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.

非技术摘要：当聚合物结晶时，只有大约一半的材料是结晶的，晶体之间的无定形材料被捕获而无法结晶。 对熔融聚合物施加流动使聚合物能够快速成核许多晶体，从而产生具有优异机械性能的更精细的尺度结构。 对于足够小的晶体，每个长聚合物链可以跨越许多晶体，晶体之间的连接称为连接链。 更强的流动性可以拉伸聚合物链并产生更多的连接链，从而实现卓越的韧性。 本研究旨在通过在结晶前应用各种强度的流动来提供关于半结晶聚合物的结构和机械性能的控制的深刻的基础理解。 如果成功，这项研究产生的基础知识将有助于理解设计各种应用的聚合物材料。 这些新知识将用于工业界定制/提高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。技术摘要：短暂的剪切流间隔可以强烈加速半结晶聚合物的成核，这极大地改变了最终的形态和机械性能。 有两种形态转变。 对于弱剪切流，更快的成核会产生精细的微米级形态，而更强的剪切流可以产生烤肉串形态。 当超过拉伸最长链所需的临界剪切速率时，剪切稀化开始，并且通过加速成核形成更小的晶体。 在临界剪切应力之上，发生向烤肉串形态的第二形态转变。 提出了使用三种聚合物类型（每种都具有严格线性链）对流动诱导结晶（FIC）进行基础研究，以确定 FIC 的哪些方面对于所有半结晶聚合物是通用的，哪些方面是聚合物特定的。 我们利用了一项关于“融化记忆”的重要实验发现；剪切间隔产生的成核前体对热循环非常稳定，只要温度永远不会升高到平衡熔化温度以上。 这使得可以使用多种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、偏光光学显微镜、原子力显微镜和机械性能。 只要温度不太高，这种前体稳定性还可能提供一种大大增强聚合物加工过程中成核动力学的方法。对此也将进行探讨。由于最长的链首先拉伸，因此将添加更多的长链来看看这是否可以实现更大的 FIC 效果。 由于纳米粒子也可以使晶体成核，因此将研究各种粒子负载，以了解粒子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智力价值和更广泛的影响审查标准进行评估，被认为值得支持。