Electrostatic and Hydrophobic Interactions in NaCMC Aqueous Solutions: Effect of Degree of Substitution

Abstract

The rheology of water soluble polyelectrolytes at intermediate and high concentrations is controlled by entanglements, hydrophobic and electrostatic interac- tions, whose influence is difficult to isolate. We investi- gate the rheology of semidilute solutions of sodium car- boxymethyl cellulose (NaCMC) with molecular weight M w ' 2 . 5 × 10 5 g/mol and varying degree of substi- tution (D.S.) as a function of polymer concentration in various solvent media: salt-free water (long ranged elec- trostatic interactions), 0.5M aqueous NaCl (screened electrostatics) and 0.5M aqueous NaOH (screened elec- trostatics, diminished hydrophobic interactions) in or- der to selectively probe the different interactions. De- creasing D.S. is found to decrease solubility and induce partial aggregation and eventual gelation. In salt-free and 0.5M NaCl solution, NaCMC with D.S. ' 1.2 ex- hibits hydrophilic polyelectrolyte and neutral polymer in good solvent behaviour respectively. Decreasing D.S. to ' 0.7-0.8 leads to hydrophobic behaviour in both solvents, becoming weak gels at high concentrations. In 0.5M NaOH (pH = 13.5) the viscosities of samples with different D.S. become identical when plotted against the overlap parameter, which we interpret as result- ing from the solubilisation of unsubstituted cellulose blocks. Small angle neutron scattering (SANS) data in- dicate that the polymer conformation is not strongly affected by hydrophobic interactions. By varying D.S., ionic strength and pH, we demonstrate the tuning of NaCMC-solvent interactions, controlling separately the electrostatic and hydrophobic effects on the solution rheology.