High β‐phase Poly(vinylidene fluoride) Using a Thermally Decomposable Molecular Splint

Abstract

of large-area thin-film, micro-patterning, low-temperature process, and flexibility. [7,8] Ferroelectric polymers such as polyvinylidene fluoride (PVDF) have been investigated as alternatives to their inorganic counterparts. It is known that C-F dipoles in PVDF align parallel to the external electric field, and remanent polarization remains after removing the field, which leads to piezoelectricity and pyroelectricity in PVDF. Depending on preparation methods, PVDF exhibited several different crystalline phases, including α, β, γ, δ, and ε. While β-phase PVDF, whose backbone chain configuration is all trans, presents the highest ferroelectricity, non-ferroelectric α-phase is dominantly observed when PVDF is prepared by thermal crystallization method since trans-gauche alternating structure is thermodynamically most stable.There have been a lot of efforts in preparing β-rich PVDF, including stretching, copolymerization with trifluoroethylene (TrFE), or adding organic/inorganic additives. [9][10][11] Stretching αor γ-phase PVDF leads to β-phase PVDF, but the enhancement of β-phase (<85%) is practically limited by relatively low drawing ratio. [12] PVDF-TrFE has several advantages exhibiting high β-phase (≈90%) and eligibility of thin-film process. However, PVDF-TrFE copolymers easily lose ferroelectricity at elevated temperatures because they An additive, 1,4-butadiene sulfone (BDS), which generates H 2 SO 3 by in situ thermal retro-Diels-Alder decompositions, is used for preparing high β-phase polyvinylidene fluoride (PVDF) films. Because of preferential multiple noncovalent interactions of H 2 SO 3 with all-trans configuration of PVDF, β-phase PVDF is spontaneously induced without mechanical drawing and/or extensive thermal annealing process. PVDF films cast from PVDF/BDS/water solutions exhibit high β-phase content (f β = 95%) when the BDS concentration is only c BDS = 1.0 wt%, which is confirmed by polarized optical microscopy (POM), SEM, Fourier transform infrared spectroscopy (FT-IR), differential scan calorimetry (DSC), and 2D grazing incidence wide-angle X-ray scattering (GIWAXS). Because of the high β-phase content, PVDF films prepared by using BDS exhibit excellent ferroelectric and piezoelectric properties (E c = 50 MV/m, P r = 5 µC/cm 2 , and d 33 = ≈-25 pm/V). Furthermore, a triboelectric nanogenerator (TENG) developed with high β-phase PVDF film exhibits enhanced performance as 2.5 times higher than neat PVDF film in output charge density, allowing reliable operation of conventional electronic devices.

A Study of Contact Electrification Process on PVDF–Metal Interface: Effect of β Phase Composition

“…According to the literature, the typical IR spectrum of semi crystalline PVDF contains peaks at 763, 795, 975, and 1384 cm − ¹ for 𝛼 phase (TGTG conformation) and 839, 1275, and 1402 cm − ¹ for 𝛽 phase PVDF (TTTT conformation). [36][37][38][39][40][41] In Figure 2a,b, the IR spectra of the films coated at 1000 rpm show characteristic to 𝛼 phase peak at 763 cm − ¹ corresponding to CF 2 bending and skeletal bending along with peak at 839 cm − ¹ for CH 2 rocking corresponding to 𝛽 phase and by a convention these peaks are used for evaluation of the 𝛽 phase composition of PVDF. [36][37][38][39][40][41] As the coating speed was increased to 2000 rpm the intensity of the peak at 763 cm − ¹ reduced relative to the absorbance peak at 839 cm − ¹ indicating occurrence of 𝛼 to 𝛽 phase transformation.…”

“…The equations below were used for quantitative estimation of the 𝛽 phase content of the PVDF films. [40] A 𝛼 = − logT 𝛼 (1)…”

“…[36][37][38][39][40][41] In Figure 2a,b, the IR spectra of the films coated at 1000 rpm show characteristic to 𝛼 phase peak at 763 cm − ¹ corresponding to CF 2 bending and skeletal bending along with peak at 839 cm − ¹ for CH 2 rocking corresponding to 𝛽 phase and by a convention these peaks are used for evaluation of the 𝛽 phase composition of PVDF. [36][37][38][39][40][41] As the coating speed was increased to 2000 rpm the intensity of the peak at 763 cm − ¹ reduced relative to the absorbance peak at 839 cm − ¹ indicating occurrence of 𝛼 to 𝛽 phase transformation. The equations below were used for quantitative estimation of the 𝛽 phase content of the PVDF films.…”

Local analysis of crystalline phases and properties of poly(vinylidene fluoride) electrospun composites with BaTiO3 nanorods

“…In recent years, scientists have developed a number of processing techniques to either directly extract the ⊎-phase from the melt or transform the ⊍-phase into a desirable structure. [12][13][14][15][16] Several studies have demonstrated that high temperatures, very high pressures or highly rapid cooling rates are necessary to produce the ⊎-phase from the melt directly. This is because PVDF is a highly crystalline material that solidifies rapidly during cooling, leading to the formation of the less desirable ⊍-phase, which has inferior ferroelectric and piezoelectric properties.…”

“…In recent years, scientists have developed a number of processing techniques to either directly extract the β ‐phase from the melt or transform the α ‐phase into a desirable structure 12–16 . Several studies have demonstrated that high temperatures, very high pressures or highly rapid cooling rates are necessary to produce the β ‐phase from the melt directly.…”