Short-term flow induced crystallization in isotactic polypropylene : how short is short?

The so-called “short-term flow” protocol is widely applied in experimental flow-induced crystallization studies on polymers in order to separate the nucleation and subsequent growth processes [Liedauer et al. Int. Polym. Proc. 1993, 8, 236–244]. The basis of this protocol is the assumption that structure development during flow can be minimized and the rheological behavior, i.e., the viscosity, does not change noticeably. In this work we explore the validity of this assumption for short but strong flows and reveal the structure formation during the early stages of crystallization. Viscosity and structure evolution of an isotactic polypropylene (iPP, Mw ≈ 365 kg/mol and Mw/Mn = 5.4) melt at 145 °C are measured during the short-flow period (0.2–0.25 s) using the combination of a slit rheometer and fast X-ray scattering measurements. For high enough (apparent) shear rates (≥240 s–1) a viscosity rise during flow is observed; i.e., the condition for “short-term flow” is not satisfied. With a time delay with re...