Molecular Aspects of Flow Effects on Crystallization in iPP

半结晶聚合物的微观结构在很大程度上取决于加工历史。在聚合物熔体结晶过程中施加流场可以加快结晶速度，导致取向晶的形成。尽管在过去的三十年中进行了深入的研究，但加工对结构发展影响的基本基础仍然是难以捉摸的。本研究探讨了潜在的分子水平过程：熔融链弛豫动力学的相互作用，流动造成的节段取向扭曲，扭曲熔体成核的动力学和各向异性，以及它们对随后的动力学和结晶形态的影响。在目前DMR的支持下，令人惊讶的发现重塑了当前的思维。定向前驱体形成的速率限制因素是分子在过冷熔体中的运动速率，这是任何现有的流动诱导结晶模型都无法预料的。大分子动力学的作用，特别是弛豫时间的分布，被证明使用相对狭窄的分布材料和它们的二元共混物。少量的长链（已知的摩尔质量和立体规则）加入到短链（大约五分之一长）的基础树脂中，极大地增强了定向前驱体的流动诱导形成。长链效应的浓度依赖性表明，长链/长链重叠起着核心作用。鉴于低浓度长链的深远影响，未来三年的研究重点将放在长链效应对其相对松弛时间和相对于体的立体规则性的依赖上，寻找机制线索。为了阐明流动诱导结构形成的最早阶段（点状前体的形成）和随后形成的线状前体之间的相互作用，将比较均质和非均质成核的模型聚合物。丝状前体在高温下消失的动力学将用于测试我们目前关于其结构的假设。聚合物的多样化和无处不在的应用很大程度上归功于“半结晶聚合物”，它占合成聚合物年产量的三分之二。每年有超过1000亿磅的塑料是由聚乙烯和聚丙烯制成的！通过控制结晶的程度和形式，它们的性质可以在很大的范围内进行调整。例如，纺丝过程中高取向晶体的发展使某些聚乙烯纤维的模量接近钢的模量。然而，由于分子结构和加工历史对材料性能的复杂影响，通过设计实现预期性能所需的基本理解仍然难以捉摸。观察结晶过程中最早事件的新方法揭示了令人惊讶的事实。与现有的模型相反，流动诱导的定向成核在链状分子运动允许的情况下发生得最快——通常的激活屏障消失了。在这个过程中最重要的聚合物链是最长的，至少需要比平均长度长四倍。分子水平理解的这些进步指导了半晶聚合物的设计，以优化其加工特性和最终材料性能，从而有利于汽车，电子，食品和医疗保健行业。

The microstructure of a semicrystalline polymer depends strongly on processing history. Imposing flow fields on a crystallizing polymer melt can accelerate the rate of crystallization and result in the formation of oriented crystallites. In spite of intensive research over the past three decades, the fundamental basis of the effects of processing on structure development remains elusive. This research probes the underlying molecular-level processes: the interplay of the relaxation dynamics of the molten chains, the distortion of segmental orientation created by flow, the kinetics and anisotropy of nucleation from a distorted melt, and their effects on the subsequent kinetics and morphology of crystallization. Under current DMR support, surprising discoveries have reshaped current thinking. The rate limiting factor in the formation of the oriented precursors that template the row-nucleated morphology is the rate of molecular motion in the subcooled melt, which is unanticipated by any existing model of flow-induced crystallization. The role of macromolecular dynamics, particularly the distribution of relaxation times, was demonstrated using relatively narrow distribution materials and their binary blends. Small amounts of long chains (with known molar mass and stereoregularity) added to a base resin of much shorter chains (approximately one-fifth as long), greatly enhanced the flow-induced formation of oriented precursors. The concentration dependence of the effect of long chains indicates that long-chain/long-chain overlap plays a central role. In view of the profound effect of low concentrations of long chains, proposed research for the next three years focuses on the dependence of the long-chain effect on their relative relaxation time and stereoregularity compared to the bulk, searching for mechanistic clues. To clarify the interplay between the earliest stages of flow-induced structure formation (the creation of point-like precursors) and the subsequent formation of threadlike precursors, homogeneous and heterogeneously nucleated model polymers will be compared. The kinetics of disappearance of the threadlike precursors when held at elevated temperature will be used to test our current hypotheses regarding their structure.The diverse and ubiquitous application of polymers is largely due to "semicrystalline polymers," which account for two-thirds of the annual production of synthetic polymers. Over 100 billion pounds a year are made of polyethylene and polypropylene alone! Their properties can be tuned over an enormous range by controlling the extent and form of crystallization. For example, the development of highly oriented crystallites during spinning gives certain polyethylene fibers a modulus approaching that of steel. However, fundamental understanding needed to achieve intended properties by design remains elusive because of the complex effects of molecular structure and processing history on material properties. New methods to observe the earliest events during crystallization are revealing surprising truths. Contrary to existing models, oriented nucleation induced by flow occurs as fast as the motion of the chain-like molecules permits-the usual activation barrier vanishes. And the polymer chains that matter most in this process are the longest ones, needing to be at least four times longer than average. Such advances in molecular-level understanding guide the design of semicrystalline polymers to optimize their processing characteristics and ultimate material properties, with benefits to the automotive, electronics, food and health care industries.