Rheology and Water Swelling of Carboxymethyl Starch Gels

A series of gels differing in ionic content was prepared by derivatizing starch with varying amounts of carboxymethyl add-on. Some derivatives were lightly crosslinked with epichlorohydrin to reduce soluble material and increase the amount of gel. The gel fraction of each batch was isolated. and viscosity 1]. shear modulus G. and swelling in water and salt solution were then determined. One gel suspension. examined in more detail. was demonstrated to behave as a closely packed gel thickening agent. thereby warranting use of the three reduced functions: reduced concentration cQ (c is weight concentration of polymer. Q is swelling capacity in excess fluidl,1]/cQ. and G/cJi:l. At cQ greater than about 2. 1]/cQ. and G/cli:l reach constant plateau values. as is typical of other examples of closely packed gel thickeners. In a comparison among the gel suspensions. plateau values of the reduced viscosity function were nearly proportional to the apparent crosslink density G/c Ii:J. in agreement with other closely packed gel thickeners. but were not affected by ionic content of dry polymer. The water swelling capacities of the different gels ranged from 47 to 450 gig and were correlated with both ionic contents and apparent crosslink densities. but the effects of these variables were less than suggested by theory. 4530