Self-Regulation in Flow-Induced Structure Formation of Polypropylene

• Published in: Macromolecular rapid communications. Vol. 36; no. 4; pp. 385 - 390
• Main Authors: Roozemond, Peter C., van Drongelen, Martin, Ma, Zhe, Spoelstra, Anne B., Hermida-Merino, Daniel, Peters, Gerrit W. M.
• Format: Journal Article
• Language: English
• Published: Germany Blackwell Publishing Ltd 01.02.2015; Wiley Subscription Services, Inc
• Subjects: Crystal structure; Crystallization; flow-induced crystallization; Formations; Injection molding; Isotactic; Kinetics; polymer processing; polypropylene; Polypropylenes; Polypropylenes - chemical synthesis; Polypropylenes - chemistry; Pressure; Rheology; Shear layers; Shear Strength; Walls; wide-angle X-ray diffraction; X-Ray Diffraction; polymer processing; polypropylene; wide-angle X-ray diffraction; rheology; flow-induced crystallization
• ISSN: 1022-1336; 1521-3927
• DOI: 10.1002/marc.201400505

Flow‐induced structure formation is investigated with in situ wide‐angle X‐ray diffraction with high acquisition rate (30 Hz) using isotactic polypropylene in a piston‐driven slit flow with high wall shear rates (up to ≈900 s−1). We focus on crystallization within the shear layers that form in the high shear rate regions near the walls. Remarkably, the kinetics of the crystallization process show no dependence on either flow rate or flow time; the crystallization progresses identically regardless. Stronger or longer flows only increase the thickness of the layers. A conceptual model is proposed to explain the phenomenon. Above a certain threshold, the number of shish‐kebabs formed affects the rheology such that further structure formation is halted. The critical amount is reached already within 0.1 s under the current flow conditions. The change in rheology is hypothesized to be a consequence of the “hairy” nature of shish. Our results have large implications for process modelling, since they suggest that for injection molding type flows, crystallization kinetics can be considered independent of deformation history. Subjecting a polymer melt below its melting point to high shear stress can create strongly oriented crystal structures. It is shown that, in channel flow, this crystalline structure does not depend on spatial coordinate or flow condition due to a self‐regulating effect, as evidenced by the crystallization kinetics.