New hydrophobically modified carboxymethylcellulose derivatives

New hydrophobically substituted carboxymethylcellulose derivatives with different percentages of modification by hexadecylamine (from 0.9 to 6% per monomeric unit acid group) were obtained by amidation of the carboxyl group by different methods. The percentage of modification (% HDA) was controlled by conductimetric titration and quantitative C-13 NMR spectra on cellulase hydrolyzed oligomers. The least degradation (checked by GPC measurements), was found on coupling the hexadecylamine with the acid form of CMC activated by N,N'-dicyclohexylcarbodiimide (DCCI) in dry dimethylsulfoxide. In the dilute range, the viscosities decreased due to the contraction of the hydrophobically modified CMC derivatives by intramolecular interactions. These effects are counterbalanced by increasing the ionic strength. Rheological measurements indicate an enhancement of the apparent viscosity for %HDA similar to 6, suggesting the establishment of intermolecular interactions, disrupted as the shear rate increases, in the semidilute range, beyond the overlap concentration (C-p*similar to 1.5 g liter(-1)). This behavior is confirmed by a moderate shear thinning effect in contrast with the precursor for which the viscosities show no shear dependence up to C-p*similar to 5.3 g liter(-1) (Newtonian behavior). (C) 1997 Elsevier Science Ltd.