Molecular Aspects of Flow Effects on Crystallization in iPP

9901403 Kornfield尽管在过去的三十年里进行了大量的研究，但加工对半结晶聚合物结构发展的影响的基本基础仍然难以捉摸。加工改变了结晶的速度、形式和各向异性。这项研究的目的是建立熔体链条的构象松弛动力学、流动引起的链段取向扭曲、扭曲熔体成核的动力学和各向异性以及它们对后续结晶动力学和形态的影响之间的相互作用。已经开发出一种独特的设备，可以创建精确控制的流动和热历史，并实时监控结构的发展。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受到精确控制的剪切间隔。在剪切过程中和剪切后的响应连续监测使用不同的结构探头的武器库。旋光、光和X射线散射将被用来实时、原位地表征松弛动力学和变形历史的相互作用，这表现在流动对结晶动力学和形貌的影响。该装置便于在实验结束时移除样品，以实现对最终结构的彻底表征。光学和电子显微镜以及x射线散射将用于非原位确定最终的凝固结构。将使用具有控制立体规律性和系统地改变链长和相应弛豫时间分布的模型聚合物系统。在多分散性强烈影响结晶对流动历史的依赖性的证据的指导下，将制备双分散样品，以揭示最慢的松弛物种(长链)在产生流动增强成核和各向异性生长中的作用。我们的社会严重依赖于具有所需性能组合的材料的发展：坚固而轻、灵活但不透、硬但韧性。半晶聚合物是一类被证明在实现这些强大的性能组合方面非常通用的材料，正是因为它们自发地形成了由非晶区分隔的板状微晶的纳米结构复合材料。这项研究将通过澄清从分子设计到加工行为，再到最终控制其材料性能的结构开发之间的联系，来提高我们优化这些聚合物的能力。潜在的影响是巨大的，无论是在经济上还是在生活质量方面，因为半结晶聚合物被非常广泛地使用-仅聚烯烃每年就在全球使用超过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.