Shearing molten isotactic polypropylene (iPP) creates metastable flow-induced precursors that accelerate subsequent crystallization; above a critical shear stress they become shish precursors that yield the highly oriented shish-kebab morphology. The 2018 Macromolecules study of Nazari, Colby, Milner and coauthors established this two-transition phenomenology - a critical shear rate set by the Rouse time (the conformational relaxation time of an unentangled chain) of the longest chains and a critical stress - but could not determine the microscopic structure of the precursors, the mechanism by which the imposed specific work multiplies nuclei, or the mechanistic route from precursor-mediated crystallization to the shish regime. This problem asks for the molecular identification of the shear-induced precursors in iPP (single stretched chains, multistem bundles, structures associated with adsorption onto nanoscale particles such as catalyst residues, or a combination), the mechanism connecting them to both observed morphology transitions including shish formation, and the generality of the longest-chain criterion. A 2022 review and 2024-2026 follow-up studies by the same authors show the question remains open: precursor structure parameters are still described as poorly understood, and particle-adsorption stabilization is still posed only as conjecture.

Background

Isotactic polypropylene (iPP) - the stereoregular form of polypropylene in which all methyl groups lie on the same side of the chain backbone - is the workhorse semicrystalline commodity plastic: on cooling from the melt its chains fold into thin crystalline lamellae that organize into spherulites, radially symmetric polycrystalline aggregates visible in polarized optical microscopy (POM). During melt processing (injection molding, film casting, extrusion) the melt is sheared before it solidifies, and shear dramatically accelerates crystallization and reshapes the final morphology; this is called flow-induced crystallization (FIC). Under sufficiently strong flow, instead of spherulites one obtains the shish-kebab morphology: shish are fibrillar, extended-chain crystals formed from chains stretched by the flow, and kebabs are folded-chain lamellae growing periodically along the shish like skewered plates. Because shish-kebab morphology imparts high modulus and strength along the flow direction, understanding which molecular events create it is both a nonequilibrium-physics question and an industrial lever. A series of studies by the Penn State group (Hamad, Colby and Milner in 2015-2016, and Nazari et al. in 2018) established a now-standard phenomenology for commercial iPP under shear. (i) Flow accelerates crystallization only when the shear rate exceeds a critical value equal to the reciprocal of the Rouse time of the longest chains in the molecular-weight distribution, the Rouse time being the time a chain needs to relax its conformation when unconstrained by entanglements; only chains with relaxation time longer than the inverse shear rate remain stretched during flow. (ii) Below a critical shear stress of order 0.1 MPa, such shear creates flow-induced precursors: stable nanoscale-oriented structures that survive after the flow stops and then nucleate many small crystallites, so the material crystallizes earlier on cooling into a fine-grained rather than coarse spherulitic texture. (iii) Above the critical stress, the flow instead creates shish precursors that develop into the oriented shish-kebab morphology; in capillary (pressure-driven) rheometry, samples sheared above the same threshold stress likewise show an abrupt increase in apparent viscosity, corroborating the transition in a second flow geometry. The mechanical specific work (dissipated work per unit volume) controls the number density of nuclei in the precursor regime: the crystallization rate and the fraction of fine-grained ('rice-grain') crystallites increase with applied work and saturate above a saturation specific work, while spherulite size is independent of applied work, but the molecular mechanism by which work multiplies nuclei is not established. Differential scanning calorimetry (DSC) measures crystallization and melting signatures, POM images the morphology, and synchrotron small- and wide-angle X-ray scattering (SAXS/WAXD) resolve nanometer-scale structure. What the 2018 paper explicitly did not settle is what the precursors are. The macroscopic data are consistent with several candidate molecular pictures discussed in the wider FIC literature: individual chains stretched nearly to full length and frozen into point-like nuclei; multistem bundles in which several chains each contribute several crystalline stems to a common nucleus; or stretched chains adsorbed onto nanoscale heterogeneous impurities such as the catalyst-residue particles present in commercial iPP, with adsorption stabilizing the stretched state. Independent evidence from the wider FIC literature (infrared spectroscopy, SAXS equatorial streaks appearing before crystal diffraction, melt-memory experiments, molecular simulation) indicates that flow first produces intramolecular conformational order (in iPP, a tendency toward the 3/1 helical crystalline conformation), intermolecular orientation order, and density fluctuations in a multistep ordering process; yet, as the 2022 review by Li and coauthors states, the structure parameters of the precursors are still poorly understood, and the sequence and coupling of these ordering steps are not established. The unresolved target is the molecular-level identification of the precursors and the mechanism linking them to both observed morphology transitions.

Problem Statement

Determine the microscopic identity and formation mechanism of the shear-induced precursors in isotactic polypropylene, as left open by Nazari et al. (2018). Concretely, establish with molecular- or segment-scale evidence: (1) what the flow-induced precursors formed below the critical shear stress are - single stretched chains, multistem bundles of stretched chain segments, structures stabilized by adsorption of stretched chains onto nanoscale heterogeneous impurities such as catalyst-residue particles, a composite of these, or a structure not among these - and which observable distinguishes the identified structure from the competing candidates; (2) the mechanism by which the applied mechanical specific work multiplies the number density of nuclei in the precursor regime, whose saturation with increasing work is established phenomenologically but not explained, and the mechanistic route by which, as specific work and stress increase, precursor-mediated nucleation of many small crystallites gives way to the shish regime and an oriented shish-kebab morphology; and (3) whether the longest-chain criterion (critical shear rate equal to the reciprocal Rouse time of the longest chains in the distribution) and the critical stress threshold hold generally across iPP molecular-weight distributions. These three components serve one objective - a molecularly resolved, mechanistic account of flow-induced precursor formation and its morphological consequences in iPP.

References

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