Effects of annealing time on physical and mechanical properties of PVDF microporous membranes by a melt extrusion‐stretching process

Abstract

Polyvinylidene fluoride-based separators for secondary battery have attracted enormous attention both in academia and in industries during the last decade. In this study, a facile, cost-effective, and environmentally friendly melt extrusion-stretching (MES) process was applied to prepare high performance PVDF microporous membranes for separator applications. In order to achieve a balance between the formation of row-nucleated lamellar structure and sufficient lamellae separation during stretching, a blend of high-and low-molecular-weight PVDF grades was chosen as raw material. Meanwhile, the effects of annealing time on physical and mechanical properties of PVDF cast films and membranes were systematically investigated, demonstrating that 6 h @145 C annealing treatment can lead to a preferable crystalline structure for the following stretching process. Therefore, PVDF membrane with high porosity, high electrolyte uptake ratio, and superior tensile strength was obtained by this MES process. Interestingly, the increase of β phase content by stretching was not significant for this PVDF membrane, revealing that the formation of micropores/ microfibrils in the stretching process would lead to stress relaxation and less crystalline structure conversion.

Recent Progress on Structure Manipulation of Poly(vinylidene fluoride)‐Based Ferroelectric Polymers for Enhanced Piezoelectricity and Applications

“…[152,153] For the annealing time, usually increasing the time can increase the lamellar thickness and degree of crystallinity. [154,155] However, it is worth noting that not all thermal treatments can induce phase transitions, and some only results in an increased degree of crystallinity (e.g., PVDF). [154,156] Furthermore, the rate at which the materials cools after annealing plays a significant role in the 𝛽 phase formation.…”

“…[154,155] However, it is worth noting that not all thermal treatments can induce phase transitions, and some only results in an increased degree of crystallinity (e.g., PVDF). [154,156] Furthermore, the rate at which the materials cools after annealing plays a significant role in the 𝛽 phase formation. [69] When the annealed PVDF is cooled rapidly, the amount of 𝛽 phase increases.…”

Challenges in Recycling Spent Lithium‐Ion Batteries: Spotlight on Polyvinylidene Fluoride Removal

“…With a good chemical stability, PVDF is not easily corroded by acid, alkali, strong oxidants, and halogens at room temperature. [42,43] Only a few chemicals, such as fuming sulphuric acid, strong alkali, and ketone, can induce swelling or partial dissolution. During the fabrication process of LIBs, the PVDF is first dissolved to colloid solution by strongly polar organic solvents such as n-methylpyrrolidone (NMP), dimethylacetamide, and dimethyl sulphoxide (Figure 2b).…”

A multifunctional composite membrane for high-safety lithium-ion batteries

“…26 Among them, PVDF is recognized as a promising polymer matrix because of its good processibility, excellent chemical stability and satisfactory thermal stability. [27][28][29] In addition, the b crystalline phase with high polarity in PVDF can increase the affinity of the electrolyte, and facilitate the dissociation and migration of lithium ions. 30 Non-solvent induced phase separation (NIPS) is the most commonly used method for manufacturing PVDF-based composite membranes.…”