Flow properties of hydroxypropyl guar gum and its long-chain hydrophobic derivatives

This paper deals with the rheological properties under continuous shear flow conditions of aqueous systems of hydroxypropyl guar gum (HPG) and three derivatives, of the same or lower molecular weight (MW) than HPG, characterized by different contents of long-chain hydrophobic pendants. The shear-dependent behavior of HPG and of the derivative with low molecular weight and low degree of hydrophobization (LMLH) resembles that of isotropic polymer solutions; accordingly, it can be fairly well described by the Cross equation, the effect of temperature discussed in terms of activation energy for viscous flow and the dependence of the Cross parameters eta(0) and lambda on polymer concentration expressed by simple power-laws. The other two derivatives considered, i.e. the one of low MW and high long-chain hydrophobic substitution and the other of a MW as high as that of HPG and a low hydrophobic pendant content (LMHH and HMLH, respectively) show an unusual behavior, characterized by the existence of a shear rate interval in which the shear viscosity undergoes a catastrophic breakdown, and which cannot be described by any simple rheological model. Nevertheless, as far as the temperature effect is concerned, all experimental data can be combined into one master curve by a simple shifting procedure. All systems exhibit marked time-dependent properties of the thixotropic type, which are described with a stretched exponential model and discussed in terms of the variation of the model parameters with temperature and polymer concentration. The peculiar features of the long-chain hydrophobic derivatives can be ascribed to a balance between inter- and intramolecular interactions, which is mainly governed by the local stress field.