Combining extensional rheology with in-situ synchrotron ultrafast x-ray scattering, we studied flow-induced phase behaviors of polyethylene (PE) in a wide temperature range up to 240 °C. Non-equilibrium phase diagrams of crystallization and melting under flow conditions are constructed in stress-temperature space, composing of melt, non-crystalline δ, hexagonal and orthorhombic phases. The non-crystalline δ phase is demonstrated to be either a metastable transient pre-order for crystallization or a thermodynamically stable phase. Based on the non-equilibrium phase diagrams, nearly all observations in flow-induced crystallization (FIC) of PE can be well understood. The interplay of thermodynamic stabilities and kinetic competitions of the four phases creates rich kinetic pathways for FIC and diverse final structures. The non-equilibrium flow phase diagrams provide a detailed roadmap for precisely processing of PE with designed structures and properties.
The sequence and coupling of intra- and interchain orderings in flow-induced crystallization (FIC) of partially cross-linked isotactic polypropylene (iPP) is studied with in situ Fourier transform infrared spectroscopy (FTIR) and synchrotron radiation X-ray scattering techniques, which reveal that multiscale structural intermediates emerge prior to the onset of crystallization. Upon imposing flow, intrachain conformational ordering or coil–helix transition (CHT) occurs first, which is directly correlated with external stress. As helical content is built up at large strain, density fluctuation happens, and sufficient long helices may result in orientation ordering before FIC. The results demonstrate that stress induced intrachain CHT is the essential structural intermediate in FIC, which can be further coupled with interchain orientation and density providing either helical content or length meets the criterions for the phase transitions. We propose that coupling among external stress, intrachain conformational, and interchain orientation and density orderings to be the molecular mechanism for FIC of polymer forming helical structures.
Thermoplastic elastomers (TPEs) combine high elasticity with melt processability due to their structural features being based on physical associations rather than chemical crosslinking. Their mechanical properties are governed by the interplay of the different dynamics present in the system (i.e., hard block associations and soft block mobility) combined with their morphology. Irrespective of their exact chemical structure or type of association (crystals, hydrogen bonds, or glassy domains), many soft TPEs show a reduction in toughness at elevated temperatures. In this study, we investigate the high‐temperature mechanical properties of a model series of industrially relevant TPEs via systematically varying composition and molecular weight. The results show an increase in temperature resistance and in large‐strain stress response as chain length increases. We underline the key parameters that influence the mechanical behavior and explain the observed effect of molecular weight on both the temperature‐ and rate‐dependent large‐strain response. A physical network‐based model is presented that can explain the experimental findings assuming an improved network connectivity and extended lifetime of the entangled segments with increasing molecular weight.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.