Molecular Aspects of Flow Effects on Crystallization in iPP

流动对 iPP 结晶影响的分子方面

• 批准号: 0216491
• 负责人: Julia Kornfield
• 金额: $ 36万
• 依托单位: California Institute of Technology
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2002
• 资助国家: 美国
• 起止时间: 2002-07-01 至 2005-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0216491&HistoricalAwards=false
• 关键词: Molecular; Aspects; Flow; Effects; Crystallization

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.

半结晶聚合物的微观结构强烈地依赖于加工历史。 在结晶聚合物熔体上施加流场可以加速结晶速率并导致取向微晶的形成。 尽管在过去的三十年里进行了深入的研究，加工对结构发展的影响的基本基础仍然难以捉摸。本研究探讨了潜在的分子水平的过程：熔融链的松弛动力学的相互作用，由流动产生的链段取向的扭曲，从扭曲的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。 在当前DMR的支持下，令人惊讶的发现重塑了当前的思维。 模板行成核形态的定向前体的形成中的速率限制因素是过冷熔体中的分子运动速率，这是任何现有的流动诱导结晶模型所未预料到的。 大分子动力学的作用，特别是弛豫时间的分布，证明了使用相对较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构规整性）添加到基础树脂的短得多的链（约五分之一长），大大增强了流动诱导形成的定向前体。 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是长链效应对相对弛豫时间和立体规整性的依赖性，寻找机制线索。 为了阐明流动诱导结构形成的最早阶段（点状前体的产生）和随后线状前体的形成之间的相互作用，将比较均匀和非均匀成核的模型聚合物。当保持在升高的温度时，丝状前体的消失动力学将被用来测试我们目前关于它们的结构的假设。聚合物的多样性和普遍存在的应用主要是由于“半结晶聚合物”，这占合成聚合物年产量的三分之二。 每年仅聚乙烯和聚丙烯就有超过1000亿磅的重量！ 通过控制结晶的程度和形式，可以在很大范围内调节它们的性能。 例如，在纺丝过程中高度取向的微晶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期事件的新方法揭示了令人惊讶的真相。 与现有的模型相反，流动诱导的定向成核与链状分子的运动一样快，通常的活化势垒消失。 在这个过程中，最重要的聚合物链是最长的，需要比平均长度至少长四倍。 分子水平上的这些进展指导了半结晶聚合物的设计，以优化其加工特性和最终材料性能，从而为汽车，电子，食品和医疗保健行业带来好处。