Self-diffusion of Linear and Star Polyacids in Layer-by-Layer Films: Impact of the Polycation Molecular Weight

We explore the effect of molecular weight of a polycation – poly(diallyldimethylammonium chloride) PDADMAC (35 kDa, 70 kDa, and 325kDa) - on the salt-triggered lateral diffusion of linear and star poly(methacrylic acid)s (L-PMAA and s-PMAA, respectively) of matched molecular weight (60 kDa) in layer-by-layer (LbL) films. Both L-PMAA and s-PMAA deposited exponentially with low molecular weight PDADMAC at pH 5, but formed thinner, linearly growing films with higher molecular weight PDADMAC, suggesting slower polymer mobility. Exposure of the assembled films to NaCl solutions revealed higher stability of films containing linear polymers compared to star-containing films, and increased stability of films constructed with high molecular weight of PDADMAC. FTIR studies showed that an increase of M_w of PDADAMC led to lower ionization of both linear and star PMAA in the films suggesting a significant dependence of density of ionic pairing on molecular weight. Fluorescence recovery after photobleaching (FRAP) technique was used to measure lateral diffusion of Alexa-488 labeled polyacids. The diffusion coefficients (D_II) of the assembled polyacids were lower for films constructed with higher-molecular-weight PDADMAC, and films of linear and star polymers exhibiting different scaling exponents for such as dependence. This work provides an insight into mobility of assembled linear and star polyelectrolytes and their dependence on the local structure of LbL films determined by a binding partner.