npj: 聚合物如何让纳米颗粒变得有序？
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大量实验证实胶体-聚合物混合物可以随聚合物浓度的改变而经历一系列无序-有序相变。来自美国的一项研究显示，聚合物需要恰当的长度才能让纳米颗粒组成一个有序的晶体结构。哥伦比亚大学的Sanat Kumar及其同事成功地解释了为什么当小颗粒悬浮在溶液中形成胶体后，聚合物会让胶体先变得有序接着又变为无序。胶体中的粒子通常可以自由移动，但聚合物的加入使它们形成晶体状结构，而添加更多的聚合物则又使胶体返回到无序状态。Kumar等采用分子动力学模拟和密度泛函理论计算表明，尽管聚合物在中等密度时通过诱导纳米颗粒相互吸引而稳定晶体，但在高密度时，因聚合物分子链较长，不能填充纳米颗粒之间的空隙，导致晶体不稳定而变得无序。论文近日发表于npj Computational Materials，英文标题及摘要如下。点击文末“阅读原文”可以自由获取论文PDF。
Reentrant equilibrium disordering in nanoparticle–polymer mixtures
Dong Meng, Sanat K. Kumar, Gary S. Grest, Nathan A. Mahynski & Athanassios Z. Panagiotopoulos
A large body of experimental work has established that athermal colloid/polymer mixtures undergo a sequence of transitions from a disordered fluid state to a colloidal crystal to a second disordered phase with increasing polymer concentration. These transitions are driven by polymer-mediated interparticle attraction, which is a function of both the polymer density and size. It has been posited that the disordered state at high polymer density is a consequence of strong interparticle attractions that kinetically inhibit the formation of the colloidal crystal, i.e., the formation of a non-equilibrium gel phase interferes with crystallization. Here we use molecular dynamics simulations and density functional theory on polymers and nanoparticles (NPs) of comparable size and show that the crystal-disordered phase coexistence at high polymer density for sufficiently long chains corresponds to an equilibrium thermodynamic phase transition. While the crystal is, indeed, stabilized at intermediate polymer density by polymer-induced intercolloid attractions, it is destabilized at higher densities because long chains lose significant configurational entropy when they are forced to occupy all of the crystal voids. Our results are in quantitative agreement with existing experimental data and show that, at least in the nanoparticle limit of sufficiently small colloidal particles, the crystal phase only has a modest range of thermodynamic stability.
原文链接：http://www.nature.com/articles/s41524-016-0005-8
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