Concentration dependent effects of urea binding to poly(N-isopropylacrylamide) brushes: a combined experimental and numerical study

Abstract

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 (cu ≤ 0.5 mol L(-1)) a preferential exclusion of urea from PNIPAM chains is observed, while in the high concentration regime (2 ≤ cu ≤ 7 mol L(-1)) a preferential binding of the osmolyte to the polymer surface is found. In both regimes, the volume phase transition temperature (Ttr) decreases with increasing urea concentration. This phenomenon derives from two different effects depending on urea concentration: (i) for cu ≤ 0.5 mol L(-1), the decrease of Ttr is explained by a decrease of the chemical potential of bulk water in the surrounding aqueous phase; (ii) for cu ≥ 2 mol L(-1), the lower Ttr 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 cu = 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.

Stabilizing effect of TMAO on globular PNIPAM states: preferential attraction induces preferential hydration

“…[37][38][39][40] The properties of osmolytes and their influence on PNIPAM were already discussed in a series of publications. 6,7,[9][10][11][12][13][14][15][41][42][43][44] Recent experiments and simulations, for example, revealed a preferential binding of urea to PNIPAM which induces bridging effects that stabilize globular PNIPAM states even below the LCST. 10,15 In contrast, more detailed studies for low urea concentrations below 1 mol L À1 indicated a preferential exclusion mechanism which stabilizes the coil state.…”

The properties of residual water molecules in ionic liquids: a comparison between direct and inverse Kirkwood–Buff approaches

“…(9) up to a specific distance d, usually including the first solvation shell. Although a direct connection between the local/bulk partition coefficient and the KB theory is questionable 98 , a preferential exclusion or a preferential binding mechanism of species indeed can be detected 44,[99][100][101] . For instance, at short distances in the local region around the reference molecule, values of K p > 1 and K p < 1 indicate a preferential binding or a preferential exclusion behavior, respectively.…”

Selective solute adsorption and partitioning around single PNIPAM chains

“…A key element to start with is first to understand the details of the solute-gel interactions on the atomic scale, attainable through atomistic computer modeling. In recent years, a number of simulation studies focused on solutepolymer interactions that affect the conformation [18], solubility [19] as well as critical solution temperature of a PNIPAM polymer [20]. A particular attention was given to understanding of cononsolvency effects [21][22][23], which appear with increasing solute concentrations [24,25].…”