Molecular Aspects of Flow Effects on Crystallization in iPP

[90901403] kornfield尽管在过去的三十年中进行了深入的研究，但加工对半晶聚合物结构发展影响的基本基础仍然是难以捉摸的。加工改变了结晶的速率、形式和各向异性。提出的研究目标是建立熔融链构象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及其对后续动力学和结晶形态的影响。开发了一种独特的装置来创建精确控制的流动和热历史，并实时监测结构的发展。它可以使过冷熔体在类似于工业过程中遇到的应力水平下精确控制剪切间隔。在剪切期间和剪切后的响应使用不同的结构探针库在现场连续监测。偏振法、光和x射线散射将在现场实时使用，以表征松弛动力学和变形历史的相互作用，这些相互作用体现在流动对结晶动力学和形貌的影响中。该装置便于在实验结束时去除样品，以便对最终结构进行彻底表征。光学显微镜和电子显微镜以及x射线散射将被用来确定最终的凝固结构。模型聚合物系统将使用具有控制立体规则和系统地改变链长分布和相应的松弛时间。基于多分散性对结晶对流动历史的依赖性有强烈影响的证据，我们将制备双分散样品，以揭示最慢弛豫物质（长链）在产生流动增强的成核和各向异性生长中的作用。我们的社会在很大程度上依赖于材料发展的进步，这些材料具有理想的性能组合：坚固但轻便，柔韧但不透水，坚硬但坚韧。半晶聚合物是一类被证明在实现这些强大的特性组合方面非常通用的材料，正是因为它们自发地形成纳米结构的复合材料，其中由非晶区域隔开的片状晶体。这项研究将提高我们优化这些聚合物的能力，通过澄清从分子设计到加工行为的联系，从那里到结构发展，最终控制它们的材料性能。无论是在经济上还是在生活质量上，潜在的影响都是巨大的，因为半晶体聚合物的应用非常广泛——全世界每年仅聚烯烃的使用量就超过1250亿磅。

9901403 KornfieldIn spite of intensive research over the past three decades, the fundamental basis of the effects of processing on structure development in semicrystalline polymers remains elusive. Processing alters the rate, form and anisotropy of crystallization. The goal of the proposed research is to establish the interplay of the conformational 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. A unique apparatus has been developed to create a precisely controlled flow and thermal history and to monitor structure development in real time. It can subject a subcooled melt to precisely controlled intervals of shear at stress levels similar to those encountered in industrial processes. The response during and after shear is continuously monitored in situ using an arsenal of different probes of structure. Polarimetry, light and x-ray scattering will be used in real time, in situ to characterize the interplay of relaxation dynamics and deformation history that is manifested in the effect of flow on crystallization kinetics and morphology. The apparatus facilitates removal of the sample at the conclusion of the experiment to enable thorough characterization of the final structure. Optical and electron microscopies and x-ray scattering will be used ex situ to determine the final solidified structure. Model polymer systems will be used that have controlled stereo regularity and systematically varied distribution of chain lengths and corresponding relaxation times. Guided by evidence that polydispersity strongly affects the dependence of crystallization on flow history, bidisperse samples will be prepared that can reveal the role of the slowest relaxing species (the long chains) in producing flow-enhanced nucleation and anisotropic growth.Our society is heavily dependent on advances in the development of materials with desired combinations of properties: strong but light, flexible but impermeable, stiff but tough. Semicrystalline polymers are a class of materials that has proved extremely versatile in achieving these powerful combinations of properties, precisely because they spontaneously form a nanostructured composite with plate-like crystallites separated by noncrystalline regions. This research will improve our ability to optimize these polymers by clarifying the link form molecular design to processing behavior, and from there to structure development, which ultimately controls their material properties. The potential impact is large, both economically and in terms of quality of life, since semicrystalline polymers are very widely used-over 125 billion pounds a year of polyolefins alone are used worldwide.