Molecular Aspects of Flow Effects on Crystallization in iPP

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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.