Understanding Polymer Crystallization at Molecular Levels: A Solid-State NMR Study

非技术总结：大约70%的工业塑料是由半结晶聚合物制成的，半结晶聚合物在工业加工时具有结晶能力。由非常长的分子链组成的聚合物分子的固有性质导致固态下高度复杂的结构层次。 控制这些分层结构的能力显著影响所得材料的机械和热性能。因此，半结晶聚合物用于各种各样的应用，包括包装、纺织品、汽车、飞机部件、能源、生物相容性材料等。为了进一步改善半结晶聚合物的性能，有必要了解其在固态下的详细结构以及分子水平上的结构形成机制。该项目的重点是对以各种受控方式结晶的样品中半结晶聚合物的分子水平结构的基本理解，并可能导致对市售聚合物材料的改进性能的预测。PI将使用新的磁共振技术来探测这种聚合物材料固态中聚合物链的分子轨迹。 这些信息将提供有关聚合物分子形态和晶体之间相互连接的前所未有的细节，并将对其机械性能产生影响。 该项目还将包括在重要技术领域对研究生进行高级科学培训。他们将经历广泛的聚合物研究，包括聚合物合成，结晶，先进仪器和分子动力学模拟。研究结果将在科学期刊上发表，并由PI和学生在全国科学会议上发表。技术概述：聚合物结晶诱导聚合物分子的结构变化，从无规卷曲到薄晶体中的折叠链。在过去的几十年里，许多出版物对结晶机理进行了理论和实验研究。然而，没有决定性的结论已达成的结晶机制和链级结构。 PI开发了13 C-13 C双量子（DQ）NMR结合选择性同位素13 C标记的先进技术。 使用这些与自旋动力学模拟能够确定的重入站点的折叠链，相邻的重入分数，连续折叠数的半结晶聚合物在散装和溶液生长的单晶作为结晶温度的函数。 这些实验揭示了在溶液生长和散装材料，三维纳米结构的形成，和结晶动力学的影响链折叠的程度上的聚合物链的邻近再入的程度。 这些方面可以与束和聚集模型和最近的分子动力学模拟结果。计划的实验将进一步研究各种重要参数对结晶区中链轨迹的影响，这些参数包括i）分子量，ii）聚合物浓度，iii）溶剂-聚合物相互作用，iv）受限空间，v）缠结和vi）熔体记忆。这些研究将提供柔性半结晶聚合物的详细结晶机制，同时有助于在技术重要领域对学生进行高级培训。

NON-TECHNICAL SUMMARY:About 70 % of industrial plastics are made of semicrystalline polymers, which have the ability to crystallize when processed industrially. The inherent nature of polymer molecules, consisting of very long molecular chains, leads to a highly complex hierarchy of structures in the solid state. The ability to control these hierarchical structures significantly influences mechanical and thermal properties of the resulting materials. Thus, semicrystalline polymers are used in a wide variety of applications including packaging, textiles, automobile, airplane parts, energy, biocompatible materials, and others. To further improve properties of semicrystalline polymers, it is necessary to understand their detailed structures in the solid state as well as the mechanisms of structural formation at the molecular level. This project focuses on fundamental understanding of molecular-level structures of semicrystalline polymers in samples crystallized in various controlled ways, and may lead to predictions of improved properties for commercially available polymeric materials. The PI will use novel magnetic resonance techniques to probe the molecular trajectory of polymer chains in the solid state of such polymer materials. This information will provide unprecedented detail about the molecular morphology of polymers and the interconnections among crystals, and would have implications on their mechanical properties. This project will also include advanced scientific training of graduate students in technologically important areas. They will experience a wide range of polymer research covering polymer synthesis, crystallization, advanced instrumentation, and molecular dynamics simulations. The research results will be reported in scientific journals and presented by the PI and students at national scientific meetings.TECHNICAL SUMMARY: Polymer crystallization induces structural changes of polymer molecules from random coils to folded chains in thin crystals. The mechanism of crystallization has been theoretically and experimentally studied in many publications over the past several decades. However, no decisive conclusion has been reached regarding the crystallization mechanism and chain-level structure. The PI has developed advanced techniques for 13C-13C double quantum (DQ) NMR combined with selective isotopic 13C labeling. Using these together with spin-dynamics simulation enables determination of the re-entrance sites of folded chains, the adjacent re-entry fraction, the successive folding number of semicrystalline polymers in the bulk and in solution-grown single crystals as a function of crystallization temperature. These experiments shed light on the extent of adjacent re-entry of polymer chains in solution-grown and bulk materials, the formation of three-dimensional nanostructures, and the effect of crystallization kinetics on the extent of chain folding. These aspects can be correlated with bundle and aggregation models and recent molecular dynamics simulation results. The planned experiments will further investigate the effects on chain trajectory in crystalline regions for a variety of important parameters including i) molecular weight, ii) polymer concentration, iii) solvent-polymer interactions, iv) confined spaces, v) entanglements, and vi) melt-memory. Such studies will provide detailed crystallization mechanisms of flexible semicrystalline polymers while contributing to the advanced training of students in technologically important areas.