Sol-gel transitions of poly(vinylidene fluoride) in organic solvents containing LiBF4

Abstract

Poly(vinylidene fluoride) (PVdF) dissolved in organic solvents containing lithium tetrafluoroborate (LiBF 4 ) forms stable gels when the solution is cooled to room temperature. Here, we describe the effects of LiBF 4 concentration and different gelation solvents on the gelation process, and characterize the resulting structural morphology using several techniques. Diethyl carbonate (DEC), propylene carbonate (PC) and c-butyrolactone (GBL) were used as gelation solvents. Time-resolved Fourier transform infrared spectroscopic measurements showed that the conformational transition from TGT G to T 3 GT 3 G occurred with the addition of lithium salts to the PVdF/DEC gel. In addition, PVdF produces thermoreversible gels in GBL and PC containing LiBF 4 , by assuming a T 3 GT 3 G conformation regardless of the LiBF 4 concentration. Scanning electron microscopy studies have indicated the presence of spherulites in the gels. Spherulite size decreased with increasing LiBF 4 concentration, whereas gel-melting temperatures increased with an increase in LiBF 4 concentration. These results indicate that gelation of PVdF in the presence of LiBF 4 occurs as polymer chains assuming the T 3 GT 3 G conformation, imparting the resulting gels with increased thermal stability. Polymer Journal (2011) 43, 540-544; doi:10.1038/pj.2011.21; published online 13 April 2011Keywords: lithium ion; molecular conformation; polymer gel electrolyte; poly(vinylidene fluoride); thermoreversible gel INTRODUCTION Polymer electrolytes have attracted much attention in lithium-battery technology because they enable fabrication of safe and reliable highenergy, high-power secondary lithium batteries. 1 Typical examples of polymer electrolytes include complexes of a lithium salt (LiX) with PEO (polyethylene oxide). The conductivity of PEO-LiX electrolytes reaches practically useful values of B10 À4 S cm À1 at temperatures of 60-80 1C but decreases to 10 À7 S cm À1 at room temperature. 2 Decrease in the conductivity on cooling is because fast ion transport occurs only in the amorphous state of the polymer.The most promising approach for improving conductivity at room temperature is to immobilize a highly conductive liquid electrolyte within a polymer matrix. This type of electrolyte is called a polymer gel electrolyte. This system demonstrates high ionic conductivity at room temperature with sufficient mechanical strength. 3,4 Among the matrices used for polymer gel electrolytes are PVdF, 5,6 poly(methyl methacrylate), 7,8 and poly(acrylonitrile). 9,10 Gel electrolytes based on PVdF are of great interest because of their good thermal, mechanical and electrochemical stability. Recent studies have shown that the ionic conductivity of PVdF gel electrolytes is related to the morphology of the gel. 11 However, a detailed characterization of these gels with regard to the gelation process and structural morphology has not been undertaken. In a previous study, we reported our investigation of the gelation process of PVdF in a number of organic solvents without a lith...

Flexible Quasi‐Solid State Ionogels with Remarkable Seebeck Coefficient and High Thermoelectric Properties

“…While at the presence of polar solvent in typical aqueous solution, the solvent molecules solvated the ions can screen the interaction among the ions and thus lower the Q* value and thus the Seebeck coefficient. [38][39][40][41] The structures of α, β, and amorphous PVDF-HFP are shown in Figure S9 the neat PVDF-HFP film. [11] The solvent screening is weak when the solvent is less polar.…”

“…To understand the ion‐dipole interactions, the Fourier‐transform infrared spectroscopy (FTIR) spectra of different ionogels were measured ( Figure 3 a). For the neat PVDF‐HFP film by spin coating, the FTIR bands at 491(δCF 2 ), 533(δCF 2 ), 614(δCF 2 + δskeleton), 761(δCF 2 + δskeleton), and 797 cm −1 (ρCH 2 ) can be assigned to the α phase of PVDF‐HFP, and the band at 875 cm −1 (ρCH 2 ) is due to amorphous structure of PVDF‐HFP . The structures of α, β, and amorphous PVDF‐HFP are shown in Figure S9 (Supporting Information).…”

Cross-Selectivity Enhancement of Poly(vinylidene fluoride-hexafluoropropylene)-Based Sensor Arrays for Detecting Acetone and Ethanol

“…PVDF-HFP has a slight interaction with DEC and is, thus, considered as a gelation solvent [37]. DEC can wash away the CB clumps or interact with the small amount of oxygen groups on CB.…”

Controlling Gel Morphology for Enhancing the Cation Mobility of Poly(vinylidene difluoride)-Based Gel Electrolytes for Lithium Secondary Batteries

“…The crystallinity of the gel is expected to be associated with the crystallinity of the constituent polymer because the PVDF-based gel formation includes the same crystal precipitation and growth process, as in the case of the solid polymer . In practice, it has been reported that the size of the polymer spherulites of the PVDF-based gel electrolytes observed via scanning electron microscopy (SEM), which reflects the crystallinity of the gel, depends on the type of solvent, salt concentration, and cooling conditions for gelation. , Although SEM observations provide direct information on the size of the crystal, in practice, it is difficult to evaluate the morphology of the gel under the equilibrium state from SEM images. This is because the morphology and composition of a gel sample can easily change in the evacuated space of the SEM equipment, as it consists of a mixed system of a solid polymer and a liquid electrolyte solution.…”