Departamento de Física Aplicada e Instituto Carlos I de Física Teórica y Computacional, Facultad de Ciencias, Universidad de Granada, Campus Fuentenueva S/N, 18071 Granada, Spain.
Área de Física de Procesos Irreversibles, División de Ciencias Básicas e Ingeniería, Universidad Autónoma Metropolitana, Ave. San Pablo 180 Col., Reynosa-Tamaulipas, 02200 Ciudad de México, Mexico.
J Chem Phys. 2017 May 21;146(19):194905. doi: 10.1063/1.4983525.
In this work, isotactic Poly (N-Isopropylacrylamide)-PNIPAM-in neat water and in electrolyte solutions is studied by means of molecular dynamics simulations. This is done for an infinitely diluted oligomer and for an assembly of several PNIPAM chains arranged into a planar membrane configuration with a core-shell morphology. We employed two different force fields, AMBER (assisted model building with energy refinement) and OPLS-AA (all atom - optimized potentials for liquid simulations) in combination with extended simple point charge water. Despite the more water insoluble character of isotactic oligomers, our results support the existence of a coil to globule transition for the isolated 30-mer. This may imply the existence of an oligomer rich phase of coil-like structures in equilibrium with a water rich phase for temperatures close but below the coil to globule transition temperature, T. However, the obtained coil structure is much more compact than that corresponding to the syndiotactic chain. Our estimations of T are (308±5) K and (303±5) K for AMBER and OPLS-AA, respectively. The membrane configuration allows one to include chain-chain interactions, to follow density profiles of water, polymer, and solutes, and accessing the membrane-water interface tension. Results show gradual shrinking and swelling of the membrane by switching temperature above and below T, as well as the increase and decrease of the membrane-water interface tension. Finally, concentration profiles for 1M NaCl and 1M NaI electrolytes are shown, depicting a strong salting-out effect for NaCl and a much lighter effect for NaI, in good qualitative agreement with experiments.
在这项工作中，通过分子动力学模拟研究了等规聚（N-异丙基丙烯酰胺）-PNIPAM 在纯水中和电解质溶液中的情况。这是针对无限稀释的低聚物和由几个 PNIPAM 链排列成具有核壳形态的平面膜构型的组装体进行的。我们使用了两种不同的力场，即 AMBER（辅助能量精修的建模）和 OPLS-AA（全原子-优化液体模拟的势能），并结合了扩展的简单点电荷水。尽管等规低聚物的亲水性较差，但我们的结果支持孤立的 30 聚体存在从线圈到球的转变。这可能意味着在接近但低于线圈到球转变温度 T 的温度下，存在富含线圈结构的低聚物相和富含水的相处于平衡状态。然而，得到的线圈结构比相应的间规链紧凑得多。我们分别估计 AMBER 和 OPLS-AA 的 T 为（308±5）K 和（303±5）K。膜构型允许包括链-链相互作用，跟踪水、聚合物和溶质的密度分布，并获取膜-水界面张力。结果表明，通过在 T 以上和以下切换温度，膜逐渐收缩和膨胀，以及膜-水界面张力增加和减少。最后，展示了 1M NaCl 和 1M NaI 电解质的浓度分布，描绘了 NaCl 的强烈盐析效应和 NaI 的轻得多的效应，与实验结果定性一致。
