Tesei Giulio, Paradossi Gaio, Chiessi Ester
Department of Chemical Sciences and Technologies, University of Rome Tor Vergata , Via della Ricerca Scientifica I, 00133 Rome, Italy.
J Phys Chem B. 2014 Jun 19;118(24):6946-55. doi: 10.1021/jp502486a. Epub 2014 Jun 10.
Poly(vinyl alcohol) (PVA) is an amphiphilic macromolecule with surfactant activity. The peculiar behavior of this polymer at the water-air interface is at the basis of its use as material for hydrated microdevices, films, and nanofibers. This work aims to investigate the behavior of PVA and water within the surface domain of highly diluted aqueous solutions by means of atomistic molecular dynamics simulations. Monodisperse atactic oligomers of 30 residues were distributed within water slabs in a vacuum box and allowed to diffuse toward the surface. After equilibration, structural features and dynamical properties of polymer chains and water in the interfacial domains were analyzed as a function of PVA surface concentration at 293 K. Surface pressure values obtained from simulations are in agreement with experimental values at corresponding polymer specific surface areas. In the explored concentration range of 6-34 μmol of residues/m(2), the chains display a transition between two states. At lower surface concentrations, elongated, quite rigid structures are adsorbed on the surface, whereas partially submerged globular aggregates, locally covered by thin water layers, are formed at higher surface concentrations. At PVA concentrations higher than about 20 μmol of residues/m(2), the percolation of chain aggregates over the interface plane produces a surface-confined polymer network with stable pores filled by water molecules. A substantial slowing of polymer and water dynamics in the interfacial domains is highlighted by the mean squared displacement time behavior of terminal residues and the interaction time of PVA-water hydrogen bonding. The diffusion coefficient of water and lifetime of hydrogen bonds between solvent molecules are halved and doubled, respectively, at the interface with the highest polymer concentration. The attenuation of water and polymer mobility concur to stabilize PVA hydrated networks in contact with air.
聚乙烯醇（PVA）是一种具有表面活性剂活性的两亲性大分子。这种聚合物在水-空气界面的特殊行为是其用作水合微器件、薄膜和纳米纤维材料的基础。这项工作旨在通过原子分子动力学模拟研究高稀释水溶液表面区域内PVA和水的行为。30个残基的单分散无规立构低聚物分布在真空箱中的水板内，并使其向表面扩散。平衡后，分析了293K时聚合物链和界面区域水的结构特征和动力学性质与PVA表面浓度的函数关系。模拟得到的表面压力值与相应聚合物比表面积下的实验值一致。在6 - 34 μmol残基/m²的探索浓度范围内，链呈现出两种状态之间的转变。在较低的表面浓度下，细长且相当刚性的结构吸附在表面，而在较高的表面浓度下，形成了部分浸没的球状聚集体，局部被薄水层覆盖。当PVA浓度高于约20 μmol残基/m²时，链聚集体在界面平面上的渗滤产生了一个表面受限的聚合物网络，其稳定的孔隙中填充有水分子。末端残基的均方位移时间行为和PVA - 水氢键的相互作用时间突出了界面区域聚合物和水动力学的显著减慢。在聚合物浓度最高的界面处，水的扩散系数和溶剂分子之间氢键的寿命分别减半和加倍。水和聚合物迁移率的衰减共同促使与空气接触的PVA水合网络稳定。
