Jürgen Engelhardt

Characterization of aqueous car☐ymethylcellulose solutions in terms of their molecular structure and its influence on rheological behaviour

A set of car☐ymethylcellulose (CMC) samples was varied in degree of substitution (DS) from 0.71 to 2.95, with the partial degree of substitution being raised uniformly at positions C2, C3 and C6, so that a homologous series of CMC was made available. A further set of samples varied in molar mass from 200 000 to 2 000 000 g mol−1 at a constant DS of ∼ 1. The chemical structural parameters were determined by n.m.r. spectroscopy, showing that quantification of the degree of substitution at position C6 may be distorted by impurities of glycolic acid. The mean molar masses, for establishing the [η]-M relationship, were determined by multiangle laser light scattering preceded by size exclusion chromatography. The influence of concentration on zero-shear viscosity was given by η0 ∝ c4.3, whereas the influence of the molar mass was determined by η0 ∝ M3.9. An increasing DS within a homologous series influences the viscosity in different solvents (H2O 0.01 M NaCl, 0.1 M NaCl) at low polyelectrolytic concentrations. Surprisingly, the viscosity for all the CMC samples can be predicted by the single equation: η0[Pas]=8.91×10−4+1.30×10−5cMW0.9+5.33×10−8c2MW1.8+4.60×10−15c4.34MW3.91 at T = 298 K in 0.01 M NaCl. Increasing the DS up to a value of ∼ 1 improves the solubility characteristics (increases the viscosity), whereas above DS ≈ 1 this effect is overlapped by degradation of the molar mass. It was also possible to quantitatively determine the viscoelasticity as a function of frequency and molecular parameters.