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

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.

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Evolutions of 1D WAXD curves on the processes of stress increase and decrease.(a) True stress-time curves of the reciprocating extension (172 °C) and the stress relaxation experiments (133 °C), respectively. The stress begins to decrease after reaching maximum strain (ε) of 2.4. (b) 1D WAXD curves of the reciprocating extension experiment at 172 °C. The marked red curves denote where structure forms or disappears and the horizontal arrow indicates the curve corresponding to the maximum strain of 2.4. The critical stresses for L→δ, δ→H, H→δ and δ→L transitions are 4.74, 6.69, 5.21 and 2.47 MPa, respectively. (c) 1D WAXD curves of the stress relaxation experiment at 133 °C. The critical stresses for O→H and H→O transitions are 6.1 and 13.5 MPa, respectively.
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f2: Evolutions of 1D WAXD curves on the processes of stress increase and decrease.(a) True stress-time curves of the reciprocating extension (172 °C) and the stress relaxation experiments (133 °C), respectively. The stress begins to decrease after reaching maximum strain (ε) of 2.4. (b) 1D WAXD curves of the reciprocating extension experiment at 172 °C. The marked red curves denote where structure forms or disappears and the horizontal arrow indicates the curve corresponding to the maximum strain of 2.4. The critical stresses for L→δ, δ→H, H→δ and δ→L transitions are 4.74, 6.69, 5.21 and 2.47 MPa, respectively. (c) 1D WAXD curves of the stress relaxation experiment at 133 °C. The critical stresses for O→H and H→O transitions are 6.1 and 13.5 MPa, respectively.

Mentions: To verify the reversibility of flow-induced phase transitions and construct melting phase diagram during stress reduction, both reciprocating extension and stress relaxation experiments were performed (see experimental procedures in the Methods). The reciprocating extension experiment at 172 °C is selected as a representative for experiments above δ-O-H triple point, while the stress relaxation experiment at 133 °C is a representative for those below δ-O-H triple point. Figure 2a presents the corresponding true stress-time curves, where stress begins to decrease after strain reaches 2.4. For conciseness, we omit SAXS patterns (see Supplementary Fig. S2) and only present 1D WAXD curves. Figure 2b shows that, with the rise of stress under extension, structural evolution follows a path of L→δ→H and H-crystal reaches its maximum content at strain of 2.4. The critical stresses for observing δ and H phases are 4.74 and 6.69 MPa, respectively, close to the results of extension with strain rate of 3 s−1 in Fig. 1b (4.78 and 7.04 MPa). Note that the specific influence of strain rate on critical stress has been discussed in Supplementary Information combining with experimental results of non-crosslinked PE. Structure melting occurs when strain reduces in reciprocal extension, where H-crystal transforms back into δ phase at 5.21 MPa and the later further melts at lower stress of 2.47 MPa, demonstrating a reversed transition of H→δ→L. Noticeably, the critical stresses for H→δ and δ→L transitions are smaller than for their reversed δ→H (6.69 MPa) and L→δ (4.74 MPa) transitions during stress increase. Therefore stress induced transitions of L↔δ and δ↔H are two pairs of non-equilibrium thermodynamically reversible phase transitions with stress hysteresis. The same phenomenon is also observed in the reversible solid-to-solid transition of O↔H as shown in Fig. 2c. The critical stress for ordering process of H→O is 13.5 MPa, significantly larger than 6.1 MPa for disordering transition of O→H. The stress gap may serve as an indicator for nucleation barrier and we name it as “overstress”. The overstresses for the reversible transitions of L↔δ, δ↔H and H↔O are 2.27 (172 °C), 1.48 (172 °C) and 7.4 MPa (133 °C), respectively. Although H- and O-crystals have rather similar molecular packings18, the H→O transition still requires a large overstress.


The non-equilibrium phase diagrams of flow-induced crystallization and melting of polyethylene
Evolutions of 1D WAXD curves on the processes of stress increase and decrease.(a) True stress-time curves of the reciprocating extension (172 °C) and the stress relaxation experiments (133 °C), respectively. The stress begins to decrease after reaching maximum strain (ε) of 2.4. (b) 1D WAXD curves of the reciprocating extension experiment at 172 °C. The marked red curves denote where structure forms or disappears and the horizontal arrow indicates the curve corresponding to the maximum strain of 2.4. The critical stresses for L→δ, δ→H, H→δ and δ→L transitions are 4.74, 6.69, 5.21 and 2.47 MPa, respectively. (c) 1D WAXD curves of the stress relaxation experiment at 133 °C. The critical stresses for O→H and H→O transitions are 6.1 and 13.5 MPa, respectively.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f2: Evolutions of 1D WAXD curves on the processes of stress increase and decrease.(a) True stress-time curves of the reciprocating extension (172 °C) and the stress relaxation experiments (133 °C), respectively. The stress begins to decrease after reaching maximum strain (ε) of 2.4. (b) 1D WAXD curves of the reciprocating extension experiment at 172 °C. The marked red curves denote where structure forms or disappears and the horizontal arrow indicates the curve corresponding to the maximum strain of 2.4. The critical stresses for L→δ, δ→H, H→δ and δ→L transitions are 4.74, 6.69, 5.21 and 2.47 MPa, respectively. (c) 1D WAXD curves of the stress relaxation experiment at 133 °C. The critical stresses for O→H and H→O transitions are 6.1 and 13.5 MPa, respectively.
Mentions: To verify the reversibility of flow-induced phase transitions and construct melting phase diagram during stress reduction, both reciprocating extension and stress relaxation experiments were performed (see experimental procedures in the Methods). The reciprocating extension experiment at 172 °C is selected as a representative for experiments above δ-O-H triple point, while the stress relaxation experiment at 133 °C is a representative for those below δ-O-H triple point. Figure 2a presents the corresponding true stress-time curves, where stress begins to decrease after strain reaches 2.4. For conciseness, we omit SAXS patterns (see Supplementary Fig. S2) and only present 1D WAXD curves. Figure 2b shows that, with the rise of stress under extension, structural evolution follows a path of L→δ→H and H-crystal reaches its maximum content at strain of 2.4. The critical stresses for observing δ and H phases are 4.74 and 6.69 MPa, respectively, close to the results of extension with strain rate of 3 s−1 in Fig. 1b (4.78 and 7.04 MPa). Note that the specific influence of strain rate on critical stress has been discussed in Supplementary Information combining with experimental results of non-crosslinked PE. Structure melting occurs when strain reduces in reciprocal extension, where H-crystal transforms back into δ phase at 5.21 MPa and the later further melts at lower stress of 2.47 MPa, demonstrating a reversed transition of H→δ→L. Noticeably, the critical stresses for H→δ and δ→L transitions are smaller than for their reversed δ→H (6.69 MPa) and L→δ (4.74 MPa) transitions during stress increase. Therefore stress induced transitions of L↔δ and δ↔H are two pairs of non-equilibrium thermodynamically reversible phase transitions with stress hysteresis. The same phenomenon is also observed in the reversible solid-to-solid transition of O↔H as shown in Fig. 2c. The critical stress for ordering process of H→O is 13.5 MPa, significantly larger than 6.1 MPa for disordering transition of O→H. The stress gap may serve as an indicator for nucleation barrier and we name it as “overstress”. The overstresses for the reversible transitions of L↔δ, δ↔H and H↔O are 2.27 (172 °C), 1.48 (172 °C) and 7.4 MPa (133 °C), respectively. Although H- and O-crystals have rather similar molecular packings18, the H→O transition still requires a large overstress.

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