Department of Chemistry, Zhejiang University, 148 Tianmushan Road, Hangzhou, Zhejiang 310028, P. R. China.
J Phys Chem B. 2020 Nov 5;124(44):9951-9957. doi: 10.1021/acs.jpcb.0c07188. Epub 2020 Oct 21.
Poly(-vinylcaprolactam) (PVCL) exhibits co-non-solvency in aqueous solutions of 2-propanol but not in methanol. What distinguishes the impact of these two cosolvents on the polymer conformational stability? We report a molecular dynamics simulation study on PVCL 50-mer and monomers dissolved in methanol-water and 2-propanol-water mixtures. We show that the alcohol-concentration dependence of the effective attraction between a pair of PVCL monomers closely resembles the conformational changes in a single PVCL 50-mer as well as the experimentally observed behavior for PVCL chains. We also found that, at the co-non-solvency maximum, the monomer-monomer attraction works over a long-range beyond the solvent-separated distance. Then, we correlate the long-range attraction to the appearance of a dense alcohol concentration accumulated between the monomers. Furthermore, we distinctly demonstrate that the co-non-solvency of PVCL monomers can be switched on/off by artificially tuning the alcohol size while keeping the energetic parameters. Thus, we conclude that the magnitude of the excluded volume effect in alcohol accompanying the gain in translational entropy of the solvent is crucial to the occurrence of PVCL polymer co-non-solvency.
聚（己内酰胺）（PVCL）在异丙醇的水溶液中表现出共非溶剂性质，但在甲醇中则没有。这两种共溶剂对聚合物构象稳定性的影响有何不同？我们报告了一项关于 PVCL 50 聚体和单体在甲醇-水和异丙醇-水混合物中溶解的分子动力学模拟研究。我们表明，一对 PVCL 单体之间的有效吸引力随醇浓度的变化与单个 PVCL 50 聚体的构象变化以及实验观察到的 PVCL 链行为非常相似。我们还发现，在共非溶剂最大值处，单体-单体吸引力在溶剂分离距离之外的长程范围内起作用。然后，我们将长程吸引力与单体之间积聚的密集醇浓度的出现相关联。此外，我们明确地证明，通过人工调整醇的大小而保持能量参数，可以打开/关闭 PVCL 单体的共非溶剂性质。因此，我们得出结论，醇中排除体积效应的大小以及溶剂的平移熵的增加对 PVCL 聚合物共非溶剂性质的发生至关重要。
