Linear versus Exponential Growth of Weak Polyelectrolyte Multilayers: Correlation with Polyelectrolyte Complexes

We report on the correlation of polyelectrolyte chain dynamics in polyelectrolyte complexes (PECs) with the deposition mode and chain mobility of polyelectrolytes (PEs) within layer-by-layer-assembled (LbL) films. The study was performed using two polyelectrolyte systems: poly­(2-(dimethylamino)­ethyl methacrylate)/poly­(methacrylic acid) (PDMA/PMAA) and completely quaternized PDMA (Q100M)/PMAA. Hydrodynamic sizes of PDMA/PMAA and Q100M/PMAA complexes in solution were followed by fluorescence correlation spectroscopy (FCS), while three different techniques were applied to probe the structure and dynamics of the same PE pairs within LbL films. Specifically, deposition of PEs at surfaces was monitored by phase-modulated ellipsometry, film internal structureby neutron reflectometry (NR), and diffusion of assembled chains in the direction parallel to the substrateby fluorescence recovery after photobleaching (FRAP). By applying these complementary techniques to PDMA/PMAA and Q100M/PMAA systems in solution and at surfaces at various pH values, we found that the dynamics of polyelectrolyte chains within PECs underwent a prominent pH-dependent transition, and that this transition in chain dynamics was closely correlated with the transition between linear and exponential film growth modes. Neutron reflectometry results confirm that, at the transition point, film structure changed from layered for linearly depositing films to highly intermixed for exponentially depositing LbLs. Moreover, FRAP indicated a several-fold difference in PE lateral diffusion coefficient for the two different film growth modes. In addition, the pH transition point was affected by steric restrictions to ionic pairing, and the pH range of exponential growth and higher chain mobility was wider for Q100M/PMAA as compared with the PDMA/PMAA system, due to the presence of a methyl spacer at the amino group, resulting in weaker ionic pairing.