Effect of molecular weight on the construction of polyelectrolyte multilayers: Stripping versus sticking

Combinations of a polyanion and a polycation, with different molar masses but narrow molar mass distributions, are employed to construct thin films by the polyelectrolyte multilayering technique. All polyelectrolytes are assembled in the presence of added salt. Even highly charged polymers in the 10(4) Da range of molar mass exhibit atypical multilayering characteristics. If the molar mass, MM, of either of the polymers is in this range, the thickness increment realized on addition of the shorter polymer is partially lost on exposure to the solution of longer polymer, as surface polyelectrolyte is stripped off by its oppositely charged partner. Complete loss of low MM polyanion on exposure to low MM polycation inhibits multilayer growth altogether. The role of salt is elucidated in the balance between multilayer growth, which relies on kinetically irreversible adsorption, and polyelectrolyte stripping, which produces a stable solution dispersion of polyelectrolyte complex. Subtleties in the irreversibility of commonly employed combinations of high molar mass polymers are revealed by in situ weveguide measurements.