Some Reactions of Microgel in Polyacrylamide Solutions
Authors: Burcik Emil J. , Thakur Ganesh C. 
Published: December 2007

Although there is disagreement concerning the role of microgel in mobility control, there is little doubt that appreciable amounts of crosslinked microgel exist in commercial preparations of partially hydrolyzed polyacrylamide, and in Dow Pusher chemical the crosslinks are to some extent anhydride bridges. Other hydrolyzable crosslinks, such as imides, may also be present, but since for our purposes here their behavior is similar to that of anhydrides, equations for only the latter will be written. The purpose of this note is to report some reactions this crosslinked material undergoes with various additives, particularly with compounds containing two or more hydroxyl groups. These reactions may be useful in mobility control technology since, as with the reaction between Pusher and formaldehyde, they are a means of Pusher and formaldehyde, they are a means of controlling the effective molecular weight of the polymer in solution.When polymer molecules are crosslinked, the size of the aggregates formed can vary over wide limits. Small particles composed of relatively few polymer molecules crosslinked together behave like true molecules in solution and are called microgel. Larger particles of crosslinked material form colloidal particles of crosslinked material form colloidal dispersions, and still larger particles become visible to the naked eye and tend to settle from the solution on standing.Uncrosslinked partially hydrolyzed polyacrylamide is composed of large chainlike molecules of high molecular weight having CONH2 groups, COONa groups and COOH groups randomly attached to every other carbon atom on a carbon backbone chain. In the preparation of these polymers, particularly in the drying operation, there is a tendency for water to split out of two COOH groups and form anhydride bridges and thus crosslink polymer molecules together into larger aggregates. An amide group and a carboxyl group call react in a similar manner to form imide crosslinks. The number of crosslinks that form, and consequently the size of the microgel, is determined by a number of factors such as the ratio of CONH2 to COONa and the temperature of drying. The pH of the solution from which the polymer was dried is also of utmost importance' since this largely determines the number of COONa groups compared with the number of COOH groups, and it is the latter, as pointed out above, that are actually capable of forming anhydride and imide bridges.When microgel crosslinked by hydrolyzable bridges is redispersed in water, there is a tendency for the crosslinks to rehydrate, with a resulting disintegration of the microgel. However not all crosslinked aggregates are equally stable. Some pairs of molecules are bound together by more crosslinks than others. Furthermore the stability of the microgel network would be determined not only by the number of crosslinks but by their arrangement as well. Thus, when two crosslinks exist between a pair of polymer molecules, the size of the ring structure formed would affect the stability, the smaller rings being the more stable.The disintegration of microgel due to rehydration of hydrolyzable crosslinks is a reaction that by its very nature is governed by the pH of the system. As the pH of the system is increased, the rate of crosslink pH of the system is increased, the rate of crosslink disintegration also increases. Owing to the presence of Na2CO3 in Pusher-type polyacrylamides, solutions prepared for field use are alkaline. prepared for field use are alkaline. JPT

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