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，广泛存在于天然和人造材料中，从DNA、木材、橡胶到管道、服装和杂货袋。许多基本的聚合物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规则地排列。结晶度在决定聚合物材料的性能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如何以及何时结晶是至关重要的。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，但来自实验和计算机模拟的证据一直是不确定的。本项目将采用模拟方法，成功地用于研究非聚合物，如水的结晶研究聚合物。该项目的目标将侧重于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，有可能导致创新性新材料的开发，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播客，重点关注科学领域的职业发展，在本科生和K-12学生之间，PI和本科生之间，在经验丰富和缺乏经验的研究生之间，以及PI和未来科学家之间创造指导机会。技术总结尽管经过数十年的研究，聚合物科学缺乏一个被广泛接受的熔融态结晶理论。精确的聚合物结晶理论的发展不仅将改变教科书，而且还将使创造具有更好性能的新聚合物和制造对环境影响更小的聚合物的工艺成为可能。成核过程是聚合物结晶的基础，但越来越多的证据表明，经典成核理论不足以解释均匀成核，导致研究人员提出有争议的替代理论。该项目的中心假设是，平衡先进的采样方法可以用来直接计算成核势垒，以测试这些新的理论。虽然新的聚合物结晶，先进的取样方法已被用于研究成核在其他领域，他们有明显的优势，比其他方法。因此，本项目将重点研究折叠链晶体的成核势垒作为聚合物分子量的函数和作为聚合物化学的函数。该项目还将为正在进行的用于计算低温聚合物熔体中自由能景观的模拟软件的开发做出贡献。该项目还将涉及综合研究和教学目标，特别关注从K-12到研究生教育水平的学生的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的劳动力的关键教育组成部分。具体的辅导活动包括近同行辅导和推广K-12学生，PI辅导的本科研究，可扩展的辅导经验，通过制作和分发播客"科学是如何发生的，“”和附近-通过化学工程学生理事会对研究生进行同侪指导。该奖项反映了NSF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。

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.