Improving Mechanical Performance of Glassy and Semicrystalline Polymers: Molecular Perspectives

非技术综述：生物可再生聚合物，如聚乳酸，有望取代以化石为基础的热塑性塑料，如聚对苯二甲酸乙二醇酯(PET)。尽管具有类似的物理性能，如相对较高的玻璃化转变温度Tg(60至70摄氏度)和缓慢的结晶动力学，但聚乳酸并未取代PET。与聚酯不同的是，结晶聚乳酸在室温下很脆，因此很难使用。非晶态的聚乳酸也很脆，因为聚乳酸可能会发生快速的物理老化。为了使替代物具有竞争力，这对国家的繁荣和福祉具有相当大的经济效益，必须取得几项科学进步。首先，必须从分子的角度更好地理解半结晶聚合物的力学性质。其次，必须在加工条件和由此产生的半结晶聚合物的机械性能之间建立更明确的关系。这项拟议的研究将调查为什么结晶不能提高玻璃化温度高于室温的聚合物的延展性。特别是，将对以下猜想进行评估：(A)这种半结晶聚合物的结晶区比非结晶区的力学强度更弱，以及(B)快速结晶是捕获链纠缠和保存(而不是耗尽)链网络所必需的。此外，还将努力建立加工、结构和性能关系，使解放军具有极强的韧性和耐热性。具体地说，将确定在保持链不交叉性的同时实现结晶的科学原理。技术摘要：过去对半结晶聚合物和流动诱导结晶的力学研究主要集中在玻璃化转变温度低于室温且结晶动力学相对较快的聚合物，例如等规聚丙烯和聚乙烯。因此，现有的知识不能用于B类半结晶聚合物(玻璃化转变温度高于室温)，如聚乳酸，因为聚乳酸在室温下的力学涉及分隔晶区的玻璃非晶区的变形。例如，著名的彼得林和Flory-Yoon各自的模型并不直接适用。玻璃态聚合物变形产生的高得多的应力提出了一个问题，即晶态区域在结构上是否与非晶态区域一样强。由于链在结晶区不在拓扑上交叉，结晶区的内聚力不能利用强共价键，因此是有限的。计划中的研究将应用聚合物玻璃熔体流变学和分子力学方面的新知识，结合成熟的表征工具，如WAXS、SAXS和DSC，以建立一个有洞察力的观点，认识到需要明确地描述和控制非晶相的状态，根据连接链相对于悬挂链和自由链的数量。此外，原位偏振光学显微镜将被用来识别存在机械脆弱的位置(球晶内、非晶区或球晶间边界)，为玻璃态半结晶聚合物的分子力学模型提供所需的信息。这项拟议的工作将探索弹性预熔变形对聚合物结晶和由此产生的聚乳酸力学性能的独特影响。在这里，每个拉伸的缠结链都有望形成一个纳米晶体，它没有链折叠，并且通过非晶态的拉伸链网连接在一起，在紧密堆积中起到刚性纳米填充物的作用。具体地说，预熔拉伸的聚乳酸的冷结晶可以产生结晶但光学透明的样品，这些样品不仅具有超高的延展性，而且具有出色的尺寸稳定性(例如，在100摄氏度下零收缩)。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。

NON-TECHNICAL SUMMARY:Bio-renewable polymers such as poly(lactic acid) (PLA) hold the promise to replace fossil-based thermoplastics such as poly(ethylene terephthalate) (PET). PLA has not replaced PET despite similar physical properties such as relatively high glass transition temperature Tg (60 to 70 degree Celsius) and slow crystallization kinetics. Unlike PET, crystalline PLA are brittle at room temperature, making the material hardly usable. Amorphous PLA is also brittle because of rapid physical aging that can take place in PLA. To make the replacement competitive, which is of considerable economic benefit to the national prosperity and welfare, several scientific advances must be made. First, a better understanding of mechanics of semicrystalline polymers must be achieved from a molecular viewpoint. Second, a clearer relationship must be established between processing conditions and resulting mechanical properties of semicrystalline polymers. The proposed research will investigate why crystallization does not enhance ductility for polymers with Tg above room temperature. In particular, research will be conducted to evaluate the conjectures that (a) crystalline regions of such semicrystalline polymers are mechanically weaker than the non-crystalline regions and (b) fast crystallization is necessary is entrap chain entanglement and preserve (instead of deplete) chain networking. Moreover, efforts will be made to establish the processing, structure and property relationships that will make PLA superbly tough and heat resistant. Specifically, scientific principles will be identified to achieve crystallization while preserving chain uncrossability.TECHNICAL SUMMARY:Past studies of mechanics of semicrystalline polymers and "flow-induced" crystallization mainly focused on polymers whose glass transition temperature are below room temperature and crystallization kinetics are relatively fast, e.g., isotactic polypropylene and polyethylene. The existing knowledge is thus not transferrable for class B semicrystalline polymers (with Tg above room temperature) such as PLA because mechanics of PLA at room temperature involves deformation of glassy amorphous regions that separate the crystalline regions. For example, the well-known respective models of Peterlin and Flory-Yoon do not directly apply. The much higher stress due to glassy polymer deformation raises the question of whether crystalline regions are structurally as strong as the amorphous regions. Since chains do not topologically cross in the crystalline regions, the cohesion of crystalline regions cannot take advantage of strong covalent bonds and is thus limited. The planned research will apply new knowledge in melt rheology and molecular mechanics of polymeric glasses, in combination with the mature characterization tools such as WAXS, SAXS and DSC to establish an insightful perspective that recognizes the need to explicitly describe and control the state of the amorphous phase in terms of the population of tie chains relative to dangling and free chains. Moreover, in situ polarized optically microscopy will be employed to identify the locations (either inside spherulites, or within amorphous regions or at inter-spherulitic boundaries) where mechanical weakness resides, providing the needed information for a molecular model of mechanics of glassy semicrystalline polymers. The proposed work will explore the unique effect of elastic pre-melt-deformation on polymer crystallization and resulting mechanical properties of PLA. Here each stretched entanglement strand is expected to nucleate a nano-crystal that is free of chain folding and acts like rigid nano-fillers in close packing, linked by non-crystalline stretched chain networking. Specifically, cold-crystallization of pre-melt-stretched PLA could result in crystalline yet optically clear samples that are not only superbly ductile but also have outstanding dimensional stability (e.g., zero shrinkage at 100 degrees Celsius)..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.