Rheology of Starch‐Acrylamide/Graft Copolymer Solutions

Abstract

Highly viscous, pseudoplastic, and viscoeleastic solutions can be made from low molecular weight polymers by designing polymers with high radii of gyration. Solutions of starch‐acrylamide, graft copolymers, one type of high radius, low molecular weight polymer, have relaxation times which increase with increasing copolymer concentration, increasing copolymer molecular weight, or, at constant molecular weight, decreasing number of grafts per starch molecule. Intrinsic viscosity, radius of gyration, and filtration data show the graft copolymers to be 1 to 6 times larger molecules than equal molecular weight polyacrylamide. None of these nonionic polymers form complexes with electrolytes or aggregate in water. Copolymer solutions lose up to 60% of their original viscosity when sheared 100 times at a shear rate of 4,300 s−1. The amount of copolymer degraded by shear increases and, thus, the loss of solution viscosity increases when copolymer molecular weight increases or, at constant molecular weight, when the number of grafts per starch molecule decreases. Solution rheology changes slowly with time for two months after solution preparation. Viscosity loss, reduction in psuedoplasticity, and reduction in viscoelasticity are all due to disentangling of polymer chains. All copolymers were prepared by redox initiated, free radical polymerization of acrylamide on starch.