Influence of Pressure on the Co-nonsolvency Effect of Homopolymer in Solutions: A Molecular Dynamics Simulation Study

Stimuli-responsive polymers capable of rapidly altering their chain conformation in response to external stimuli exhibit broad application prospects. Experiments have shown that pressure plays a pivotal role in regulating the microscopic chain conformation of polymers in mixed solvents, and one notable finding is that increasing the pressure can lead to the vanishing of the co-nonsolvency effect. However, the mechanisms underlying this phenomenon remain unclear. In this study, we systematically investigated the influence of pressure on the co-nonsolvency effect of single-chain and multi-chain homopolymers in binary mixed good-solvent systems using molecular dynamics simulations. Our results show that the co-nonsolvency-induced chain conformation transition and aggregation behavior significantly depend on pressure in all single-chain and multi-chain systems. In single-chain systems, at low pressures, the polymer chain maintains a collapsed state over a wide range of co-solvent fractions (x-range) owing to the co-nonsolvency effect. As the pressure increases, the x-range of the collapsed state gradually narrows, accompanied by a progressive expansion of the chain. In multichain systems, polymer chains assemble into approximately spherical aggregates over a broad x-range at low pressures owing to the co-nonsolvency effect. Increasing the pressure reduces the x-range for forming aggregates and leads to the formation of loose aggregates or even to a state of dispersed chains at some x-range. These findings indicate that increasing the pressure can weaken or even offset the co-nonsolvency effect in some x-range, which is in good agreement with the experimental observations. Quantitative analysis of the radial density distributions and radial distribution functions reveals that, with increasing pressure, (1) the densities of both polymers and co-solvent molecules within aggregates decrease, while that of the solvent molecule increases; and (2) the effective interactions between the polymer and the co-solvent weaken, whereas those between the polymer and solvent strengthen. This enhances the incorporation of solvent molecules within the chains, thereby weakening or even suppressing the chain aggregation. Our study not only elucidates the regulatory mechanism of pressure on the microscopic chain conformations and aggregation behaviors of polymers, but also may provide theoretical guidance for designing smart polymeric materials based on mixed solvents.