Conformational changes and phase transformation mechanisms in PVDF solution‐cast films

Abstract

The supramolecular crystal structure in poly(vinylidene fluoride) (PVDF) solution-cast films is studied through changing crystallization conditions in two solvents of different structures and polarities. The crystalline-state chain conformations of isothermally solution-crystallized PVDF in N, N-dimethylacetamide (DMAc), and cyclohexanone are studied through the specific FTIR absorption bands of ␣, ␤, and ␥ phase crystals. There are no changes in the FTIR spectra of cyclohexanone solution-crystallized films in the temperature range of 50 -120°C. In the case of DMAc solutioncrystallized films, low temperature crystallization mainly results in formation of trans states (␤ and ␥ phases), whereas at higher temperatures gauche states become more populated (␣ phase). This is due to the variations in solvent polarity and ability to induce a specific conformation in PVDF chains, through the changes in chain coil dimensions. This indicates that in spite of cyclohexanone solutions, the intermolecular interactions between PVDF and DMAc are temperature-sensitive and more important in stabilizing conformations of PVDF in crystalline phase than temperature dependence of PVDF chain end-to-end distance Ͻr 2 Ͼ. The high-resolution 19 F NMR spectroscopy also showed little displacement in PVDF characteristic chemical shifts probably due to changes in PVDF chain conformation resulting from temperature variations. Upon uniaxial stretching of the prepared films under certain conditions, contribution of trans state becomes more prominent, especially for the originally higher ␣ phase-containing films. Due to formation of some kink bands during film stretching and phase transformation, ␣ phase absorption bands are still present in infrared spectra. Besides, uniaxial stretching greatly enhances piezoelectric properties of the films, maybe due to formation of oriented ␤ phase crystals, which are of more uniform distribution of dipole moments.

RMB Internationalization and Energy Prices: Evidence for Co-integration, Causality and Interaction Relationship

“…Sadorsky [9] uses co-integration method and Granger causality test to reveal the relationship between the energy futures price of crude oil and the U.S. trade weighted index, and found that the U.S. dollar exchange rate will impact on energy futures prices. Other scholars have similar conclusions, like Salimi and Yousefi [10], Lizardo and Mollick [11] and Reboredo, Rivera-Castro and Zebende [12]. Eichengreen, Chitu and Mehl [13] think that the pricing of oil will break through the dollar based pricing system, and move toward a variety of currencies, and will be implemented for a period of time.…”

Ferroelectric Properties and Polarization Switching Kinetic of Poly (vinylidene fluoride-trifluoroethylene) Copolymer

“…Its ferroelectric properties originate from the large difference in electronegativity between fluorine, carbon and hydrogen, which have Pauling's values of 4.0, 2.5 and 2.1, respectively [Pauling, 1960]. Most of the electrons are attracted to the fluorine side of the polymer chain and polarization is created [Salimi & Yousefi, 2004;Fujisaki et al, 2007]. The Curie temperature of PVDF is estimated to be above the melting temperature at 195-197 o C [Lovinger, 1986, as cited in Kepler, 1995].…”

Enhanced thermal properties of poly(vinylidene fluoride) composites with ultrathin nanosheets of MXene

“…[33][34][35] Poly(vinylidene) uoride (PVDF) is a semi-crystalline polymer with ve different crystalline forms (namely a, b, g, d and 3). 36,37 Due to its excellent piezoelectric, pyroelectric and ferroelectric properties, PVDF has applications in many elds, including sensors, actuators, membranes and smart scaffolds. [38][39][40][41][42] However, the thermal conductivity of PVDF is low, which limits its use in elds where heat transfer is important.…”