Multiscale Theory For Semicrystalline Polymers

技术总结该奖项支持聚合物结晶的理论研究和教育。聚合物分子的长链性质决定了聚合物晶体采用链状折叠的片层形式，但晶体如何成核的基本问题尚不清楚。有实验证据表明，最常见的半结晶聚合物聚乙烯实际上是通过中间旋转体的中间相成核的。然而，目前还没有理论基础来评估这一有趣的假设，或确定这种现象在其他聚合物中的普遍程度。要取得进展，需要一种关于中间相的理论，不仅能够计算它们相对于熔体和晶体相的自由能，而且能够计算熔体与中间相或晶体之间界面的自由能。半晶体聚合物中晶体-熔体界面的有效理论也可以预测连接相邻晶片的？结链的浓度，并导致塑料的韧性和延展性。此外，流动对结晶动力学和形貌的影响也引起了人们的极大兴趣。这在半结晶聚合物的商业应用中至关重要，因为它们对流动效应非常敏感。如果没有一个好的静态结晶理论，就很难理解流动是如何加速成核的。PI旨在为理解聚乙烯中的中间相提供理论基础。这将导致对体能和表面自由能的预测，并将确定聚乙烯中的成核是否真的可以通过中间相发生。考虑到问题本身的多尺度性质，将采用一种变革性的综合技术：有序相的原子模拟、表征中间相区壁的固体模拟的新使用、部分有序中间相的介观离散自旋模拟以及有序聚合物相与相邻熔体之间界面的新的接枝链刷理论。这种独特的策略组合将导致更好地理解聚合物晶体是如何成核的，并将使我们更接近于实现这些真正现代材料的最佳性能。这一具有实际重要性的智力丰富的问题为教育提供了一个极好的机会，为学生和博士后提供了广泛的接触理论、分析方法、原子和介观模拟的机会。PI正在开发一门2009年秋季的聚合物和复杂流体的本科课程，这与本提案的多尺度方法非常一致。化学工程系研究生中的少数族裔和性别比例很高，约有30%是女性。工程学院有一个活跃的女性工程课程和一个多元文化工程课程。所有模拟都将使用开源软件进行，以消除任何障碍，供其他人使用。非技术总结该奖项支持有关聚合物如何结晶的理论和计算研究和教育。半晶聚合物虽然相对年轻，但却是现代最普遍的材料。现在全世界每年生产的这种材料的数量超过了钢铁的产量。即便如此，它们想要的机械和物理性能的最终潜力还没有实现。这是因为，与有着数百年历史的冶金领域相比，半结晶聚合物的科学基础在很大程度上仍在进行中，许多关键成果是在过去几十年才取得的。同样，通过催化方面的进展来改进对聚合物分子结构的控制也是最近才出现的。PI将使用理论技术来探索通过中间聚合物相结晶的可能的微观机制。这一具有实际重要性的知识丰富的问题为教育提供了一个极好的机会，为学生和博士后提供了广泛的接触理论、分析方法、原子和中尺度模拟的机会。PI正在开发一门2009年秋季的聚合物和复杂流体的本科课程，这与本提案的多尺度方法非常一致。化学工程系研究生中的少数族裔和性别比例很高，约有30%是女性。工程学院有一个活跃的女性工程课程和一个多元文化工程课程。所有模拟都将使用开源软件进行，以消除任何障碍，供其他人使用。

TECHNICAL SUMMARYThis award supports theoretical research and education on polymer crystallization. The long-chain nature of polymer molecules dictates that polymeric crystals adopt a chain-folded lamellar form, but the basic question of how the crystals nucleate is ill understood. There is experimental evidence that polyethylene, the most common semicrystalline polymer, actually nucleates via an intermediate ?rotator" mesophase. However, no theoretical basis yet exists to assess this intriguing hypothesis, or to ascertain how widespread this phenomenon might be in other polymers. Progress requires a theory of the mesophases, able to compute not only their free energy relative to the melt and crystal phases, but also the free energy of the interface between the melt and the mesophase or crystal.An effective theory of the crystal-melt interface in semicrystalline polymers would also predict the concentration of ?tie chains", which link together adjacent crystalline lamellae and lead to the toughness and ductility of plastics. Also, there is much current interest in the effect of flow on crystallization kinetics and morphology. This is crucial in commercial use of semicrystalline polymers, which are very sensitive to flow effects. Without a good theory of quiescent crystallization, any effort to understand how flow speeds up nucleation is severely handicapped.The PI aims to develop the theoretical basis to understand the mesophases in polyethylene. It will lead to predictions for bulk and surface free energies, and it will determine whether nucleation in polyethylene can indeed occur via a mesophase. Given the inherently multiscale nature of the problem, a transformative synthesis of techniques will be employed: atomistic simulation of ordered phases, novel use of ?solid-state simulations" to characterize mesophase domain walls, mesoscopic discrete-spin simulation of partially ordered mesophases, and a new adaptation of grafted chain ?brush" theory of the interface between ordered polymer phases and adjacent melt. This unique combination of strategies will lead to a better understanding of how polymer crystals nucleate, and will bring us closer to achieving optimal properties of these truly modern materials.This intellectually rich problem of practical importance provides an excellent opportunity for education, offering students and postdocs broad exposure to theory, analytical methods, atomistic and mesoscale simulation. The PI is developing an undergraduate course for Fall 2009 in Polymers and Complex Fluids, which is well aligned with the multiscale approach of this proposal. The Chemical Engineering Department has a strong record of minority and gender representation among its graduate students, with about 30 percent women. The College of Engineering has an active Women In Engineering Program and a Multicultural Engineering Program. All simulations will be performed with open source software, to remove any barrier to use by others. NON-TECHNICAL SUMMARYThis award supports theoretical and computational research and education on how polymers crystallize. Semicrystalline polymers, although relatively young, are the most ubiquitous materials of the modern age. The mass of such materials now produced worldwide each year exceeds the production of steel. Even so, their ultimate potential for desirable mechanical and physical properties is as yet unfulfilled. This is because, in contrast to the centuries-old field of metallurgy, the science base for semicrystalline polymers is still very much a work in progress, with many key results obtained only in the past few decades. Likewise, improved control of polymer molecular structure through advances in catalysis has emerged only relatively recently.The PI will use theoretical techniques to explore a possible microscopic mechanism for crystallization through an intermediate polymer phase. This intellectually rich problem of practical importance provides an excellent opportunity for education, offering students and postdocs broad exposure to theory, analytical methods, atomistic and mesoscale simulation. The PI is developing an undergraduate course for Fall 2009 in Polymers and Complex Fluids, which is well aligned with the multiscale approach of this proposal. The Chemical Engineering Department has a strong record of minority and gender representation among its graduate students, with about 30 percent women. The College of Engineering has an active Women In Engineering Program and a Multicultural Engineering Program. All simulations will be performed with open source software, to remove any barrier to use by others.