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The non-equilibrium phase diagrams of flow-induced crystallization and melting of polyethylene

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ABSTRACT

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.

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In-situ WAXD/SAXS characterization and crystallization phase diagram.(a) Representative true stress (σ)-time curve during extension with strain rate of 3 s−1 at 172 °C. The inserted images display selected 2D WAXD and SAXS patterns collected at the numbered points. Sample fracture happens at time of 930 ms indicated by sudden reduction of stress. The extensional direction is vertical as shown by the arrow. (b) The non-equilibrium crystallization phase diagram of crosslinked PE in stress-temperature space. Four phases of melt (L), non-crystalline shish (δ), H-crystal (H) and O-crystal (O) are included.
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f1: In-situ WAXD/SAXS characterization and crystallization phase diagram.(a) Representative true stress (σ)-time curve during extension with strain rate of 3 s−1 at 172 °C. The inserted images display selected 2D WAXD and SAXS patterns collected at the numbered points. Sample fracture happens at time of 930 ms indicated by sudden reduction of stress. The extensional direction is vertical as shown by the arrow. (b) The non-equilibrium crystallization phase diagram of crosslinked PE in stress-temperature space. Four phases of melt (L), non-crystalline shish (δ), H-crystal (H) and O-crystal (O) are included.

Mentions: Crosslinked PE with a gel fraction of 43.5% was employed as the model sample due to its accessibility of high stress without fracture even at temperature far above the melting point. It was prepared by γ-ray radiation on a commercial PE with a quiescent equilibrium melting point of 141.4 °C17. It should be mentioned that we have done a similar research on the non-crosslinked PE and verified the conclusions in this paper (Supplementary Figs S4–8). The experimental details and data treating can be found in the Methods. Figure 1a presents a representative true stress-time curve of crosslinked PE melt under extension with strain rate of 3 s−1 at 172 °C as well as the selected two-dimensional (2D) wide-angle x-ray diffraction (WAXD) and small-angle x-ray scattering (SAXS) patterns. At the beginning of extension at low stress, only melt deformation is observed and no ordered structure appears (No. 1). At extension time of around 600 ms (No. 2), two symmetric streaks perpendicular to extensional direction show up in SAXS pattern, commonly regarded as scattering from shish or rod-like structure. Whilst absence of crystalline reflection in the simultaneous WAXD pattern indicates the non-crystalline nature of obtained shish. We name this non-crystalline shish as δ phase, which was assumed to be a vague kinetic state before and will be discussed later. Highly oriented crystal appears at around 770 ms (No. 3), evidenced by weak diffraction spots in WAXD pattern. Integrating 2D WAXD pattern into one-dimensional (1D) intensity curve, crystal forms of PE can be specified (Supplementary Fig. S1, x-ray wavelength is 0.103 nm). Single diffraction peak at 2θ of around 13.9° belongs to (100)h reflection of hexagonal crystal (H-crystal), whilst two peaks simultaneously appearing at 2θ of around 14.4° (strong) and 15.5° (weak) correspond to (110)o and (200)o reflections of orthorhombic crystal (O-crystal), respectively. Under extension at 172 °C, the structural evolution is melt→non-crystalline δ→H-crystal, while no O-crystal is induced. Here we define the onset stress for observing characteristic WAXD or SAXS signal as the critical stress for relevant structure formation.


The non-equilibrium phase diagrams of flow-induced crystallization and melting of polyethylene
In-situ WAXD/SAXS characterization and crystallization phase diagram.(a) Representative true stress (σ)-time curve during extension with strain rate of 3 s−1 at 172 °C. The inserted images display selected 2D WAXD and SAXS patterns collected at the numbered points. Sample fracture happens at time of 930 ms indicated by sudden reduction of stress. The extensional direction is vertical as shown by the arrow. (b) The non-equilibrium crystallization phase diagram of crosslinked PE in stress-temperature space. Four phases of melt (L), non-crystalline shish (δ), H-crystal (H) and O-crystal (O) are included.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5016777&req=5

f1: In-situ WAXD/SAXS characterization and crystallization phase diagram.(a) Representative true stress (σ)-time curve during extension with strain rate of 3 s−1 at 172 °C. The inserted images display selected 2D WAXD and SAXS patterns collected at the numbered points. Sample fracture happens at time of 930 ms indicated by sudden reduction of stress. The extensional direction is vertical as shown by the arrow. (b) The non-equilibrium crystallization phase diagram of crosslinked PE in stress-temperature space. Four phases of melt (L), non-crystalline shish (δ), H-crystal (H) and O-crystal (O) are included.
Mentions: Crosslinked PE with a gel fraction of 43.5% was employed as the model sample due to its accessibility of high stress without fracture even at temperature far above the melting point. It was prepared by γ-ray radiation on a commercial PE with a quiescent equilibrium melting point of 141.4 °C17. It should be mentioned that we have done a similar research on the non-crosslinked PE and verified the conclusions in this paper (Supplementary Figs S4–8). The experimental details and data treating can be found in the Methods. Figure 1a presents a representative true stress-time curve of crosslinked PE melt under extension with strain rate of 3 s−1 at 172 °C as well as the selected two-dimensional (2D) wide-angle x-ray diffraction (WAXD) and small-angle x-ray scattering (SAXS) patterns. At the beginning of extension at low stress, only melt deformation is observed and no ordered structure appears (No. 1). At extension time of around 600 ms (No. 2), two symmetric streaks perpendicular to extensional direction show up in SAXS pattern, commonly regarded as scattering from shish or rod-like structure. Whilst absence of crystalline reflection in the simultaneous WAXD pattern indicates the non-crystalline nature of obtained shish. We name this non-crystalline shish as δ phase, which was assumed to be a vague kinetic state before and will be discussed later. Highly oriented crystal appears at around 770 ms (No. 3), evidenced by weak diffraction spots in WAXD pattern. Integrating 2D WAXD pattern into one-dimensional (1D) intensity curve, crystal forms of PE can be specified (Supplementary Fig. S1, x-ray wavelength is 0.103 nm). Single diffraction peak at 2θ of around 13.9° belongs to (100)h reflection of hexagonal crystal (H-crystal), whilst two peaks simultaneously appearing at 2θ of around 14.4° (strong) and 15.5° (weak) correspond to (110)o and (200)o reflections of orthorhombic crystal (O-crystal), respectively. Under extension at 172 °C, the structural evolution is melt→non-crystalline δ→H-crystal, while no O-crystal is induced. Here we define the onset stress for observing characteristic WAXD or SAXS signal as the critical stress for relevant structure formation.

View Article: PubMed Central - PubMed

ABSTRACT

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.

No MeSH data available.


Related in: MedlinePlus