Fundamental Studies of Flow-Induced Polymer Crystallization

非技术总结：当聚合物结晶时，只有大约一半的材料是晶体，晶体之间的非晶态物质被困住而不能结晶。对熔融聚合物施加气流使聚合物能够迅速成核许多晶体，从而产生具有优异机械性能的更精细的尺度结构。如果晶体足够小，每个长聚合物链可以跨越许多晶体，晶体之间的连接被称为结链。更强的流动拉伸聚合物链，并产生更多的结链，以获得更高的韧性。本研究旨在通过在结晶前应用不同强度的流动，对半结晶聚合物的结构和力学性能的控制提供深入的基础理解。如果成功，从这项研究中产生的基础知识将导致能够为各种应用设计聚合物材料所需的理解。这一新知识将用于工业定制/改善工程热塑性材料的机械性能。该项目还包括对各级学生的广泛教学和培训，以及学术和工业合作。技术概述：短时间的剪切流动可以强烈地加速半结晶聚合物的成核，这极大地改变了最终的形态和力学性能。有两种形态转变。对于弱剪切流动，更快的成核会形成精细的微米级形态，而更强的流动会形成羊肉串形态。超过拉伸最长链所需的临界剪切速率，剪切变薄开始，通过加速成核形成更小的晶体。在临界剪切应力以上，第二次形态转变为羊肉串形态。提出了一项流动诱导结晶（FIC）的基础研究，使用三种具有严格线性链的聚合物类型，以确定FIC的哪些方面适用于所有半结晶聚合物，哪些方面是聚合物特异性的。我们利用了一个关于“熔体记忆”的重要实验发现；剪切的间隔产生的成核前体对热循环非常稳定，只要温度不高于平衡熔化温度。这允许使用广泛的实验方法来研究这些前体对后续结晶和形态发展的影响。这些方法包括差示扫描量热法、闪光扫描芯片量热法、x射线散射、偏振光学显微镜、原子力显微镜和机械性能。只要温度不太高，这种前驱体的稳定性也可能提供一种在聚合物加工过程中大大提高成核动力学的手段；这也将被探讨。由于最长的链首先拉伸，因此将添加更多的长链，以查看这是否能够实现更大的FIC效应。由于纳米颗粒也可以成核晶体，因此将研究各种颗粒负载，以了解颗粒成核和流动增强成核之间的竞争。该奖项反映了NSF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。

NON-TECHNICAL SUMMARY: When polymers crystallize, only about half of the material is crystalline, with amorphous material in between crystals that gets trapped and cannot crystallize. Applying a flow to the molten polymer enables the polymer to nucleate many crystals rapidly, resulting in a finer scale structure with superior mechanical properties. With small enough crystals, each long polymer chain can span many crystals, with connections between crystals referred to as tie chains. Stronger flows stretch polymer chains and create more tie chains for superior toughness. This study aims to provide deep fundamental understanding regarding the control of structure and mechanical properties of semicrystalline polymers by applying flows of various strengths prior to crystallization. If successful, the fundamental knowledge generated from this research will result in the understanding needed to be able to design polymeric materials for a variety of applications. This new knowledge will be of use to industry to tailor/improve the mechanical properties of engineering thermoplastic materials. The project also includes extensive teaching and training of students at all levels and academic and industrial collaborations.TECHNICAL SUMMARY: Brief intervals of shear flow can strongly accelerate nucleation of semicrystalline polymers, and this drastically changes the final morphology and mechanical properties. There are two morphology transitions. For weak shear flows, faster nucleation creates a fine micron-scale morphology, while stronger flows can create a shish-kebab morphology. Above a critical shear rate needed to stretch the longest chains, shear thinning starts and smaller crystals are formed by accelerating nucleation. Above a critical shear stress, a second morphology transition to shish-kebab morphology occurs. A fundamental study of flow-induced crystallization (FIC) is proposed using three polymer types that each have strictly linear chains, to decide which aspects of FIC are universal to all semicrystalline polymers and which are polymer-specific. We exploit a vital experimental finding regarding “melt memory”; the interval of shear creates nucleation precursors that are very stable to thermal cycling as long as temperature is never raised above the equilibrium melting temperature. That allows using a wide array of experimental methods to study the effects of those precursors on subsequent crystallization and morphology development. These methods include differential scanning calorimetry, flash scanning chip calorimetry, X-ray scattering, polarized optical microscopy, atomic force microscopy, and mechanical properties. This precursor stability also potentially provides a means to greatly enhance nucleation kinetics during polymer processing, as long as the temperature is not too high; this will also be explored. Since the longest chains stretch first, more long chains will be added to see whether this enables a larger FIC effect. Since nanoparticles can also nucleate crystals, various particle loadings will be studied to understand the competition between particle nucleation and flow-enhanced nucleation..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.