Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08544, USA.
J Phys Chem B. 2012 Mar 1;116(8):2385-95. doi: 10.1021/jp2112582. Epub 2012 Feb 21.
A molecular model of silica nanoparticles grafted with poly(ethylene oxide) oligomers has been developed for predicting the transport properties of nanoparticle organic-hybrid materials (NOHMs). Ungrafted silica nanoparticles in a medium of poly(ethylene oxide) oligomers were also simulated to clarify the effect of grafting on the dynamics of nanoparticles and chains. The model approximates nanoparticles as solid spheres and uses a united-atom representation for chains, including torsional and bond-bending interactions. The calculated viscosities from Green-Kubo relationships and temperature extrapolation are of the same order of magnitude as experimental data but show a smaller activation energy relative to real NOHMs systems. Grafted systems have higher viscosities, smaller diffusion coefficients, and slower chain dynamics than the ungrafted ones at high temperatures. At lower temperatures, grafted systems exhibit faster dynamics for both nanoparticles and chains relative to ungrafted systems, because of lower aggregation of particles and enhanced correlations between nanoparticles and chains. This agrees with the experimental observation that NOHMs have liquidlike behavior in the absence of a solvent. For both grafted and ungrafted systems at low temperatures, increasing chain length reduces the volume fraction of nanoparticles and accelerates the dynamics. However, at high temperatures, longer chains slow down nanoparticle diffusion. From the Stokes-Einstein relationship, it was determined that the coarse-grained treatment of nanoparticles leads to slip on the nanoparticle surfaces. Grafted systems obey the Stokes-Einstein relationship over the temperature range simulated, but ungrafted systems display deviations from it.
已开发出一种接枝聚氧化乙烯（PEO）低聚物的二氧化硅纳米粒子的分子模型，用于预测纳米颗粒有机杂化材料（NOHM）的输运性质。还模拟了在 PEO 低聚物介质中的未接枝二氧化硅纳米粒子，以澄清接枝对纳米粒子和链动力学的影响。该模型将纳米粒子近似为固体球体，并使用包括扭转和键弯曲相互作用的联合原子表示法来表示链。从格林-库珀关系和温度外推计算出的粘度与实验数据处于同一数量级，但与实际的 NOHM 系统相比，其活化能较小。在高温下，与未接枝的系统相比，接枝系统的粘度更高，扩散系数更小，链动力学更慢。在较低的温度下，与未接枝的系统相比，接枝系统的纳米粒子和链的动力学更快，这是由于颗粒的聚集程度较低以及纳米粒子和链之间的相关性增强。这与实验观察结果一致，即 NOHM 在没有溶剂的情况下表现出类液态行为。对于低温下的接枝和未接枝系统，增加链长都会减小纳米粒子的体积分数并加速动力学。但是，在高温下，较长的链会降低纳米粒子的扩散速度。根据 Stokes-Einstein 关系，确定粗粒化处理的纳米粒子会导致纳米粒子表面的滑移。在所模拟的温度范围内，接枝系统遵循 Stokes-Einstein 关系，但未接枝系统会偏离该关系。
