Compression induced phase transition of nematic brush: A mean-field theory study.
Compression induced phase transition of nematic brush: A mean-field theory study.
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DOI：
10.1063/1.4936324
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发表时间：
2015-11
期刊：
影响因子：
--
通讯作者：
Jiuzhou Tang;Xinghua Zhang;Dadong Yan
中科院分区：
文献类型：
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作者：
Jiuzhou Tang;Xinghua Zhang;Dadong Yan
聚合物刷对外部压力的响应行为是其应用的最重要特征之一。对于柔性聚合物刷，在低接枝密度的情况下，基于高斯链模型的理论被广泛研究，压缩导致链的均匀变形。然而，在高接枝密度的情况下，刷子变得各向异性并且将形成向列相。正常压缩往往会破坏向列序，从而导致复杂的响应行为。在弱压缩下，向列刷中的链条被弯曲，弯曲能和Onsager相互作用产生弹性。在深度压缩下，向列聚合物刷的响应行为取决于链刚性。对于压缩的刚性聚合物刷，链倾向于随机重新取向以使取向熵最大化，并且其向列序被破坏。对于压缩的柔性聚合物刷，链倾向于向后折叠以保持向列顺序。在压缩过程中发生屈曲折叠转变。对于压缩的半柔性刷，链条集体向某个方向倾斜，这导致横向平面上旋转对称性的破坏。向列刷的这些响应行为与高度受挫蠕虫状链的性质有关，由于难以求解修正的扩散方程，因此很难用传统的自洽场论来研究。为了克服这个困难，目前的工作中开发了平均场理论中的单链，结合了蒙特卡罗模拟和蠕虫链模型的平均场理论。该方法在受限条件下显示了整个链条刚度区域的高性能。
Responsive behavior of polymer brush to the external compression is one of the most important characters for its application. For the flexible polymer brush, in the case of low grafting density, which is widely studied by the Gaussian chain model based theory, the compression leads to a uniform deformation of the chain. However, in the case of high grafting density, the brush becomes anisotropic and the nematic phase will be formed. The normal compression tends to destroy the nematic order, which leads to a complex responsive behaviors. Under weak compression, chains in the nematic brush are buckled, and the bending energy and Onsager interaction give rise to the elasticity. Under deep compression, the responsive behaviors of the nematic polymer brush depend on the chain rigidity. For the compressed rigid polymer brush, the chains incline to re-orientate randomly to maximize the orientational entropy and its nematic order is destroyed. For the compressed flexible polymer brush, the chains incline to fold back to keep the nematic order. A buckling-folding transition takes place during the compressing process. For the compressed semiflexible brush, the chains are collectively tilted to a certain direction, which leads to the breaking of the rotational symmetry in the lateral plane. These responsive behaviors of nematic brush relate to the properties of highly frustrated worm-like chain, which is hard to be studied by the traditional self-consistent field theory due to the difficulty to solve the modified diffusion equation. To overcome this difficulty, a single chain in mean-field theory incorporating Monte Carlo simulation and mean-field theory for the worm-like chain model is developed in present work. This method shows high performance for entire region of chain rigidity in the confined condition.