Influence of hydrophobic association in the aqueous media on the rheology and polymer conformation of associative polymers

The concentration and hydrophobicity-dependent hydrophobic associations are responsible for distinct rheological behavior for associative polymers (AP) to prove its application in various industrial operations. In this study, commercially available water-soluble, AP C319, P329, and D118 with similar weight-average molar masses with varying hydrophobicity were characterized by the field-flow fractionation method to determine molecular weight distribution and size distribution, steady shear flow, and the uniaxial elongation in capillary breakup experiments. The domination of intramolecular and intermolecular hydrophobic associations was found to be responsible for the polymer shrinkage (with hydrophobicity) and aggregation (with concentration), respectively. The tumbling of polymer macromolecules under the shear field resulted in a similar shear behavior for all polymers. However, higher shear viscosities due to the formation of polymer aggregates in the semi-dilute regime justify the domination of intermolecular hydrophobic associations. These associations also contributed to broader MWD for polymer solutions having higher relaxation time and extensional viscosity compared to the corresponding solutions in the dilute regime. For C319, the formation of flexible aggregates at 2000 ppm resulted in higher strain hardening compared to 1000 ppm, whereas, at 1000 ppm, the higher entanglements between polymer chains due to the lowest hydrophobicity for C319 resulted in the higher strain hardening. At high hydrophobicity, the coiled structure became rigid due to reduced solvent holding inside the compacted structure formed due to the domination of intramolecular hydrophobic associations. Therefore, the relaxation time and extensional viscosity decrease with an increase in the hydrophobicity in both regimes.