Polyacrylamide is widely used in industrial applications such as enhanced oil recovery and water treatment due to its excellent ability to regulate the rheological properties of solutions. However, during these applications, progressive hydrolysis alters the molecular structure of polyacrylamide, leading to a unique biphasic change in viscosity that remains poorly understood. In this study, density functional theory calculations and molecular dynamics simulations were systematically conducted to elucidate the molecular mechanism of polyacrylamide hydrolysis and clarify the origin of its biphasic viscosity response at molecular level. The density functional theory calculations and molecular dynamics simulation results reveal that initial hydrolysis enhances structural viscosity by promoting polymer aggregation, whereas further hydrolysis leads to polymer chain dispersion, resulting in a decrease in structural viscosity and an increase in frictional viscosity. At high hydrolysis levels, chain recoiling driven by salt ion interactions reduces both structural and frictional viscosities. This study not only elucidates the fundamental mechanisms governing polyacrylamide hydrolysis and its biphasic effects on viscosity but also provides valuable insights for designing polymers with optimized rheological properties to meet the demands of diverse industrial applications.

Polyacrylamide (PAM) is a widely utilized polymer in various industrial applications, such as enhanced oil recovery (EOR) and wastewater treatment , valued for its exceptional ability to modulate solution rheology. In practical applications, PAM undergoes hydrolysis—a chemical reaction that converts its amide groups into carboxylate groups. Although the mechanisms and processes of hydrolysis have been extensively investigated, the effects of hydrolysis on the rheological properties of PAM solutions are complicated and have yet to be fully understood. Especially, previous experimental studies have reported a distinct biphasic trend in solution viscosity as hydrolysis progresses : viscosity initially increases but declines beyond a certain threshold. This phenomenon is consistently observed across different temperatures and salinities (Fig. 1a) and plays a critical role in PAM's industrial performance. The lack of molecular-level understanding of this unique biphasic effect has hindered the precise design of molecular structures and solution formulations in polymer science and industrial applications. This knowledge gap motivated the present study.

Here, we use the application of PAM in EOR as an example to highlight the critical impact of the biphasic effect of PAM hydrolysis on industrial performance. PAM-based polymer flooding is a widely used EOR technique that enhances oil displacement efficiency by reducing viscous fingering and enhancing pore permeability. Before injection, PAM is typically converted into partially hydrolyzed polyacrylamide (HPAM) to enhance solution viscosity . However, during polymer flooding, solution viscosity and oil displacement efficiency often decline significantly. Previous studies attributed this performance loss primarily to the high-temperature and high-salinity conditions of oil reservoirs, leading to the development of temperature-resistant and salt-resistant PAM-based polymers . Yet, even with these modifications, significant viscosity loss during polymer flooding still persists , suggesting that structural variations in PAM molecules due to further hydrolysis during application play a more critical role than temperature and salinity alone. Thus, a deeper understanding of PAM hydrolysis and its effects on viscosity is vital for optimizing EOR efficiency and designing more resilient polymer formulations.

Essentially, the viscosity of polymer solutions is fundamentally governed by molecular interactions. With the advancement of computational methods, molecular simulations have become a powerful tool in polymer science for elucidating the interactions among polymers, ions, and water . For example, Abdel-Azeim et al. investigated the effect of sulfonation on the PAM side chain regarding its phase behavior and interfacial properties using equilibrium molecular dynamics (MD) simulations and well-tempered metadynamics. They found that the sulfonated polymer exhibits improved salt tolerance and better stability under high-salinity conditions due to its weak interactions with brine cations . Similarly, Wang et al. employed all-atom molecular dynamics simulations to analyze the structural characteristics and salt-tolerant performance of sulfonic acid-modified HPAM with varying branch chain lengths. They concluded that moderate increases in branch chain length enhance salt resistance and flexibility, whereas excessive branching leads to polymer folding due to entanglement . These simulation studies have significantly advanced our understanding of the microscopic characteristics of PAM and its behavior in solution. However, to the best of our knowledge, no systematic simulation study has yet been conducted to directly investigate the relationship between the degree of PAM hydrolysis and solution viscosity.

In this study, we employ classical MD simulations and density functional theory (DFT) calculations to elucidate the microscopic mechanisms governing the effects of hydrolysis on the viscosity of PAM solutions under high-temperature, high-salinity, and alkaline conditions. The study is structured into three key components: (i) DFT and MD simulations were performed to study the hydrolysis mechanisms and monomer disparities under alkaline conditions; (ii) the periodic perturbation method in non-equilibrium MD was employed to examine the effect of hydrolysis degree on the rheological properties of solutions; (iii) detailed analyses were conducted to understand the atomic interactions among water, ions, and PAM with various hydrolysis degrees. Collectively, this study successfully explains why the solution viscosity varies with the degree of PAM hydrolysis in a biphasic trend. The insights gained offer a valuable framework for predicting and designing PAM-based polymers tailored to meet the rheological requirements in various industrial applications.