Correlation between Flow-Induced Nucleation Morphologies and Strain in Polyethylene: From Uncorrelated Oriented Point-Nuclei, Scaffold-Network, and Microshish to Shish

Investigation of the synergistic effect of melt-extension and nanofiller on the crystal-crystal phase transition from form II to I of isotactic polybutene-1

New questions and conjectures are raised on the crystal-crystal phase transition of isotactic polybutene-1 (iPB-1) containing nanofiller in the flow field. In this work, we investigate the phase transition from flow-induced oriented form II to I in iPB-1 blends with multi-walled carbon nanotubes (MWCNTs) with a homemade two-drum extensional rheometer combined with in situ wide-angle X-ray diffraction (WAXD) measurements. The MWCNTs show a limited promoting effect on the phase transition kinetics under quiescent conditions, while the phase transition kinetic is highly accelerated with the impose of melt-extension. When the loading extension strain is 0.5 or 2.0, the half time of phase transition (t_1/2) is shortened from tens of hours to a few hours, depending on the melt-extension strain and the MWCNTs content in iPB-1. When the extension strain increases to 3.5, t_1/2 decreases to about 30 min, which is independent of the MWCNTs content in all iPB-1 blends except in blends with MWCNTs content of 1%, where the phase transition rate in the middle and late stages is restrained. It's speculated that flow-induced molecular orientation or shish-kebab morphology affects the internal stress or stress transfer. The addition of a nanofiller enlarged the effect of melt-extension through strengthening the localized intensity of flow field. In general, the combination of nanofiller and melt-extension can obviously promote the phase transition kinetics.

Polymer crystallization under external flow

The general aspects of polymer crystallization under external flow, i.e., flow-induced crystallization (FIC) from fundamental theoretical background to multi-scale characterization and modeling results are presented. FIC is crucial for modern polymer processing, such as blowing, casting, and injection modeling, as two-third of daily-used polymers is crystalline, and nearly all of them need to be processed before final applications. For academics, the FIC is intrinsically far from equilibrium, where the polymer crystallization behavior is different from that in quiescent conditions. The continuous investigation of crystallization contributes to a better understanding on the general non-equilibrium ordering in condensed physics. In the current review, the general theories related to polymer nucleation under flow (FIN) were summarized first as a preliminary knowledge. Various theories and models, i.e., coil-stretch transition and entropy reduction model, are briefly presented together with the modified versions. Subsequently, the multi-step ordering process of FIC is discussed in detail, including chain extension, conformational ordering, density fluctuation, and final perfection of the polymer crystalline. These achievements for a thorough understanding of the fundamental basis of FIC benefit from the development of various hyphenated rheometer, i.e., rheo-optical spectroscopy, rheo-IR, and rheo-x-ray scattering. The selected experimental results are introduced to present efforts on elucidating the multi-step and hierarchical structure transition during FIC. Then, the multi-scale modeling methods are summarized, including micro/meso scale simulation and macroscopic continuum modeling. At last, we briefly describe our personal opinions related to the future directions of this field, aiming to ultimately establish the unified theory of FIC and promote building of the more applicable models in the polymer processing.

Revealing the detailed structure in flow-induced crystallization of semicrystalline polymers

We systematically investigate the detailed structure in the flow-induced crystallization of a lightly cross-linked high-density polyethylene as a model semicrystalline polymer sample by combining the small-angle X-ray scattering (SAXS) and the spherical harmonic expansion (SHE) method. The SHE divides the two-dimensional SAXS pattern into several components according to the deformation geometry, and allows extraction of the most relevant information. Employing the first two anisotropic components in the expansion and a comprehensive model, we determine the crystalline morphological parameters, such as the long period, the lamellar diameter and thickness, and their polydispersities. In particular, we find that the lamellar diameter exhibits bimodal distributions at high strains. Lamellae with similar diameters tend to gather rather than to randomly distribute with others, suggesting the existence of heterogeneity in the semicrystalline structure. Moreover, we observe the strong polydispersities of the lamellar structure at low strains. The structural heterogeneity and polydispersities could be related to the inhomogeneities in crystal growth and nucleation processes.

Molecular dynamics simulation of shish-kebab crystallization of polyethylene: Unraveling the effects of molecular weight distribution

By means of molecular dynamics simulations, extensional flow was performed on five polyethylene models with different molecular weight distributions (MWDs) precisely designed in view of Grubbs, metallocene, Ziegler-Natta, and chromium-based catalysts, while ignoring the sequence distributions of short branches to shed light on the molecular mechanism of MWD on shish-kebab formation. The formation of shish-kebab crystallites can be divided into three stages: the emergence of precursors, evolution from precursors to shish nuclei, and the formation of lamellar crystallites. The results demonstrated that the precursors initiated from trans-rich segments with local order and minor crystallinity grew into large shish nuclei and eventually evolved into lamellae. There were more inconsecutively trans-state bonds occurring in long chains rather than in short chains, which promoted an easier transformation from precursors to shish nuclei. Therefore, broader MWDs make positive contributions to the formation of shish nuclei, increase the crystallization speed, and the generation of a more regular, compact, and thicker lamella with less tie molecule fractions, while the final crystallinity is independent of MWD.

A portable extruder for in situ wide angle x-ray scattering study on multi-dimensional flow field induced crystallization of polymer

We have designed and constructed a portable extruder with a rotatable mandrel, which can be employed to study the multi-dimensional flow field (MDFF) induced crystallization of polymer combined with in situ wide angle x-ray scattering (WAXS). With the piston driving the melt sample to flow along the channel, a direct axial shear field is achieved. At the same time, the central mandrel keeps rotating under a stable speed, providing the sample with an additional circumferential shear field. By presetting different proportions of the two shear fields, namely, axial and circumferential, various flow states of the sample can be obtained, which makes it capable of investigating the effects of MDFF on polymer crystallization. We have performed an in situ WAXS experiment of MDFF induced crystallization of isotactic polypropylene based on the portable extruder at the beam line BL16B in Shanghai Synchrotron Radiation Facility. The rheological and structural information is collected simultaneously, which manifests the viability of the portable extruder on regulating MDFF and can provide guidance for polymer processing.

Structural and morphological transitions in extension-induced crystallization of poly(1-butene) melt

Structural and morphological transitions of flow-induced crystallization (FIC) in poly(1-butene) (PB-1) melt have been studied by combining extensional rheology and in situ synchrotron radiation ultrafast wide- and small-angle X-ray scattering (WAXD/SAXS) measurements. Unexpectedly, metastable Form III is crystallized directly from the PB-1 melt by high-speed extension, which has a short lifetime of several tens of milliseconds and manifests the thermodynamic and kinetic competition among Form III, Form II and melt under flow. Relative crystallinity evolution of Form II after extension reveals a crystal melting dominated process within the observation time of 120 s even under high supercooling. This is opposite to the common case of FIC but supports the idea that flow alters the obtained crystal size and its thermodynamic stability. Additionally, a morphological transition from a flow-induced network to shish is observed by SAXS with increasing extension temperature from below to above the melting point of Form II. With above observations, we construct nonequilibrium structural and morphological diagrams of FIC in strain rate-temperature space, which may guide the industrial processing of the PB-1 material.

Polymer-Ion Interaction Weakens the Strain-Rate Dependence of Extension-Induced Crystallization for Poly(ethylene oxide)

The crystallization of poly(ethylene oxide) (PEO)-sodium iodine (NaI) composites is investigated by differential scanning calorimetry (DSC), extensional rheology, and in situ small-angle X-ray scattering (SAXS) with the aim of demonstrating versatile roles played by polymer-ion interactions. In the isothermal quiescent crystallization process, a decrease in the crystal growth rate is observed for PEO-NaI and is attributed to slow chain movement caused by the coordination between cations and polymer. In situ SAXS on extensional flow-induced crystallization (FIC) exhibits enhanced kinetics and orientation for both PEO and PEO-NaI with increasing strain rate. However, an overall weaker strain-rate dependence of FIC is observed for PEO-NaI, which can be interpreted as a synergistic consequence of promoted nucleation under flow and impeded crystal growth by polymer-ion interaction. A possible microscopic mechanism is proposed to account for the experimental observation based on the formation of transient cross-linking points in PEO-NaI and their influence on the entanglement network of polymer under various flow fields. The disclosed strain-rate dependence and various ion effects on the behavior of PEO-salt composites contribute to a comprehensive understanding of polymer-ion solid polyelectrolytes.

A highly-deformable piezoresistive film composed of a network of carbon blacks and highly oriented lamellae of high-density polyethylene

The electrical resistance change of highly extensible films consisting of a network of carbon blacks in high-density polyethylene, with different regularity of stacked lamellae, is investigated.