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

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 (MS ~ 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.