Effect of Long Chains on the Threshold Stresses for Flow-Induced Crystallization in iPP: Shish Kebabs vs Sausages

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• 作者：Lucia Fernandez-Ballester ; Derek W. Thurman ; Weijun Zhou ; Julia A. Kornfield
• 刊名：Macromolecules
• 出版年：2012
• 出版时间：August 28, 2012
• 年：2012
• 卷：45
• 期：16
• 页码：6557-6570
• 全文大小：765K
• 年卷期：v.45,no.16(August 28, 2012)
• ISSN：1520-5835

文摘

The addition of small concentrations (2 wt % or less) of ultrahigh molecular weight isotactic polypropylene (M_L 3500 kg/mol) to a matrix of lower molecular weight chains (M_S 186 kg/mol, e.g. M_L/M_S 20) substantially decreases the critical stress for inducing a highly oriented skin under flow-induced crystallization conditions鈥攕ignificantly more than for blends of M_L/M_S 5 (Seki et al.)鈥攁nd promotes the formation of point precursors and oriented 鈥渟ausage-like鈥?structures not observed for M_L/M_S 5. These differences correlate with the onset of long chain stretching during shear: the ratio of long chains鈥?Rouse time to short chains鈥?disengagement time indicates that 3500 kg/mol chains can easily stretch if tethered onto a point nuclei and even when untethered. Adding 3500 kg/mol chains has strong effects that saturate beyond the overlap concentration, suggesting that an uninterrupted supply of long chains greatly accelerates formation of threads. A conceptual model is proposed that distinguishes between a critical stress for shish initiation and that for propagation.