Although the microporous membrane prepared based on the melt stretching mechanism has been commercialized for more than 20 years, the formation process from the initial lamellae structure to final fiber connecting bridges and pores is still unclear. In this work, to clarify the transformation mechanism, in situ SAXS and WAXS were carried out during hot stretching at 130 °C to 100%. The scattering patterns from the annealed film, cold stretched film to 20% (stretched at room temperature), and heating to 130 °C were also collected. The preparation technology was similar to that during the commercial fabrication. It was found that during cold stretching to 20% many long and narrow crazes are formed between separated lamellae clusters, and a part of destroyed crystals appeared. After heating to 130 °C, oriented structure and needlelike voids appeared, which was related to the shrinkage of oriented amorphous chains along the transverse direction, due to the tension stress effect. Also some oriented crystal structure was formed. During hot stretching to 20%, the lamellae which are close to the craze wall are rotated as the fibril crystal as the axle and the connecting bridges were formed among the separated lamellae cluster. Further stretching to 100%, these connecting bridges transformed to fiber bridges, contributed by strain-induced crystallization. During the whole hot stretching, the amorphous chains oriented along the machine direction and also crystallized into fiber bridges. This is the first time to clearly describe the lamellae to fiber bridges transformation during the preparation of microporous membrane.
This paper proposes a multi-objective optimization method for the start-up strategy of pumped storage units (PSU) for the first time. In the multi-objective optimization method, the speed rise time and the overshoot during the process of the start-up are taken as the objectives. A precise simulation platform is built for simulating the transient process of start-up, and for calculating the objectives based on the process. The Multi-objective Particle Swarm Optimization algorithm (MOPSO) is adopted to optimize the widely applied start-up strategies based on one-stage direct guide vane control (DGVC), and two-stage DGVC. Based on the Pareto Front obtained, a multi-objective decision-making method based on the relative objective proximity is used to sort the solutions in the Pareto Front. Start-up strategy optimization for a PSU of a pumped storage power station in Jiangxi Province in China is conducted in experiments. The results show that: (1) compared with the single objective optimization, the proposed multi-objective optimization of start-up strategy not only greatly shortens the speed rise time and the speed overshoot, but also makes the speed curve quickly stabilize; (2) multi-objective optimization of strategy based on two-stage DGVC achieves better solution for a quick and smooth start-up of PSU than that of the strategy based on one-stage DGVC.
In order to study the effect of interfacial wettability of separator on electrochemical properties for lithium–ion batteries, two different kinds of polyvinylidene fluoride-hexafluoropropylene (PVDF–HFP) solution are prepared and used to coat onto a polypropylene (PP) microporous membrane. It is found that the cell performance of a coated separator using aqueous slurry (WPS) is better than that of the coated separator using acetone (APS) as the solvent. The separator with flat and pyknotic surface (PP and APS) has a strong polar action with the electrolyte, where the polar part is more than 80%. To the contrary, the WPS has a roughness surface and when the PVDF–HFP particles accumulate loose, it makes the apolar part plays a dominate role in surface free energy; the dispersive energy reaches to 40.17 mJ m−2. The WPS has the lowest immersion free energy, 31.9 mJ m−2 with the electrolyte, and this will accelerate electrolyte infiltration to the separator. The loose particle accumulation also increases the electrolyte weight uptake and interfacial wettability velocity, which plays a crucial role in improving the cell performance such as the ionic conductivity, discharge capacity and the C-rate capability.
In this article, the pre‐oriented polylactide (PLA) films were prepared by extrusion casting under melt‐stretching flow and cast roll temperature field. Shish‐kebab like structure was induced and the elongation at break under room temperature of PLA film prepared with melt‐draw ratio of 40 was high to 270%. To clarify the origin of this ductile behavior, the pre‐oriented PLA structure evolution during stretching at room temperature was followed. It was found that melt‐stretching makes the chains fully extended to form amount of rigid amorphous fraction (RAF). The low chain entanglement induces weak connection strength between RAF and mobile amorphous fraction. The craze is easily formed at the interface between these two phases. The initial ductile deformation during stretching of pre‐oriented PLA films comes from the rigid extended chain slip and free amorphous chain network deformation. When the strain is higher than 40%, the RAF region appears periodic ordered structure. With the strain increasing, the RAF gradually transforms to mesophase. The molecular chains are stretched to extend in the crazes. The strain‐hardening behavior comes from the crystal destroy and chain extension. The room temperature stretching of pre‐oriented PLA film can only increase chain's orientation and package tightness, but could not induce the formation of highly perfect crystals.
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