Intramolecular and Intermolecular Packing in Polymer Crystallization

超过三分之二的商用聚合物是半结晶的，包括我们生活中大量和各种各样的材料以及各种高科技应用。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整聚合物结晶已经得到了改进。然而，在分子水平上理解聚合物结晶的机理是一个具有广泛重要性的基本问题。许多因素，如分子缠结、迁移率、分子量、溶剂-聚合物或聚合物-聚合物相互作用、聚合物浓度等，在结晶过程中影响半结晶聚合物的链级结构。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晶聚合物的分子内和分子间结构。本研究将为以下方面的研究奠定基础：(i)熔体生长晶体中链折叠结构的缠结效应，（ii）单晶中链折叠基序的分子量效应，（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)立体规则聚烯烃型材料的分子动力学。所建立的基础对于理解结晶机制和设计新型高分子材料将是重要的。在这个项目中训练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发表技术演讲和海报。长而柔韧的半结晶聚合物在结晶过程中有不同程度的折叠。除了分子间的堆积外，折叠还产生分子内的堆积。了解分子内和分子间的堆积结构在长聚合物链的结晶过程中是必不可少的。本项目将重点利用核磁共振（NMR）技术评价13C标记的半晶聚合物在溶液和熔融生长晶体中的分子内和分子间填充结构。我们将以可生物降解的聚合物聚乳酸（PLA）作为模型体系，因为建立的分解路线使我们能够再利用13C标记的单体，有效地合成各种13C标记的PLA，这些PLA的分子量从2K到100K g/mol以上都是精确控制的。首先，该项目旨在通过研究(i) Mw对链折叠结构的影响以及（ii）熔融生长晶体中低Mw组分对长链链折叠结构的混合影响，来研究缠结对聚l -乳酸分子内堆积结构的影响。第二个重点是阐明(i) Mw对溶液生长晶体中折叠链的分子尺寸的影响和（ii）稀溶液中的前驱体结构。最近发展的动态核极化核磁共振显著提高了13C标记聚合物信号的灵敏度（约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。该实验将在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填充对聚合物结晶的重要性。一个PLA立体配合物的模型系统将被用来研究分子内和分子间填充结构的Mw效应。最后，详细介绍了新开发的聚烯烃的分子动力学和力学性能。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。

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.