Intramolecular and Intermolecular Packing in Polymer Crystallization

第一部分：非技术总结超过2/3的商用聚合物是半结晶的，包括我们生活中以及各种高科技应用中的大量和种类的材料。在过去的半个世纪里，通过大分子链结构或通过高阶结构组装来调节聚合物结晶的方法已经得到了改进。然而，一个具有广泛重要性的基本问题是在分子水平上理解聚合物结晶的机理。分子缠结、迁移率、分子质量、溶剂-聚合物或聚合物-聚合物相互作用、聚合物浓度等因素都会影响半结晶聚合物结晶过程中的链级结构。本项目旨在表征从熔体或溶液中生长的晶体中特殊标记的半结晶聚合物的分子内和分子间结构。这项研究有望加强以下方面的基础：(I)熔融晶体中链状折叠结构的纠缠效应，(Ii)单晶中链折叠模体的分子量效应，(Iii)稀溶液中聚合物结晶的早期阶段，(Iv)螺旋聚合物的手性识别，以及(V)立体规则聚烯烃类材料的分子动力学。所建立的基础将对理解结晶机理和设计新型聚合物材料具有重要意义。受过该项目培训的研究生将获得广泛的实验技能，包括聚合物合成、材料表征、高级核磁共振表征和数值模拟。他们将有机会在国家和区域会议上介绍技术演讲和海报。第二部分：技术总结长和柔性半结晶聚合物在结晶过程中不同程度地折叠。除了分子间堆积外，折叠还会产生分子内堆积。在长链聚合物的结晶过程中，分子内和分子间堆积结构的理解是必不可少的。本项目将利用核磁共振波谱技术评估13C标记的半结晶聚合物在溶液和熔融生长晶体中的分子内和分子间堆积结构。生物可降解聚合物聚乳酸(PLA)将被用作模型体系，因为已建立的分解路线允许我们重复使用13C标记单体来有效地合成各种13C标记PLAs，其相对分子质量从2K到100K g/mol都是精确可控的。首先，本课题通过研究(I)分子量对聚乳酸分子内折叠结构的影响和(Ii)低分子量组分的掺杂对熔体晶体中长链折叠结构的影响来研究分子内堆积结构的纠缠效应。第二个推力旨在阐明(I)分子量对溶液生长晶体中折叠链的分子尺寸的影响以及(Ii)稀溶液中的前体结构。最近发展起来的动态核极化核磁共振显着提高了13C标记聚合物信号的灵敏度(~400倍)，从而可以直接分析13C标记聚合物在冷冻溶液状态下的链级结构。本实验将从分子水平上揭示聚合物结晶的早期阶段。第三个重点是了解分子间和分子内堆积对聚合物结晶的重要性。一个聚乳酸立体络合物的模型系统将被用来研究分子内和分子间堆积结构上的分子量效应。最后，将探讨最近开发的聚烯烃的详细分子动力学和机械性能。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

PART 1: NON-TECHNICAL SUMMARYMore than 2/3 of all commercially available polymers are semicrystalline including an enormous number and variety of materials in our lives as well as in all kinds of high-technology applications. Tuning of polymer crystallization through the macromolecular chain structures or through high-order structural assemblies has been improved over the last half century. However, a fundamental problem of widespread importance is to understand the mechanism of polymer crystallization at the molecular level. Many factors, such as molecular entanglement, mobility, molecular weight, solvent-polymer or polymer-polymer interactions, polymer concentration, and others influence the chain-level structure of semicrystalline polymers during crystallization. This project is aimed at characterizing intramolecular and intermolecular structures of specially labeled semicrystalline polymers in crystals grown from the melt or from solution. The research is expected to enhance foundations about: (i) entanglement effects on chain-folding structure in melt-grown crystals, (ii) molecular weight effects on chain-folding motif in single crystals, (iii) early stage of polymer crystallization in dilute solutions, (iv) chiral recognition of helical polymers, and (v) molecular dynamics of stereoregular polyolefin-type materials. The established foundations will be important for understanding crystallization mechanisms and for designing novel polymer materials. Graduate students trained in this project will acquire broad experimental skills covering polymer synthesis, materials characterization, advanced NMR characterization, and numerical simulations. They will have opportunities to present technical talks and posters at national and regional meetings. PART 2: TECHNICAL SUMMARYLong and flexible semicrystalline polymers fold to various extents during crystallization. Folding generates intramolecular packing in addition to intermolecular packing. Understanding of intramolecular and intermolecular packing structures is essential in the crystallization process of long polymer chains. This project will focus on evaluation of the intramolecular and intermolecular packing structures of 13C labeled semicrystalline polymers in solution and melt-grown crystals by using nuclear magnetic resonance (NMR) spectroscopy. A biodegradable polymer, poly(lactic acid) (PLA) will be used as a model system, since established decomposition route allows us to re-use 13C labeled monomer to effectively synthesize a variety of 13C labeled PLAs with precisely controlled and diverse molecular weights from 2K to over 100K g/mol. First, the project is aimed at entanglement effects on the intramolecular packing structure of poly(L-lactic acid) by investigating (i) Mw effect on the chain-folding structure and (ii) blending effect of low Mw component on the chain-folding structure of long chains in melt-grown crystals. A second thrust aims at elucidating (i) Mw effect on the molecular dimension of the folded chains in solution-grown crystals and (ii) precursor structure in dilute solutions. Recently developed Dynamic Nuclear Polarization NMR significantly enhances sensitivity of the 13C labeled polymer signals (~ 400 times) and thus may allow to directly analyze the chain-level structure of 13C labeled polymer in the frozen solution state. This experiment will shed light on the early stage of polymer crystallization at the molecular level. A third thrust aims at understanding the importance of both intermolecular and intramolecular packing on polymer crystallization. A model system of PLA stereocomplex will be used to address Mw effects on both intramolecular and intermolecular packing structures. Finally, detailed molecular dynamics and mechanical properties of recently developed polyolefins will be explored.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.