Electrostatic and hydrophobic interactions in NaCMC aqueous solutions: effect of degree of substitution and solvent medium on structure and rheology.

Sodium carboxymethyl cellulose (NaCMC) is an anionic, weak, semiflexible polyelectrolyte, extensively used in industry. We employ SANS, light scattering, and rheology to probe the conformation and dynamics of NaCMC solutions across a wide range of molecular weight (Mw), degree of substitution, salt and polymer concentrations. In salt-free solution, the overlap concentration and correlation length show the expected c* ∝ N^–2 and x ~ c^-1/2 dependences. 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. A second crossover, to a steep concentration dependence for specific viscosity (η_sp ∝ c^3.5±0.2), commonly assigned to the concentrated regime, follows c** ∝ N^–0.6±0.2 which thus suggests instead a dynamic crossover related to entanglement. The scaling of c* and c_e in high salt solution show neutral polymer in good solvent behaviour, characteristic of highly screened polyelectrolyte solutions. The crossover between the salt-free and high salt solution is not well described by current scaling theories for flexible or semiflexible polyelectrolytes. Backbone hydrophobicity becomes important for poorly substituted samples at high concentrations.