The Topology of Polymer Brushes Determines Their Nanoscale Hydration.

Time-of-flight neutron reflectometry (ToF-NR) performed under different relative humidity conditions demonstrates that polymer brushes constituted by hydrophilic, cyclic macromolecules exhibit a more compact conformation with lower roughness as compared to linear brush analogues, due to the absence of dangling chain ends extending at the polymer-vapor interface. In addition, cyclic brushes feature a larger swelling ratio and an increased solvent uptake with respect to their linear counterparts as a consequence of the increased interchain steric repulsions. It is proposed that differences in swelling ratios between linear and cyclic brushes come from differences in osmotic pressure experienced by each brush topology. These differences stem from entropic constraints. The findings suggest that to correlate the equilibrium swelling ratios at different relative humidity for different topologies a new form of the Flory-like expression for equilibrium thicknesses of grafted brushes is needed.

Application of polymeric macroporous supports for temperature-responsive chromatography of pharmaceuticals.

A macroporous particulate support prepared previously by reactive gelation under shear and functionalized with poly(N-isopropylacrylamide), PNIPAM, brushes of variable length is applied for temperature-responsive chromatography, whereby temperature modulates hydrophobic interactions. Several different analytes, including small pharmaceuticals, peptides, proteins and monoclonal antibodies are employed. Contrary to the most commonly observed behavior in conventional chromatography, increasing retention is observed at elevated temperatures. Peak broadening is quantified using the peak standard deviation, which depends on both the polymer chain conformation and analyte adsorptivity. The favorable effect of grafted polymer thickness on retention becomes progressively less pronounced for thicker grafted PNIPAM layers. The effect of eluent composition on solute-sorbent interactions was investigated by introducing NaCl, methanol, dioxane and by varying the pH. Salt or organic solvent addition affects apart from the analytes solution properties, the hydrophobicity of the stationary phase itself. Frontal analyses performed at different temperatures to determine dynamic binding capacities, indicate small mass transfer resistances imposed by this novel packing material.