Rheological Properties of Associative Polyelectrolytes Synthesized by Solution Polymerization
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Rheological Properties of Associative Polyelectrolytes Synthesized by Solution Polymerization Areli I. Velazquez1, Alejandro Coronado2 and Enrique J. Jiménez1 Centro de investigación en Química Aplicada CIQA, Blvd. Enrique Reyna 140, 25294, Saltillo, México. 2 Facultad de Ciencias Químicas, Universidad Autónoma de Coahuila, Blvd. Venustiano Carranza and C. Ing. José Cárdenas Valdez, 25280, Saltillo, México 1
ABSTRACT Water-soluble associative polyelectrolytes of methacrylic acid [MAA] and ethyl acrylate [EA] (1:1 molar ratio), hydrophobically modified with small amounts of stearyl metacrylate [MM18], were synthesized by means of solution polymerization. Polyelectrolytes with two different molecular structures: multisticker, with hydrophobic groups randomly distributed along the hydrophilic chain and combined, with the hydrophobic groups along the chain and as terminal groups of the backbone, were obtained. Steady shear behavior and linear viscoelastic properties were studied as a function of polymer microstructure and hydrophobic group concentrations on salt-free aqueous solution using a cone-and-plate rheometer. Concentration regimes were obtained for each synthetized polymer. Viscoelastic study shows that the maximum thickening effect corresponds to the combined structure followed by multisticker structure. These polyelectrolytes exhibit high thickening power on aqueous solutions due to the synergy between the hydrophobic attractive interactions and coil expansion phenomena. INTRODUCTION Water soluble polymers are of great importance at industrial level (as thickening and gelling agents in paints, cosmetics, oil fluids, etc.). Their thickening power of these polymers are directly related to the hydrodynamic volume of the macromolecule in solution, therefore high molecular weights are commonly observed. However, their main drawback is suffering mechanical degradation at high shear rates, which leads to a irreversible decrease in their viscosity. 4 decades ago appeared a new class of water-soluble polymers: associative polymers, which have good thickening properties even with relatively small molecular weights [1]. Watersoluble associative polymers are macromolecules composed of a hydrophilic monomer (in excess) and a hydrophobic monomer (small proportion, 0.5-5 mol%) distributed along the chain or as terminal groups. In aqueous solution and above a certain polymer concentration, the hydrophobic groups interact and form tridimensional transient networks of intra and intermolecular associations that increases the viscosity of the solution Associative polymers are classified according to the hydrophobe position with respect to the hydrophilic backbone as: multisticker, microstructure with hydrophobic blocks randomly distributed along the hydrophilic chain [1-3]; telechelic, in which the hydrophobic groups are located in terminal position; and combined, displaying hydrophobes at both inside and terminal positions of the backbone [4-8]. Another way to increase their viscosity in aqueous solution is adding ionic charges
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