BRUSH FORMATION FROM MIXTURES OF SHORT AND LONG END-FUNCTIONALIZED CHAINS IN A GOOD SOLVENT

Using surface force measurements, we have determined the structure of polymer brushes formed onto mica, from dilute solutions of zwitterion-terminated polystyrene chains of molecular weights 26 500 and 375 000 in the good solvent toluene. We find that brushes of the longer amphiphiles, formed from a solution of these polymers alone, are rapidly replaced by shorter chains when the latter are added to the solution. This requires a certain minimum concentration of the shorter amphiphiles, but substantial replacement of long-by-short chains can occur even when the monomer concentrations of the shorter polymers is only (1/500) that of the longer ones. By analyzing the force profiles for the mixed brushes thus formed, we obtain a quantitative measure of their structural details. We evaluate these observations in terms of a simple scaling model for the free energy of brushes that are in equilibrium with free end-functionalized chains in solution.