Department of Chemical Engineering , University of Patras & FORTH-ICE/HT , Patras GR26504 , Greece.
Particle Technology Laboratory, Department of Mechanical and Process Engineering , ETH Zürich , CH-8092 Zürich , Switzerland.
J Phys Chem B. 2019 May 16;123(19):4204-4219. doi: 10.1021/acs.jpcb.9b01696. Epub 2019 May 1.
Long MD simulations are carried out using a detailed all-atom force field to investigate the effect of pH or, equivalently, degree of ionization α (= 0, 50, 100%) and degree of polymerization N (= 20, 23, 46, 70, and 110) on the structure and dynamics of poly(acrylic acid) (PAA) at infinite dilution. To ensure the validity and add to the reliability of our research conclusions, a systematic validation of several molecular mechanics force fields is performed. It is observed that the generalized AMBER force field in combination with the RESP charge fitting method best describes both the structural and dynamical behavior of PAA in comparison to experimentally obtained data. It is found that ⟨ R⟩changes with N as ⟨ R⟩ ∼ N, with ν = 0.27 at α = 0% degree of ionization (acidic conditions), ν = 0.94 at α = 50% degree of ionization (neutral conditions), and ν = 0.87 at α = 100% degree of ionization (basic conditions), which is in perfect agreement with theory. The global shape of the PAA chain in the solution is quantified in terms of the three eigenvalues of the average radius-of-gyration tensor, the relative shape anisotropy κ, and the asphericity parameter b. It is revealed that at α = 0%, the chain adopts a spherelike conformation, while at α = 50 and 100%, its conformation is flattened and flexible. In addition, it is revealed that as the degree of ionization increases, the persistence length L increases, which suggests that PAA chains become stiffer with increasing pH. The global and local conformational changes of the PAA chain with the degree of ionization are found to be highly related to the solvation of the polymer. Finally, it is revealed that the diffusion coefficient D of the center of mass of PAA also exhibits a power law scaling with N, D ∼ N, with ν = 0.25 at α = 0% degree of ionization, ν = 0.46 at α = 50% degree of ionization (neutral conditions), and ν = 0.44 at α = 100% degree of ionization (basic conditions), in excellent agreement with recent experimental data and theoretical predictions.
使用详细的全原子力场进行长时间分子动力学模拟，以研究 pH 值（或等效的电离度α（= 0、50、100%）和聚合度 N（= 20、23、46、70 和 110）对无限稀释条件下聚丙烯酸（PAA）结构和动力学的影响。为了确保研究结论的有效性和可靠性，我们对几种分子力学力场进行了系统验证。结果表明，与实验数据相比，通用 AMBER 力场与 RESP 电荷拟合方法相结合，能够最好地描述 PAA 的结构和动力学行为。结果发现，与 N 的关系为 ⟨R⟩∼N，在α=0%（酸性条件）时ν=0.27，在α=50%（中性条件）时ν=0.94，在α=100%（碱性条件）时ν=0.87，这与理论完全一致。通过平均回转半径张量的三个特征值、相对形状各向异性 κ 和非球形参数 b 来量化 PAA 链在溶液中的整体形状。结果表明，在α=0%时，链呈球形构象，而在α=50%和 100%时，其构象扁平化且灵活。此外，结果表明随着电离度的增加，PAA 链的持久性长度 L 增加，这表明 PAA 链随着 pH 值的增加变得更加僵硬。随着电离度的变化，PAA 链的整体和局部构象变化与聚合物的溶剂化密切相关。最后，结果表明 PAA 质心的扩散系数 D 也与 N 呈幂律关系，在α=0%时ν=0.25，在α=50%时ν=0.46（中性条件），在α=100%时ν=0.44（碱性条件），与最近的实验数据和理论预测非常吻合。
