Characterization of co- and post-hydrolyzed polyacrylamide molecular weight and radius distribution under saline environment

The structural changes in the hydrolyzed polyacrylamide (PAM) under different saline conditions show significant effects on the rheological properties. At the higher shear rates, these conformational changes cannot be discerned in the shear rheological field but the extensional rheology can probe the polymer's microstructure can provide a better description. In this paper, two types of polymers: co-hydrolyzed PAM and post-hydrolyzed PAM were tested under different saline conditions. The polymer conformation results were interpreted to ascertain the underlying mechanisms of polymer and cation interactions and their effect on the extensional properties. The results showed that the interactions of polymer chains with divalent cations lead to the formation of the cross-linked chelating structures corresponded to the narrow molecular weight distribution (MWD) with a lower radius of Gyration (R_g). However, the uniform carboxylate ion density across the co-hydrolyzed PAM restricted shrinkage of the polymer chains after interactions with cations showed the high strain hardening due to polymer chain entanglement at high Hencky strain. Whereas, monovalent cation interactions with the carboxylate groups resulted in the monodentate structures that sustained the broader MWD that explains the observed higher strain hardening and extensional relaxation time.