Fundamental Studies of Flow-Induced Polymer Crystallization

流动诱导聚合物结晶的基础研究

• 批准号: 2218775
• 负责人: Ralph Colby
• 金额: $ 66.77万
• 依托单位: Pennsylvania State Univ University Park
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2022
• 资助国家: 美国
• 起止时间: 2022-08-01 至 2026-07-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2218775&HistoricalAwards=false
• 关键词: Fundamental; Studies; Flow; Induced; Polymer

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.

非技术性总结：当聚合物结晶时，只有大约一半的材料是结晶的，在晶体之间的非晶材料被捕获并且不能结晶。 向熔融聚合物施加流动使得聚合物能够快速成核许多晶体，从而产生具有上级机械性能的更精细尺度结构。 在足够小的晶体中，每个长的聚合物链可以跨越许多晶体，晶体之间的连接被称为系链。 更强的流动拉伸聚合物链，并创造更多的上级韧性的连接链。 本研究的目的是通过在结晶前施加不同强度的流动来控制半结晶聚合物的结构和力学性能，从而提供深入的基本理解。 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。 这些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。技术总结：短暂的剪切流动可以强烈加速半结晶聚合物的成核，这会极大地改变最终的形态和机械性能。 有两种形态转变。 对于弱剪切流，更快的成核产生精细的微米级形态，而更强的流可以产生串肉串形态。 在拉伸最长链所需的临界剪切速率以上，剪切稀化开始，并且通过加速成核形成较小的晶体。 在临界剪切应力以上，发生第二形态向串晶形态的转变。 流动诱导结晶（FIC）的基础研究提出了使用三种聚合物类型，每种都有严格的线性链，以确定FIC的哪些方面是通用的所有半结晶聚合物，哪些是聚合物的具体。 我们利用一个重要的实验发现，关于“熔体记忆”的剪切的间隔创建成核前体，是非常稳定的热循环，只要温度是从来没有超过平衡熔化温度上升。 这允许使用广泛的实验方法来研究这些前体对随后的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，偏光显微镜，原子力显微镜，和机械性能。 这种前体稳定性还潜在地提供了一种在聚合物加工期间大大增强成核动力学的手段，只要温度不太高;这也将被探索。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使晶体成核，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，并被认为是值得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的支持。