Electrostatic layer-by-layer self-assembly of poly(carboxybetaine)s: Role of zwitterions in film growth

We report on self-assembly of a weak zwitterionic polymer - poly(carboxybetaine), PCB5 - within a polyelectrolyte multilayer film. Multilayer growth and ionization of carboxylic groups are monitored in situ using ATR-FTIR. Deposition of PCB5 within the film is contrasted for two pH regimes: (1) at slightly acidic pH values where PCB5 carries overall cationic charge, but a significant number of dipoles provided by ionized carboxylic groups also exists within the polymer chain, and (2) at neutral and basic pH values where these polymers become zwitterionic. In the latter case, no multilayers could be produced with either polyanions or polycations. In the former case, PCB5 can be deposited within the polymer film via alternating adsorption with a variety of polyanions, such as heparin, DNA, poly(carboxylic acid)s, or poly(styrenesulfonate). The produced films can be erased from the solid surface by exposure to solutions with higher pH values, when the fraction of charged carboxylic groups in PCB5 is increased. The presence of zwitterions in PCB5 chains allowed building hybrid layers which contained PCB5 bound with both a polyanion and a polycation. In contrast to PCB5/polyanion films, hybrid films showed a stability region extended to neutral pH values and dissolved at both pH > 7 and pH < 4. The PCB5-containing films show promise for bioseparation and biotechnology applications when used as sacrificial layers or for controlled release of incorporated small molecules.