Diffusion of Linear and Star Polyacids in Solution and Multilayer Assemblies

We studied diffusion of linear (LPMAA) and 4-, 6- and 8-arm star poly(methacrylic acids) (4PMAA, 6PMAA and 8PMAA, respectively) of matched molecular weights in solution and within electrostatic layer-by-layer (LbL) films. Linear and star PMAA (LPMAA and sPMAA, respectively) exhibited significant differences in mobility that were dependent on pH and salt concentration. Fluorescence correlation spectroscopy (FCS) showed that at high pH star polymers were more compact, with ~15% smaller hydrodynamic radius (R_h) of 8PMAA as compared to its linear counterpart. In contrast, hydrodynamic size of PMAA was independent on molecular architecture at low pHs where molecular conformations were determined by intermolecular hydrogen bonding. In the case of LbL films of the polyacids assembled with a linear polycation, molecular architecture did not impact film growth, but strongly affected PMAA mobility measured by fluorescence recovery after photobleaching (FRAP). For all the films, the dependences of PMAA diffusion coefficients on NaCl concentration were exponential, but sPMAA’s mobility was more responsive to the presence of ions. As a result, two regimes and a crossover salt concentration were identified, with LPMAA diffusing faster than sPMAA at low salt concentrations, and the opposite order of diffusion rates at higher salt contents. Possible contribution of the ionic-bonding-induced entanglements to this phenomenon will be discussed.