CAREER: Evaluating Theories of Polymer Crystallization by Directly Calculating the Nucleation Barrier in a Polymer Melt

聚合物是一种长链分子，广泛存在于各种天然和人造材料中，从DNA、木材、橡胶到管道、衣服和杂货袋。许多重要的高分子材料具有至少部分结晶的结构，这意味着分子内的原子在空间中有规则地排列自己。结晶度在决定聚合物材料的性质方面起着至关重要的作用，因此科学家和工程师能够理解和操纵聚合物结晶的方式和时间至关重要。尽管进行了广泛的研究，但聚合物结晶背后的确切机制仍然未知。有几个相互竞争的理论，但实验和计算机模拟的证据还没有定论。本项目将应用一种成功用于研究非聚合物（如水）结晶的模拟方法来研究聚合物。该项目的目标将集中在研究分子的长度和不同类型的聚合物如何影响结晶机制。这种新方法的成功可能会彻底改变我们对聚合物如何形成晶体的基本理解，有可能导致创新新材料的开发，并减少现有材料对环境的影响。该项目还将包括与研究活动相结合的教育目标。具体来说，该项目将通过创建一个专注于科学职业发展的播客，在本科生和K-12学生之间、在PI和本科生之间、在经验丰富和经验不足的研究生之间、以及在PI和未来的科学家之间创造指导机会。尽管经过了几十年的研究，聚合物科学仍然缺乏一种被广泛接受的熔体结晶理论。精确的聚合物结晶理论的发展不仅会改变教科书，而且还能创造出性能更好的新型聚合物，以及制造对环境影响更小的聚合物的工艺。成核过程是聚合物结晶的基础，但越来越多的证据表明，经典成核理论不能充分解释均匀成核，导致研究人员提出有争议的替代理论。本项目的中心假设是，平衡先进的采样方法可以直接计算成核势垒来验证这些新理论。先进的采样方法虽然是聚合物结晶的新技术，但已在其他领域用于研究成核，并且具有明显的优势。因此，本项目将重点研究折叠链晶体的成核屏障与聚合物分子量和聚合物化学的关系。该项目还将有助于正在进行的模拟软件的开发，用于计算低温聚合物熔体中的自由能景观。该项目还将涉及综合研究和教学目标，特别侧重于为从K-12到研究生教育水平的学生提供有效和可扩展的指导机会。指导新的科学家和工程师，特别是代表性不足的少数民族，是培养具有全球竞争力和多样化劳动力的关键教育组成部分。具体的指导活动包括对K-12学生的近同伴指导和拓展，pi指导的本科生研究，通过制作和分发播客“科学是如何发生的”进行可扩展的指导体验，以及通过化学工程学生委员会对研究生进行近同伴指导。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

NONTECHNICAL SUMMARYPolymers are long-chain molecules found in a wide array of natural and man-made materials, ranging from DNA, wood, and rubber to piping, clothing, and grocery bags. Many essential polymer materials have a structure that is at least partially crystalline, meaning the atoms within the molecule arrange themselves regularly in space. Crystallinity plays a crucial role in determining the properties of polymer materials, and thus it is critical that scientists and engineers be able to understand and manipulate how and when polymers crystallize.Despite extensive research, the exact mechanism behind polymers crystallization remains unknown. There are several competing theories, but evidence from experiments and computer simulations has been inconclusive. This project will apply a simulation method that was successfully used to study the crystallization of non-polymers such as water to study polymers. The project objectives will focus on studying how the length of the molecules and the different types of polymers affect the mechanism of crystallization. Success with this new approach could revolutionize our basic understanding of how polymers form crystals, potentially leading to the development of innovative new materials and reduced environmental impacts from existing ones.The project will also include educational objectives that are integrated with research activities. Specifically, the project will create mentoring opportunities between undergraduates and K-12 students, between the PI and undergraduates, between more and less experienced graduate students, and between the PI and future scientists through the creation of a podcast that focuses on career development in the sciences.TECHNICAL SUMMARYDespite decades of research, polymer science lacks a widely accepted theory for crystallization from the melt state. The development of an accurate theory of polymer crystallization would not only change textbooks, but it would also enable the creation of new polymers with better properties and processes for making polymers with less environmental impact.Nucleation processes are foundational to polymer crystallization, but accumulating evidence suggests that Classical Nucleation Theory inadequately explains homogeneous nucleation, leading researchers to propose controversial alternative theories. The central hypothesis of this project is that equilibrium advanced sampling methods can be used to directly calculate the nucleation barrier to test these new theories. While new to polymer crystallization, advanced sampling methods have been used study nucleation in other fields, and they have distinct advantages over other methods. Accordingly, this project will focus on a study of the nucleation barrier for folded-chain crystals as a function of polymer molecular weight and as a function of polymer chemistry. This project will also contribute to the ongoing development of simulation software for computing free energy landscapes in low-temperature polymer melts.The project will also involve integrated research and teaching aims specifically focusing on effective and scalable mentoring opportunities for students spanning from K-12 through graduate education levels. Mentoring new scientists and engineers, especially underrepresented minorities, is a key educational component of cultivating a globally competitive and diverse workforce. Specific mentoring activities include near-peer mentoring and outreach to K-12 students, PI-mentored undergraduate research, scalable mentoring experiences through the production and distribution of the podcast ``How Science Happens,'' and near-peer mentoring of graduate students through a chemical engineering student council.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.