Entanglement of semiflexible polyelectrolytes: Crossover concentrations and entanglement density of sodium carboxymethyl cellulose

Viscosity data for aqueous solutions of sodium carboxymethyl cellulose (NaCMC) as a function of polymer concentration (c), added salt concentration (c S), and degree of polymerization (N) are presented. In dilute solution, NaCMC adopts an expanded coil conformation in excess salt (R ∝ N 0.59, [η] ∼ N 0.786) and a rodlike conformation (R ∝ N, [η] ∼ N 2) in salt-free water. The total persistence length in 0.1M NaCl is calculated from the N dependence of the intrinsic viscosity to be l p ≃ 5.5 nm. The entanglement crossover, evaluated from the c and N dependence of the specific viscosity, is found to be independent of c S. Our results suggest that polymer conformation (e.g., l p or solvent quality) within the range of parameters studied does not affect the formation of entanglements. The scaling model of Dobrynin et al. correctly describes the nonentangled rheology of NaCMC in salt-free and excess salt solutions but does not explain the c S independence of the entanglement crossover. Modifications to this model are proposed, which result in better but still limited agreement with experiments.