Postdoc: Phase Transitions in Diblock Copolymer Systems

本项目的重点是开发可扩展的平行分子动力学（MD）代码，以研究剪切作用下双嵌段共聚物的行为。快速并行算法已经开发用于简单流体和短程MD，但可以处理聚合链的算法还不可用。此外，目前的算法只处理平衡问题，而有必要研究加入剪切流的问题。几个研究小组用模拟的方法研究了二嵌段共聚物的平衡相图。模拟还没有被用来研究它们在实验中观察到的、相当复杂的动态响应。平衡态对称双块在有序-无序转变温度（TODT）下由低温有序（片层）相向高温无序相转变。剪切作用改变了TODT的值。即使在静态TODT之上，也可以观察到有序相。对于超长聚合物，理论上已经预测了TODT的变化。对于短块——克莱默在康奈尔大学的小组正在进行的实验——理论预测是不存在的，人们只能依靠模拟来代替。本项目还将研究影响薄片取向的因素。实验上，已经观察到不同的方向，但是哪种机制在首选方向的选择中起作用，到目前为止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.