「Frontier Insights · Vol.3」Block Structure Modulates the Molecular Chain Conformation of Cationic Polyacrylamide — Shouxin's Phased Research Achievements

Shouxin R&D Center

In the molecular design of cationic polyacrylamide (CPAM), the block ratio—the proportion between neutral blocks and cationic blocks—is a critical structural parameter determining polymer performance. However, the underlying influence of this ratio on the microscopic chain conformation has long lacked systematic atomic-scale understanding. This knowledge gap has hindered the transition of CPAM product design from "empirical screening" to "rational design."

In recent years, the Shouxin R&D team has focused on the quantitative relationship between the block structure of cationic polyacrylamide and its molecular chain conformation. We have established a polymer molecular dynamics (MD) simulation platform alongside a polymer synthesis validation platform. To date, the team has achieved phased progress in the following areas:

Quantitative impact of block ratio on molecular chain conformation: Our simulation data demonstrate that the ratio of neutral blocks (A_m) to cationic blocks (B_n) significantly affects the degree of chain extension. At a block ratio of 8:12 (neutral:cationic), the molecular chain adopts its most extended conformation—with the ratio of mean-square end-to-end distance to mean-square radius of gyration reaching a peak, cationic groups (—N⁺(CH₃)₃) are fully exposed to the solvent, and ion accessibility is optimal. When the cationic block proportion is too low (e.g., 16:4), the chain charge density is insufficient, electrostatic repulsion weakens, and hydrophobic aggregation between chains occurs. This finding reveals a "non-monotonic" regulatory pattern of the block ratio on molecular conformation—higher charge density does not necessarily yield a more extended chain; rather, an optimal balance exists.

During the course of this research, we took note of an independent study published in 2025 in the top international chemical engineering journal Chemical Engineering Journal(Vahid et al., titled "Block ratio optimized cationic polyacrylamides for enhanced nitrate rejection under applied potential"). Using molecular dynamics simulations, that study not only examined the molecular conformations of DCPAM with various block ratios in bulk solution-overlapping with Shouxin's research direction—but also expanded into two additional dimensions: (1)Ion selectivity—the study found that NO₃⁻ accumulates at significantly higher densities near CPAM charged groups compared to Cl⁻, demonstrating preferential adsorption of CPAM toward nitrate ions; (2)Electric-field coupled performance-when CPAM was placed in an electric field-assisted reverse osmosis system, the 12:8 ratio achieved the highest water flux while completely rejecting NO₃⁻, with salt rejection rates ranging from 78% to 100%. The quantitative conclusions of that study are highly consistent with Shouxin's finding that the 8:12 block ratio yields optimal molecular conformation. Moreover, its extensions into ion selectivity and electric-field coupling point the way for Shouxin's subsequent R&D efforts.

 Figure 1. Chain conformation parameters and molecular snapshots for different block ratios

 Figure 2. Distribution of CPAM with different block ratios in a confined electric field and ion density profiles

Building on the cross-validation of our own results and external literature, Shouxin has initiated the following R&D translation efforts:

Establishing a CPAM synthesis platform with tunable block ratios, enabling precise molecular customization for different application scenarios;

Drawing on the literature methodology to launch extended research on ion selectivity and electric-field coupling, bridging fundamental molecular conformation data to practical application performance prediction;

Developing a molecular simulation-assisted product design workflow to accelerate the R&D cycle from molecular design to process scale-up.

Looking ahead, Shouxin will continue to deepen fundamental research and application expansion of cationic polyacrylamide, driving product iteration through molecular-level mechanisms, and providing more scientific and efficient polymer solutions for the water treatment industry.

Source：Chemical Engineering Joumal 513 (2025) 162346

Original Title：Block ratio optimized cationic polyacrylamides for enhanced nitrate rejection under applied potential

Original Author：Hossein Vahid, Arsalan Hashemi, Mohammad Khavani, Abhinav Sharma, Mohammad R.K. Mofrad, Tapio Ala-Nissila