报告时间：2014年6月17日（星期二）14:00—15:00

 

报告地点：龙赛理科楼南楼202报告厅

 

报告人：Prof. Julie Kornfield

 

报告人简介：

Julie Kornfield, Ph.D. Professor of Chemical Engineering at Caltech, an expert in polymer physics. She has won numerous awards, including the Dillon Medal of the American Physical Society, and is a Fellow of the American Association for the Advancement of Science.

 

报告摘要：

 

The dramatic effects of flow on crystallization kinetics and morphology of semicrystalline polymers enable the ongoing increase in living standards.  Processing flow can accelerate crystallization by orders of magnitude compared to the quiescent case, allowing low-cost, high-volume manufacturing. Flow can also induce highly oriented crystal morphologies that allow simple polymers to attain exceptional properties.  We seek molecular insight into the formation and growth of the oriented crystalline structures using in situ techniques (rheo-optical and rheo-WAXD) in combination with ex situ microscopy. To expose the role of very long chains in the formation of oriented precursors in isotactic polypropylene (iPP), we examined model binary blends of well-defined, long chains at very low concentrations in a matrix of much shorter---but still highly enthangled---chains. Specifically, long chain concentration (c) is 2% or less, and two long chains lengths (1M or 3.5M) are examined that are 5- and 18-times longer than the matrix chains (1.86x105 g/mol). As c increases (at fixed shearing conditions), oriented precursors form sooner, the degree of orientation increases, and the parent:daughter ratio increases.  Long chains appear to act cooperatively (effects increase nonlinearly with c up to their overlap concentration c*, 1% for 1M and 0.4% for 3.5M) and their effect saturates beyond c*. The threshold stress required to induce highly oriented crystallization decreases as c increases to c*: is reduced 10% for 1M and 50% for 3.5M.  Low concentrations of long chains allow experiments to be performed at nearly identical conditions (shear stress, shearing time, total strain and total work done one the system) to isolate the effects of long chain concentration c and length. The results do not support models that relate oriented precursors to the specific work imposed on the melt.  Instead, the results motivate models that account for the dynamics of the longest chains and their interactions with growing precursors and with neighboring chains.

 

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