schroer, Martin A.; Holm, Christian; Klitzing, Regine von; Smiatek, Jens
The binding effects of osmolytes on the conformational behavior of grafted polymers are studied in this work. In particular, we focus on the interactions between urea and poly(N-isopropylacrylamide) (PNIPAM) brushes by monitoring the ellipsometric brush thickness for varying urea concentrations over a broad temperature range. The interpretation of the obtained data is supported by atomistic molecular dynamics simulations, which provide detailed insights into the experimentally observed concentration-dependent effects on PNIPAM-urea interaction. In particular, in the low concentration regime (c(u) <= 0.5 mol L-1) a preferential exclusion of urea from PNIPAM chains is observed, while in the high concentration regime (2 <= c(u) <= 7 mol L-1) a preferential binding of the osmolyte to the polymer surface is found. In both regimes, the volume phase transition temperature (T-tr) decreases with increasing urea concentration. This phenomenon derives from two different effects depending on urea concentration: (i) for c(u) <= 0.5 mol L-1, the decrease of T-tr is explained by a decrease of the chemical potential of bulk water in the surrounding aqueous phase; (ii) for c(u) >= 2 mol L-1, the lower T-tr is explained by the favorable replacement of water molecules by urea, which can be regarded as a cross-linker between adjacent PNIPAM chains. Significant effects of the concentration-dependent urea binding on the brush conformation are noticed: at c(u) <= 0.5 mol L-1, although urea is loosely embedded between the hydrated polymer chains, it enhances the brush swelling by excluded volume effects. Beyond 0.5 mol L-1, the stronger interaction between PNIPAM and urea reduces the chain hydration, which in combination with cross-linking of monomer units induces the shrinkage of the polymer brush.
