Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
J Phys Chem B. 2013 Apr 4;117(13):3634-47. doi: 10.1021/jp3099973. Epub 2013 Mar 25.
Diffusion of small molecules in amorphous polymers is known to follow a form of so-called hopping motion: penetrant molecules are trapped in microscopic cavities for extended time periods; diffusion is made possible by rare but fast jumps between neighboring cavities. Existing understanding of the hopping mechanism is based on the inspection of molecular images during individual molecular-dynamics trajectories. We focus on the diffusion of water molecules in a hydrophilic polymer below its glass transition temperature. The transition path ensemble of one hopping event is sampled with aimless shooting, a type of transition path sampling technique. In these trajectories, configurations of both the penetrant and the polymer change during the transition. Statistical analysis of the ensemble using likelihood maximization leads to a reaction coordinate of the transition, whose key components include the penetrant configuration and distances between penetrant-host atom pairs that have strong electrostatic interactions. Polymer motions do not contribute directly to the reaction coordinate. This result points toward a transition mechanism dominated by the penetrant movement. Molecular insights from this study can benefit the development of computational tools that better predict material transport properties, facilitating the design of new materials, including polymers with engineered drying properties.
渗透分子在微观腔室中被捕获很长时间；扩散是通过相邻腔室之间罕见但快速的跳跃来实现的。对跳跃机制的现有理解是基于在单个分子动力学轨迹期间检查分子图像。我们专注于在亲水聚合物低于玻璃化转变温度下水分子的扩散。通过无目的射击（一种跃迁路径采样技术）来采样一个跳跃事件的跃迁路径集合。在这些轨迹中，渗透物和聚合物的构型在跃迁过程中都会发生变化。使用似然最大化对集合进行统计分析，得到跃迁的反应坐标，其关键组成部分包括渗透物构型和具有强静电相互作用的渗透物-宿主原子对之间的距离。聚合物运动不会直接贡献于反应坐标。这一结果表明，跃迁机制主要由渗透物的运动决定。这项研究的分子见解可以为更好地预测材料传输特性的计算工具的发展提供帮助，从而促进新的材料的设计，包括具有工程干燥性能的聚合物。
