Postdoc: Phase Transitions in Diblock Copolymer Systems

该项目的重点是开发可扩展的并行分子动力学（MD）代码，以研究剪切下的二嵌段共聚物的行为。 快速并行算法已经开发了简单的流体和短程MD，但算法，可以处理聚合物链是不可用的。 此外，目前的算法只处理平衡问题，而有必要研究的问题与附加剪切流。 几个研究小组已经通过模拟方法研究了两嵌段共聚物的平衡相图。 模拟还没有被用来研究他们的实验观察到的，而复杂的，动态响应。 处于平衡的对称二嵌段在有序-无序转变温度（TODT）下从低温有序（层状）相转变为高温无序相。 剪切的应用改变了TODT的值。 甚至在静止的TODT之上也观察到有序相。 理论上已经预测了非常长的聚合物的TODT的变化。对于短的二嵌段--康奈尔大学的克雷默小组正在进行实验--理论预测是不存在的，人们必须依靠模拟来代替。 本项目还将研究影响薄板取向的因素。 在实验上，已经观察到不同的方向，但哪些机制在选择首选方向中起作用还没有得到很好的理解。

This projects focuses on development of scaleable parallel Molecular Dynamics (MD) code to study the behavior of diblock copolymers under shear. Fast parallel algorithms have been developed for simple fluids and for short range MD, but algorithms that can handle polymeric chains are not available. Besides, current algorithms only deal with equilibrium problems, whereas there is a need to study problems with added shear flow. Several groups, have studied the equilibrium phase diagram of diblock copolymers by means of simulations. Simulations have not as yet been used to study their experimentally observed, rather complex, dynamic response. Symmetric diblocks in equilibrium make a transition from a low temperature, ordered (lamellar) phase to high temperature, disordered phase at the order-disorder transitions temperature (TODT). Application of shear changes the value of TODT. Ordered phases are observed even above the quiescent TODT. The shift in TODT has been predicted theoretically for very long polymers. For short diblocks- on which experiments are underway in Kramer's group at Cornell-theoretical predictions is not exist and one has to rely on simulations instead. This project will also study factors that influence the orientation of lamellae. Experimentally, different orientations have been observed, but which mechanisms play a role in the choice of preferred direction is not, as yet, well understood.