The Nonlinear Behavior Of Isotactic Polypropylene In Flow Field And Its Influence To Crystallization

In polymer material processing, external flow field induces the change of chain conformation and difference of rheological behaviors, which lead to variation of combination path as crystallization. To understand flow-induced crystallization needs the clear point of relationship among external flow field, polymer rheological behavior and crystallization While polymer melt is almost in nonlinear rheological region in processing. Local flow field perhaps largely differs from apparent condition due to instability of flow, which makes relationship between flow field and crystallization more complicated. Shear thinning is always in a homogenous way under external flow while wall slip, shear banding and delayed fracture often occur during both experiments and industrial processing in heterogeneous ways. The correlation between these nonlinear rheological behaviors and crystallization makes us understand in depth and provide much more ways to control external condition to acquire ideal samples/products.Recent observations of shear banding and delayed fracture in entangled polymer liquid have attracted a great deal of attention in polymer community, which challenge our current understanding of polymer dynamics. Though the interpretation on such non-linear behaviors is still a matter of debate, the observations have been confirmed by different groups (including our group). An immediate question is raised up:how do such non-linear rheological behaviors affect flow-induced crystallization (FIC) of polymer? In the study of FIC of polymer, a homogeneous flow field is generally assumed and non-linear rheological behaviors like shear banding and delayed fracture have not been taken into account, which may leads to a mismatch between flow field and crystallization behavior.In this study, we combine particle-tracking velocimetry (PTV) and synchrotron radiation microbeam wide angle X-ray diffraction (SR-uWAXD) to study FIC of isotactic polypropylene. Interestingly, post-shear movement and delayed fractures are observed with PTV after a step strain shear when large shear rates and strains are imposed. With ex-situ SR-μWAXD, we found that the layers moving forward keep high crystal orientations while the layers moving backward show low orientations, which manifests a non-uniform distribution of crystal orientation across the sample thickness. The correlation between post-shear movement and crystal orientation indicates that delayed fracture observed in current work is due to interplay between inertia and elastic retraction, where inertia drives forward while retractive force pulls backward and dissipates the effect of shear. Though current work only establishes a qualitative correlation between real flow and crystallization behavior, to the best of the authors’knowledge, it may be the first effort in this direction, which gives a clear warning that simply correlating crystallization behavior with apparent flow may lead to misinterpretations and hinder us to unveil the real physics of FIC.