Comparative rheological behavior of some cellulosic ether derivatives

The shear flow of cellulosic derivatives (methylcellulose and hydroxypropylmethylcellulose with different degrees of methoxyl and hydroxypropyl substituents) has been studied. The static light scattering and intrinsic viscosity measurements have allowed the determination of the weight-average molecular weight (M-w), radius of gyration (R-g), critical overlap concentration C*, intrinsic viscosity [eta], and Huggins constant. These results indicate that the studied methylcellulose A4M and hydroxypropylmethylcellulose F4M have comparable molecular weight and an intrinsic viscosity of the same magnitude in pure water. However, hydroxypropylmethylcellulose K4M possesses a greater molecular weight and a corresponding high value of [eta]. Even if hydroxypropylmethylcellulose F4M and K4M have different molecular weights and different [II], they show a rather similar C* value. Although the critical parameter is the same for A4M and F4M, the C-r* concentration characterizing the end of the dilute regime is lower in the case of methylcellulose derivative, indicating that the absence of any hydroxypropyl substituent on the polymer chain is responsible for the A4M chains to develop associative interactions among themselves more favorably. This may be explained by their tendency to escape from the aqueous medium. Although the studied hydroxypropylmethylcellulose samples differ from their methoxyl degree of substitution and their hydroxypropyl molar substitution (F4M, DS = 1.76, MS = 0.14; K4M, DS = 1.39, MS = 0.21), their aqueous solutions display a rather similar rheological behavior in steady flow, suggesting that the distribution of the various substituents along the polymer chain does not induce a great difference on the interactions that lead to entangled structures of comparable density and cohesion.