Polyolefin Miscibility: New Insights from an Experimental Molecular Perspective

聚烯烃是一类在经济和技术上都很重要的材料，作为所有聚合物的一个子集，它们通常是众所周知的，但茂金属聚合催化剂的不断改进表明，通过共聚、共混和复合材料可以提高物理性能。因此，聚烯烃及其共混物的应用范围很广，从低成本的一次性食品包装到用于航空航天结构设计的多组分复合材料。在所有涉及不仅仅是纯聚合物的情况下，这是大多数，共混物或复合材料的形态和局部结构决定了材料的最终物理和性能特性。令人惊讶的是，聚烯烃共混物的第一原理混合规则是未知的，因为这些化学上简单的聚合物违背了溶解度相似的惯例。 智力优势。在分子或链水平上的实验数据的明显缺乏表明在本体聚烯烃混合物中链-结构/可混性的关系还远未被认识。微观到介观（即埃到几十纳米）的长度尺度可通过固体NMR实验来解决这一缺陷。通过获取特定于链的数据（动力学和距离）来解决长期存在的关于链堆积和链结构的相对重要性的问题，可以检查每个在确定熵与熵对整体相行为的贡献中的作用。 更广泛的影响。围绕聚烯烃相行为的基本科学问题，特别是对于熔融或固态的无定形聚烯烃，是深远的，因为这些非极性、非结晶聚合物的混合物构成了大分子热力学的限制类。因此，聚烯烃及其共混物是对聚合物科学中一些长期存在的问题进行实验研究的理想体系。例如，本体无定形聚烯烃是具有许多自由度的浓缩体系。鉴于不同的链结构可能与聚烯烃，可以明确的自由度内和分子之间（合理引入动态异质性波动）被identified和相关的相行为/相变？非晶态大分子相变的长度尺度和时间尺度，如液晶中的相变，已在构型熵参数的范围内描述。我们能否利用本体聚烯烃及其共混物，在局部链水平（1-10 nm）提供非侵入性实验证实这一观点？如果非相互作用分子中玻璃的形成是由构型熵的损失驱动的，那么分子水平（小于聚合物的回转半径）的实验能否检测和定义局部的Tg？这些数据是否可以用来帮助拓宽目前聚合物相行为的熵模型的范围，使其包括构型熵的贡献？ PI已经并将继续通过该项目积极参与各级教育。PI的努力通过NCSU化学系的公共宣传主任得到了充分利用，在小学、初中和高中的场地上展示了物理科学和聚合物科学的多个演示。此外，该项目的多学科方面（光谱学，聚合物化学，聚合物物理学，材料科学）继续吸引该部门的顶尖本科生和研究生。

Polyolefins comprise an economically and technologically important class of materials.As a subset of all polymers, they are in general well-known, but continuing improvements inmetallocene polymerization catalysis have demonstrated that enhanced physical properties areaccessible through copolymerization, blending, and composites. Consequently, polyolefins andtheir blends span the gamut of possible applications, ranging from low-cost disposable foodpackaging to multicomponent composites for aerospace structural design. In all cases involving more than just a pure polymer, which is the majority, the morphology and local structure of the blend or composite determines the final physical and performance properties of the material.Surprisingly, first-principle mixing rules for polyolefin blends are not known, as thesechemically simple polymers defy like dissolves like solubility conventions. Intellectual Merit. A conspicuous lack of experimental data at the molecular or chain levelsuggests that chain-structure/miscibility relationships in bulk polyolefin mixtures are far frombeing realized. The microscopic to mesoscopic (i.e. angstroms to tens of nanometers) lengthscales accessible by solids NMR experiments can address this deficiency in a non-invasivemanner. Through acquisition of the chain-specific data (dynamics and distance) needed toresolve long-standing questions about the relative importance of chain packing and chainarchitecture, the role of each in determining entropic versus enthalpic contributions to the overallphase behavior may be examined. Broader Impact. The fundamental scientific questions surrounding polyolefin phase behavior,particularly for amorphous polyolefins in the melt or solid states, are far reaching in thatmixtures of these nonpolar, noncrystalline polymers constitute a limiting class ofmacromolecular thermodynamics. As such, polyolefins and their blends are ideal systems for theexperimental study of some long-standing questions in polymer science. For example, bulkamorphous polyolefins are concentrated systems, with many degrees of freedom. Given thevarying chain architectures possible with polyolefins, can distinct degrees of freedom within andbetween molecules (rational introduction of dynamic heterogeneity fluctuations) be identifiedand related to phase behavior/phase transitions? The length-scale and time-scale of phasetransitions in amorphous macromolecules, like those in liquid crystals, have been described inthe context of configurational entropy arguments. Can we, using bulk polyolefins and theirblends, provide non-invasive experimental confirmation of this view at the local chain level (1-10 nm)? If glass formation in non-interacting molecules is driven by loss of configurationalentropy, can molecular level (less than the polymer radius of gyration) experiments detect anddefine local Tg's? Can this data be used to help broaden the scope of current thermodynamicmodels of polymer phase behavior to include configurational entropy contributions? The PI has been, and will continue to be, an active participant at all levels of educationthrough this project. The PI, whose efforts are leveraged through the public outreach director inthe NCSU chemistry department, has presented multiple demonstrations in physical science, andpolymer science, at elementary, middle, and high school venues. Moreover, themultidisciplinary aspect of this project (spectroscopy, polymer chemistry, polymer physics,materials science) continues to attract top undergraduate and graduate students in the department.