Poly（vinylidene fluoride）（PVDF） exhibits pronounced polymorphs. Its γ phase is attractive due to the electroactive properties. The γ-PVDF is however difficult to obtain under normal crystallization condition. In...Poly（vinylidene fluoride）（PVDF） exhibits pronounced polymorphs. Its γ phase is attractive due to the electroactive properties. The γ-PVDF is however difficult to obtain under normal crystallization condition. In a previous work, we reported a simple melt-recrystallization approach for producing γ-phase rich PVDF thin films through selective melting and subsequent recrystallization. We reported here another approach for promoting the αγ′ phase transition to prepare γ-phase rich PVDF thin films. To this end, a stepwise crystallization and subsequent annealing process was used. The idea is based on a quick generation of a large amount of α-PVDF crystals with some of their γ-PVDF counterparts at suitable crystallization temperature and then annealing at a temperature above the crystallization temperature for enhancing the molecular chain mobility to overcome the energy barrier of phase transition. It was found that crystallizing the PVDF melt first at 152 °C for 4 h, then quenching to room temperature and finally annealing the sample at 160 °C for 100 h was the most efficient to produce γ-PVDF rich films. This is related to the melting and recrystallization of the α-PVDF crystals produced during quenching in the annealing process at 160 °C, which favors the formation of γ-PVDF crystals for triggering the αγ′ phase transition.展开更多
The deformation gradients in multilayered hot roll-bonded composite materials incorporating two dissimilar face-centered cubic metals, i.e., brass and Cu, were investigated by characterizing the deformation microstruc...The deformation gradients in multilayered hot roll-bonded composite materials incorporating two dissimilar face-centered cubic metals, i.e., brass and Cu, were investigated by characterizing the deformation microstructure, hardness and texture at different thickness positions of the composites. For the constitutive metals, the center part of each metal sheet forms the substructure containing coarse grains, while the ultrafine grains and significant shear banding form in the outer part. As deformation increases, the cross-interface shear occurs in the Cu sheet of the composites. Then, grain fragmentation, shear banding and dynamic recovery become the main factors that influence hardness of the metal. Moreover, in the co-deformed composites, the interface between brass and Cu plays a role in texture developments of the individual metals.展开更多
文摘Poly（vinylidene fluoride）（PVDF） exhibits pronounced polymorphs. Its γ phase is attractive due to the electroactive properties. The γ-PVDF is however difficult to obtain under normal crystallization condition. In a previous work, we reported a simple melt-recrystallization approach for producing γ-phase rich PVDF thin films through selective melting and subsequent recrystallization. We reported here another approach for promoting the αγ′ phase transition to prepare γ-phase rich PVDF thin films. To this end, a stepwise crystallization and subsequent annealing process was used. The idea is based on a quick generation of a large amount of α-PVDF crystals with some of their γ-PVDF counterparts at suitable crystallization temperature and then annealing at a temperature above the crystallization temperature for enhancing the molecular chain mobility to overcome the energy barrier of phase transition. It was found that crystallizing the PVDF melt first at 152 °C for 4 h, then quenching to room temperature and finally annealing the sample at 160 °C for 100 h was the most efficient to produce γ-PVDF rich films. This is related to the melting and recrystallization of the α-PVDF crystals produced during quenching in the annealing process at 160 °C, which favors the formation of γ-PVDF crystals for triggering the αγ′ phase transition.
基金financially supported by the National Natural Science Foundation of China(Nos.51202256 and51201028)the Fundamental Research Funds for the Central Universities(No.N130510001)the Program for New Century Excellent Talents in University(No.NCET-13-0104)
文摘The deformation gradients in multilayered hot roll-bonded composite materials incorporating two dissimilar face-centered cubic metals, i.e., brass and Cu, were investigated by characterizing the deformation microstructure, hardness and texture at different thickness positions of the composites. For the constitutive metals, the center part of each metal sheet forms the substructure containing coarse grains, while the ultrafine grains and significant shear banding form in the outer part. As deformation increases, the cross-interface shear occurs in the Cu sheet of the composites. Then, grain fragmentation, shear banding and dynamic recovery become the main factors that influence hardness of the metal. Moreover, in the co-deformed composites, the interface between brass and Cu plays a role in texture developments of the individual metals.
