Stiff chains in tight spots: confinement and semiflexibility in semicrystalline polymers and entangled melts

摘要最常见的塑料聚合物，聚乙烯和聚丙烯，是由精炼和天然气原料中获得的简单分子制成的。这些聚合物价格低廉，重量轻，用途广泛，而且无处不在——然而，考虑到它们的分子结构，它们的材料性能仍远未达到最佳。有改进空间的一个原因是我们对塑料特性的分子起源的理解非常不完整。塑料的一个关键特性是，当你拉它们时，它们会“断裂”，而不是断裂成两半。对于手和眼睛来说，这就是使塑料成为塑料的原因。但是，固体材料是如何做到“给予”而不是“折断”的呢？为了找到答案，PI的小组将使用计算机模拟来观察组成普通塑料的晶体内部，在这些晶体中，长聚合物分子像铅笔一样聚集在一起。晶体填料中可能存在缺陷——被困住的扭曲，称为“扭曲孤子”，它们可以沿着分子的长度移动，就像地毯下的肿块一样。了解聚合物晶体如何在不断裂的情况下改变形状，可以帮助我们了解如何为更广泛的用途定制廉价、轻量化的聚合物材料。设计新的聚合物分子是昂贵的。分子模拟可以用来预测分子结构的关键材料性质，指导尝试哪些新分子。聚合物的一个关键特性是它们在熔融时的流动方式，因为聚合物通常是通过熔融加工制成成品的。现代理论在预测聚合物流动行为方面做得很好，如果人们知道两个关键参数：聚合物的“纠缠”程度，以及它的“光滑”程度。为了预测给定分子结构的这些参数，PI的团队将模拟由单个长自纠缠环分子组成的密集熔体。利用这些参数，可以预测真实聚合物的流动行为，并用于指导新分子的设计。技术摘要聚合物链受到相邻分子的排斥势的限制在半结晶聚合物和纠缠熔体的物理性质中起着重要作用。在这些不同的系统中，在受限几何中保持活跃的初等激励是一个关键的共同因素。在纠缠的熔体中，这些激发态是“管”内的运动。在半结晶聚合物的晶片中，激发态是扭转孤子。链刚度增强了约束效应，使管束变窄，增加了孤子能量。该建议利用分子动力学和蒙特卡罗模拟结合拓扑方法来表征这些高度受限系统中最重要的运动。这些结果将有助于预测关键的材料特性，包括成核屏障、熔体的缠结长度和摩擦系数，以及半结晶聚合物中的非晶橡胶样模量。本提案的第一部分建立在使用原子分子动力学模拟来表征聚乙烯中的扭孤子的成功基础上。扭转孤子允许层状晶体中的链式运动，使塑料变形成为塑料。孤子在无序的“旋转”相中增殖，这在半晶聚乙烯的成核中起着关键作用。聚丙烯是世界上第二大最常见的聚合物，仅次于聚乙烯。它以单链螺旋而不是全反式带的形式结晶。聚乙烯中的扭孤子和旋转相是一种侥幸，还是一种范式？模拟将用于发现扭曲孤子和旋转相是否也在聚丙烯中起关键作用。第二部分利用PI？S集团在纠缠聚合物熔体方面的专业知识。新方法将应用于一些基本的重要问题：长纠缠链的动力学机制、支链聚合物臂松弛时形成的“发夹”结构，以及半结晶聚合物中非晶态区域的橡胶样模量。了解聚烯烃的结晶可以为竞争激烈的全球市场带来先进的产品。非晶区被困的缠结很少被了解，但对于大应变塑性变形而不失效至关重要。这种关键的材料属性使塑料薄膜的环保“减径”成为可能，用更少的材料完成同样的工作。从小规模模拟中确定基本材料参数的有效方法可以开启计算机辅助材料设计的新时代。

Nontechnical AbstractThe most common plastic polymers, polyethylene and polypropylene, are made from simple molecules obtained from refining and natural gas feed stocks. These polymers are inexpensive, lightweight, versatile, and ubiquitous - and yet, given their molecular structures, their material properties are still far from optimal. One reason there is room for improvement is that our understanding of the molecular origin of plastic properties is very incomplete. A key property of plastics is that when you pull on them, they "give", rather than snapping in two. To the hand and eye, that is what makes a plastic plastic. But how does a solid material manage to "give" rather than snap? To find out, the PI's group will use computer simulations to look inside the crystals that make up common plastics, in which the long polymer molecules pack together like pencils in a box. There can be defects in the crystal packing - trapped twists, called "twist solitons", that can move along the length of the molecule, like a bump under a rug. Understanding how polymer crystals change shape in response to stress without snapping may give insight into how to tailor inexpensive, weight-saving polymer materials for a wider range of uses.Designing new polymer molecules is expensive. Molecular simulations can be used to predict key material properties of molecular structures, to guide which new molecules to try. A key property of polymers is how they flow when molten, because polymers are often made into finished products by melt processing. Modern theory does a good job of predicting polymer flow behavior, if one knows two key parameters: how "entangled" a polymer is, and how "slippery" it is. To predict these parameters for a given molecular structure, the PI's group will model a dense melt consisting of a single long self-entangled ring molecule. With these parameters, flow behavior of real polymers can be predicted, and used to guide design of new molecules.Technical AbstractConfinement of polymer chains by the repulsive potentials of neighboring molecules plays an essential role in the physics of semicrystalline polymers and entangled melts. The elementary excitations that remain active in the constrained geometry are a key common element in these disparate systems. In entangled melts, these excitations are motions within the "tube". In the crystalline lamellae of semicrystalline polymers, the excitations are twist solitons. Chain stiffness intensifies the effects of confinement, narrowing the tube and increasing the soliton energy. This proposal exploits molecular dynamics and Monte Carlo simulations combined with topological methods to characterize the most important motions in these highly constrained systems. These results will enable predictions for key material properties, including nucleation barriers, entanglement length and friction factor for melts, and the amorphous rubberlike modulus in semicrystalline polymers.The first part of this proposal builds on success of using atomistic molecular dynamics simulations to characterize twist solitons in polyethylene. Twist solitons permit chain motion in lamellar crystallites, enabling the deformations that make plastics plastic. Solitons proliferate in disordered "rotator" phases, which play a key role in nucleation of semicrystalline polyethylene. Polypropylene is the second most common polymer worldwide after polyethylene. It crystallizes in a packing of single-chain helices instead of all-trans ribbons. Are twist solitons and rotator phases in polyethylene a fluke, or a paradigm? Simulations will be used to discover if twist solitons and rotator phases also play a key role in polypropylene.The second part exploits PI?s group expertise in entangled polymer melts. New methods will be applied to problems of fundamental importance: dynamical regimes for long entangled chains, structure of "hairpins" formed as branched polymer arms relax, and the rubberlike modulus of the amorphous region in semicrystalline polymers.Understanding crystallization in polyolefins could lead to advanced products in a competitive global marketplace. Trapped entanglements in the amorphous region are little understood, yet critical for large-strain plastic deformation without failure. This key material attributes enables environmentally friendly "downgauging" of plastic films, doing the same job with much less material. Efficient methods to determine fundamental materials parameters from small scale simulations could unleash a new era in computer-aided materials design.