Phase behavior and rheology in water and in model paint formulations thickened with HM-EHEC: influence of the chemical structure and the distributionof hydrophobic tails

The phase behavior and rheology of aqueous solutions of hydrophobically mod ified ethyl(hydroxyethyl) cellulose have been investigated. Effects of vari ations in the chemical structure of the hydrophobic tails grafted to the po lymer backbone were followed. When the length of the polymer hydrophobic ta ils was increased the effects caused by association between different polym er chains became more pronounced. This was manifested by an increased tende ncy of the solution to phase separate, a higher viscosity, and a more elast ic rheological response. The higher elasticity and viscosity was ascribed t o slower polymer dynamics following from stronger hydrophobic associations. A separation of chemically different polymer chains into two coexisting ph ases was strongly promoted by modification with long hydrophobic tails. It was found that one of the coexisting phases contained highly substituted po lymer chains, while in the other phase, less substituted polymer chains wer e found. It is proposed that this type of phase separation occurs because t he highly substituted polymer chains have a pronounced tendency to form a n etwork. Model paint formulations prepared with the different polymers showed that a n increasing length of the polymer hydrophobic tails slowed down the dynami cs of the formulation. This was manifested as a higher thickening efficienc y (a smaller amount of polymer material was needed to obtain the desired vi scosity), and a more pronounced shear-thinning behavior of formulations com prising polymers with long hydrophobic tails. Compared to the simpler syste ms, which only contained polymer and water, the model paint formulations we re less prone to phase separation. It is suggested that, in the paint formu lation, surfactants, latex particles, pigment, and fillers increase the num ber of possible association sites for the polymer hydrophobic tails. (C) 20 00 Elsevier Science Ltd. All rights reserved.