复杂高分子熔体拉伸流变行为的计算机模拟研究

Low melt strength and thus poor processability of simple linear polymer melts act as a bottleneck for a successful engineering of high-performance polymer materials for use of packaging. A molecular understanding of extensional rheology of polymer melts enables us, through a molecular design, to essentially enhance a strain hardening of them under extensional flow. So far, it has been well-known that long-chain branching and polydispersity of linear polymers are two main strategies for imposing a strain hardening property on polymer melts. However, the relationship between molecular structure and extensional rheology of polymers hasn’t been completely established, essentially because molecular mechanism of strain hardening of polymer melts under elongation still remains incompletely understood. Particle-based computer simulation plays an indispensible role in understanding and predicting viscoelastic properties of polymer melts starting from their molecular structures. Studying polymer rheology via computer simulation first requires preparation of well-equilibrated configurations of entangled polymer melts, which however is still a practical challenge in the community. Here, we will extend our previously developed hierarchical simulation method to cases of long-chain branched and polydisperse linear polymer melts, and generate well-equilibrated configurations of them. Then, by employing large-scale nonequlibrium molecular dynamics simulations, we aim to clarify unambiguously the underlying mechanisms of strain hardening of the above-mentioned, complex polymer melts under extensional flow. Accomplishment of this study will not only provide a theoretical guidance for an efficient materials design in polymer packaging industries, but also generate new knowledge and understanding for nonlinear extensional rheology of polymers.

简单线型高分子的低熔体强度以及由此导致的可加工性差，是当前高性能聚合物包装材料开发中的主要瓶颈。从分子水平上理解高分子熔体的拉伸应变硬化行为，并以分子结构调控提升该性能是突破这一瓶颈的关键。在高分子体系中引入链的支化与分子量的多分散性是解决该问题的主要途径，但是这些体系的分子结构及组成同其熔体拉伸流变行为之间的关系尚不完全清楚，其拉伸应变硬化行为的分子机制还比较模糊。粒子模拟是从分子结构出发理解与预测高分子熔体粘弹行为的重要方法。然而，作为该方法研究的首要步骤，高效制备高分子熔体的平衡态构型仍然是个技术上的挑战。为此，我们拓展业已建立的高效构建线型高分子平衡态构型的层级模拟方法至这些复杂体系中，首先获得体系平衡态构型，继而采用大规模非平衡态分子动力学模拟等手段剖析这些体系拉伸流变行为的分子机理。该研究不仅能够为工业界开发高性能包装材料提供有益指导，亦能对非线性高分子拉伸流变学提供新理解。

高分子链的拓扑受限（缠结）是高分子物理学中一个重要的基本概念，它在决定高分子体系包括力学在内的众多物理性能方面发挥着关键的角色。近年来，以拓扑缠结为键合方式的新型高分子（机械互锁高分子）进入了人们的视野。拓扑键的引入给该体系带来了新的构象动力学模式与构象熵的来源，因此预期体系会涌现出完全不同的性质。本课题中，我们聚焦环型高分子、聚索烃、聚轮烷三类典型的复杂拓扑高分子体系，研究了它们的平衡态链构象性质、单链弹性力学等；澄清了环型高分子在亚浓溶液中链构象尺寸与浓度的标度律，揭示了线型和花瓣型聚索烃在稀溶液中的链构象特性、线型单链聚索烃的弹性力学，以及假聚轮烷环覆盖率的调控机制。这些研究丰富了复杂拓扑高分子的基本物理内容，为实验设计基于它们的功能材料提供了有益的帮助。