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

• Published in: Macromolecules Vol. 45; no. 16; pp. 6557 - 6570
• Main Authors: Fernandez-Ballester, Lucia, Thurman, Derek W, Zhou, Weijun, Kornfield, Julia A
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 28.08.2012
• Subjects: Applied sciences; Crystallization; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Properties and characterization; Binary mixture; Shish kebab polymer; Melt crystallization; Shear flow; Morphology; Isotactic polymer; Propylene polymer; Kinetics; Experimental study; Modeling; Mechanism
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma3000384

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significantly more than for blends of M L/M S ∼ 5 (Seki et al.)and promotes the formation of point precursors and oriented “sausage-like” 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’ Rouse time to short chains’ 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.