Viscosity and scaling of semiflexible polyelectrolyte NaCMC in aqueous salt solutions

We investigate the viscosity dependence on concentration and molecular weight of semiflexible polyelectrolyte sodium carboxymethylcellulose (NaCMC) in aqueous salt-free and NaCl solutions. Combining new measurements and extensive literature data, we establish relevant power laws and crossovers over a wide range of degree of polymerization (N) as well as polymer (c) and salt (c_s) concentrations. In salt-free solution, the overlap concentration shows the expected c^∗ ∝ N^-2 dependence, and the entanglement crossover scales as c_e ∝ N^-0.6±0.3, in strong disagreement with scaling theory for which c_e ∝ c^∗ is expected, but matching the behavior found for flexible polyelectrolytes. A second crossover, to a steep concentration dependence for specific viscosity (η_sp ∝ c³.5±0.2), commonly assigned to the concentrated regime, is shown to follow c^∗∗ ∝ N^-0. ±0.2 (with c^∗∗/c_e 〈 6) which thus suggests instead a dynamic crossover, possibly related to entanglement. The scaling of c^∗ and c_e in 0.01 and 0.1 M NaCl shows neutral polymer in good solvent behavior, characteristic of highly screened polyelectrolyte solutions. This unified scaling picture enables the estimation of viscosity of ubiquitous NaCMC solutions as a function of N, c, and c_s and establishes the behavior expected for a range of semiflexible polyelectrolyte solutions.