星形高分子流体流变行为的微观机理

Star polymers’ unique properties, such as high thermostability and elastic modulus, make them promising materials in commercial applications. The production and the final performance of materials are closely related to the polymer processing, during which polymers usually exhibit a fluid state. Hence, rheological properties are increasingly important, which is deeply rooted in the microscopic dynamics and structure. As the number of arms increases, a new relaxation mode, named as local structural relaxation, is introduced in star polymer melts due to the formation of "hard core". The effect of this new relaxation mode on the rheological behaviors is unknown so far, which is of great significance both from a fundamental and an applied viewpoint. In this project, we plan to investigate the unentangled and entangled star polymer melts with various functionality under the condition of equilibrium and nonequilibrium (including start-up, steady and oscillatory shear) state. We expect to elucidate the evolution of local structural relaxation, entanglement network and terminal relaxation, reveal the coupling between above dynamical modes, and finally construct the physical picture for the rheological properties.

星形高分子材料，具有高耐热性、高弹性模量等优异性能，应用前景广阔。加工成型过程对于高分子材料的制备及优异性能的实现至关重要，而流变学研究正是致力于解决该过程中涉及的基本科学问题。流变行为与微观动力学密切相关，星形链随着臂数的增加在分子链末端松弛之外引入了一种新的松弛模式---局域结构松弛，揭示微观松弛与宏观性质之间的关联是理解流变行为微观机理的关键，具有应用和基础研究的双重意义。本项目主要针对不同臂数的非缠结和缠结星形高分子体系，拟开展平衡态和非平衡态（包括启动、稳态和振荡等剪切模式）的模拟研究，探讨体系中局域结构松弛、缠结网络（缠结体系）和分子链松弛的响应演化，揭示不同松弛机制之间的相互耦合，最终阐明上述关键因素影响流变行为的微观机理。

星形聚合物随着臂数的增加会在中心区域形成紧密排列的“硬核”，这些硬核的出现使得星形聚合物出现了附加的松弛模式——局域结构松弛，从而使星形聚合物在流场下存在两个水平上的结构响应，即链水平和局域结构水平，其流变行为也可能较传统的线形聚合物存在很大的不同。利用分子动力学模拟，我们研究了星形高分子在瞬态启动剪切和稳态剪切外场下的流变性质。研究表明：拥有大量臂的星形聚合物熔体与少量臂的星形聚合物熔体的应力应变响应截然不同，多臂星形高分子表现的更像软球，而少臂星形高分子更像线形链的无规线团。多臂星形高分子流体表现出奇异的应力过冲行为和剪切变稀行为，而针对体系微观结构和动力学的研究表明局域结构松弛和链结构松弛的共同作用是上述奇异流变行为的微观起源。