Full Characterization of Multiphase, Multimorphological Kinetics in Flow-Induced Crystallization of IPP at Elevated Pressure

Abstract

Understanding the complex crystallization behavior of isotactic polypropylene (iPP) in conditions comparable to those found in polymer processing, where the polymer melt experiences a combination of high shear rates and elevated pressures, is key for modeling and therefore predicting the final structure and properties of iPP products. Coupling a unique experimental setup, capable to apply wall shear rates similar to those experienced during processing and carefully control the pressure before and after flow is imposed, with in situ X-ray scattering and diffraction techniques (SAXS and WAXD) at fast acquisition rates (up to 30 Hz), a well-defined series of short-term flow experiments are carried out using 16 different combinations of wall shear rates (ranging from 110 to 440 s–1) and pressures (100–400 bar). A complete overview on the kinetics of structure development during and after flow is presented. Information about shish formation and growth of α-phase parents lamellae from the shish backbones is extracted from SAXS; the overall apparent crystallinity evolution, amounts of different phases (α, β, and γ), and morphologies developing in the shear layer (parent and daughter lamellae both in α and γ phase) are fully quantified from the analysis of WAXD data. Both flow rate and pressure were found to have a significant influence on the nucleation and the growth process of oriented and isotropic structures. Flow affects shish formation and the growth of α-parents; pressure acts on relaxation times, enhancing the effect of flow, and (mainly) on the growth rate of γ-phase. The remarkably high amount of γ-lamellae found in the oriented layer strongly indicates the nucleation of γ directly from the shish backbone. All the observations were conceptually in agreement with the flow-induced crystallization model framework developed in our group and represent a unique and valuable data set that will be used to further validate and implement our numerical modeling, filling the gap for quantitatively modeling crystallization during complicated processing operations like injection molding.

High‐pressure induced formation of isotactic polypropylene mesophase: Synergistic effect of pressure and pressurization rate

“…The quiescent crystallization favors formation of α‐phase, the formation of mesophase has to suppress that of α‐phase, which needs a very fast cooling rate of 100 °C/s. Differently, for the crystallization under pressure, γ‐phase is more favorable than α‐phase, then the mesophase formation is competing with that of γ‐phase . In our work, when the pressurization rate is less than 2.5 MPa/s, the maximum crystallinity of γ‐phase was about 40%, which, however, decreased to around 20% with increasing the pressurization rate to 5 MPa/s.…”

“…The extended‐chain morphologies of polyethylene, poly(ethylene terephthalate) (PET), and polyamide 6 appear in crystallization under elevated pressure . Furthermore, pressure is even able to vary the relative stability between various crystal modifications and therefore induces appearance of new phase .…”

Quantification of pressure‐induced γ‐crystals in isotactic polypropylene: The influence of shear and carbon nanotubes

“…Isotactic polypropylene (iPP) is one of the most important thermoplastic polymers owing to its well‐balanced physical and mechanical properties . Moreover, iPP is a polymorphic material which can crystallize in 3 known crystalline forms: monoclinic α, hexagonal β, and orthorhombic γ . Their chain conformations are all the classical 3 1 helix.…”

Effect of high‐temperature annealing on the microstructure and mechanical properties of polypropylene with shish kebab or spherulite structure

“…Polypropylene (PP) can crystallize into two distinct crystal morphologies, namely, spherulite and oriented shish kebab superstructures. Spherulites are formed under quiescent crystallization conditions; shish kebabs can only be observed during elongational or shear‐flow‐induced crystallization, wherein the stretched chains in the melt act as the “shish” to induce the epitaxial growth of the “kebabs.” A shish kebab is a self‐reinforced structure that can simultaneously improve impact strength, tensile strength, modulus, stiffness, and thermal stability …”

“…[1][2][3][4] A shish kebab is a self-reinforced structure that can simultaneously improve impact strength, tensile strength, modulus, stiffness, and thermal stability. [5][6][7][8] Annealing below the melting temperature (T m ) is a common method to improve the microstructure and subsequently the mechanical properties. In the past few decades, numerous works were performed to study the effects of annealing on the development of microstructures and iPP properties.…”