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

The phase behavior and rheology of aqueous solutions of hydrophobically modified ethyl(hydroxyethyl) cellulose have been investigated. Effects of variations in the chemical structure of the hydrophobic tails grafted to the polymer backbone were followed. When the length of the polymer hydrophobic tails was increased the effects caused by association between different polymer chains became more pronounced. This was manifested by an increased tendency of the solution to phase separate, a higher viscosity, and a more elastic rheological response. The higher elasticity and viscosity was ascribed to slower polymer dynamics following from stronger hydrophobic associations. A separation of chemically different polymer chains into two coexisting phases was strongly promoted by modification with long hydrophobic tails. It was found that one of the coexisting phases contained highly substituted polymer chains, while in the other phase, less substituted polymer chains were found. It is proposed that this type of phase separation occurs because the highly substituted polymer chains have a pronounced tendency to form a network. Model paint formulations prepared with the different polymers showed that an increasing length of the polymer hydrophobic tails slowed down the dynamics of the formulation. This was manifested as a higher thickening efficiency (a smaller amount of polymer material was needed to obtain the desired viscosity), and a more pronounced shear-thinning behavior of formulations comprising polymers with long hydrophobic tails. Compared to the simpler systems, which only contained polymer and water, the model paint formulations were less prone to phase separation. It is suggested that, in the paint formulation, surfactants, latex particles, pigment, and fillers increase the number of possible association sites for the polymer hydrophobic tails. (C) 2000 Elsevier Science Ltd. All rights reserved.