Salt thickening performance and mechanism of an N-Vinyl-2-Pyrrolidinone based amphiphilic polymer

Enhancing salt tolerance is a critical requirement for polymer flooding in the petroleum production process. This work introduced a novel salt thickening polymer called NDC16, which is a cationic polymer composed of poly(N-Vinyl-2-Pyrrolidinone-co-Double hexadecyldimethylallyl ammonium chloride). Notably, NDC16 does not contain traditional Acrylamide (AM) or Acrylic acid (AA). Its salt thickening performance and mechanism in sodium chloride and calcium chloride solutions were separately analyzed at aggregation and molecular levels by photophysical method, micromorphology, and molecular dynamics (MD) simulation. The results demonstrated that the novel polymer was able to increase the viscosity of the sodium chloride solution to 20 times higher than the initial one. However, its performance in increasing viscosity in a calcium chloride solution was only half as effective. The inclusion experiments indicated that almost all increasing viscosity could be contributed to the hydrophobic association effects of the polymer molecules. The results of the dynamic analysis revealed that the NDC16 aggregates showed a more orderly arrangement in NaCl solutions, while the network structure appeared as relatively disordered clusters in CaCl2. Besides, there existed an optimal salinity at which the network density of the hydrophobic structure reached the highest level. The variation in thickening performance can be elucidated by the molecular interaction. Due to their smaller molecular interactions, the Na+ ions exhibited a stronger potential to permeate into the polymer clusters. It stretched the polymer molecules and allowed them to display a structured arrangement. The large Radius of Gyration (Rg) and spatial distance of the polymers in two solutions indicated that the polymer clusters were much elongated in Na+ solvents, thereby confirming their permeation effect. These results enhance the comprehension of the process by which salt thickening occurs due to the interaction between cationic hydrophobic associative polymers and various salt solutions.