Molecular modelling of flow-induced crystallisation in polymers

半结晶塑料制品在我们的日常生活中随处可见。从食品和饮料容器到高性能塑料部件，半结晶塑料构成了商用塑料的最大群体。塑料的结晶受其分子形状的强烈影响。这是因为塑料是由长链分子或聚合物组成的。聚合物分子的连接性质迫使它们结晶成有序结晶区域的混合物，其中穿插着更随机排列的链区域。非晶材料和结晶材料的比例，以及晶体的排列和取向，统称为形态。晶体形态强烈影响强度、韧性、渗透性、表面纹理、透明度和几乎任何其他实际感兴趣的性能。众所周知，形态可以由塑料结晶时所经历的流动条件决定。通常，这些流动发生在塑料制品成型的过程中。例如，在将塑料注入模具或将其吹成薄膜时发生的流动。因此，通过了解流动如何影响结晶，原则上有可能通过仔细控制加工方式来提高产品的最终性能。不幸的是，在分子水平上对聚合物结晶的详细了解，特别是在流动状态下，很难获得。这是因为聚合物中的流动诱导结晶取决于几个复杂因素的微妙相互作用。首先，聚合物在流动过程中的结晶是由流动迫使分子形成的形状控制的，直到最近，关于聚合物在强流动下如何移动的精确理论还不够准确。其次，聚合物的结晶总是不完全的；聚合物分子的连接性质阻碍了材料达到最低能态的努力，因此平衡概念不能应用。实际上，最终状态是由结晶动力学控制的。在这个项目中，我们采用了一种新的流动诱导结晶方法来克服这两个问题。最近导出的分子流动模型已被证明可以可靠地预测聚合物分子在流动中的构型，我们将这些模型作为我们模型的起点。为了捕捉结晶动力学，我们采用了一种有效的动力学蒙特卡罗模拟技术来模拟晶体形成的早期阶段。实验表明，对这些早期阶段的影响是流动控制结晶的主要方法。这些模拟的结果将提高我们对流动诱导结晶的理解，并将为我们提供一个模板，以推导更简单的基于微分方程的模型，这将适用于塑料加工的流动建模。

Products made of semi-crystalline plastics are found everywhere in our everyday lives. From food and drinks containers to high performance plastic components, semi-crystalline plastics comprise the largest group of commercially useful plastics. The crystallisation of plastic is strongly affected by its molecular shape. This is because plastics are made-up of long-chain molecules, or polymers. The connected nature of polymer molecules forces them to crystallise into a mixture of ordered crystalline regions, which are interspersed with regions where the chains are more randomly arranged. The proportion of amorphous and crystalline material, along with the arrangement and orientation of the crystals, is collectively known as the morphology. The crystal morphology strongly influences strength, toughness, permeability, surface texture, transparency and almost any other property of practical interest. It is known that morphology can be determined by the flow conditions that a plastic experiences as it crystallises. Typically, these flows occur during the process that shapes a plastic product. For example, flows occurring while injecting a plastic into a mould or blowing it into a film. Thus, by understanding how flow affects crystallisation it is possible, in principle, to enhance the final properties of a product by careful control of how it is processed. Unfortunately, a detailed understanding of polymer crystallisation at a molecular level, particularly under flow has been difficult to acquire. This is because flow-induced crystallisation in polymers depends on the subtle interplay of several complicating factors. Firstly, polymer crystallisation during flow is controlled by the shapes that flow forces the molecules to form, and precise theories for how polymers move under strong flow have, until recently, not been sufficiently accurate. Secondly, crystallisation is polymers is always incomplete; the connected nature of polymer molecules frustrates the materials efforts to reach the lowest energy state so equilibrium concepts cannot be applied. In fact the final state is controlled by the crystallisation kinetics. In this project we take a new approach to flow induced crystallisation to overcome these two problems. Recently derived molecular flow models have been shown to reliably predict the configuration of polymer molecules under flow, and we use these as the starting point of our model. To capture the crystallisation kinetics we employ an efficient kinetic Monte Carlo simulation technique to simulate the early stages of crystal formation. Influence over these early stages, experiments suggest, are the primary method by which flow controls crystallisation. Results from these simulations will improve our understanding of flow-induced crystallisation and will provide a template for us to derive more simple differential equation based models, which will be suitable for flow modelling of plastic processing.