Effect of flow history on poly(vinylidine fluoride) crystalline phase transformation

Abstract

This study was devoted to the effect of extensional flow during film extrusion on the formation of the b-crystalline phase and on the piezoelectric properties of the extruded poly(vinylidine fluoride) (PVDF) films after cold drawing. The PVDF films were extruded at different draw ratios with two different dies, a conventional slit die and a two-channel die, of which the latter was capable of applying high extensional flow to the PVDF melt. The PVDF films prepared with the two-channel die were drawn at different temperatures, strain rates, and strains. The optimum stretching conditions for the achievement of the maximum b-phase content were determined as follows: temperature ¼ 908C, strain ¼ 500%, and strain rate ¼ 0.083 s À1. The samples prepared from the dies were then drawn under optimum stretching conditions, and their b-phase content and piezoelectric strain coefficient (d 33 ) values were compared at equal draw ratios. Measured by the Fourier transform infrared technique, a maximum of 82% b-phase content was obtained for the samples prepared with the two-channel die, which was 7% higher than that of the samples prepared by the slit die. The d 33 value of the two-channel die was 35 pC/N, which was also 5 pC/N higher than that of the samples prepared with the slit die.

A New Approach of Polyvinylidene Fluoride (PVDF) Poling Method for Higher Electric Response

“…According to the reference works [43,44] and our experiment, stretching ratio over 3 is good enough for later poling. After stretching and/or poling process, α-phase will transform into β-phase.…”

A comparison of two different methods for formation of the beta phase in nanocomposites based on vinylidene fluoride-hexafluoropropylene copolymer

“…, where: A α and A β -are the heights of the peaks at 764 and 840 cm −1 , respectively, and the coefficient 1.26 represents the ratio of the absorption coefficients at 764 and 840 cm −1 [9][10][11].…”

Melt spinning of poly(vinylidene fluoride) fibers and the influence of spinning parameters on β‐phase crystallinity

“…PVDF is known to exist in at least four crystal forms, which are designated α, β, γ, and δ. The crystalline structure and the formation of the piezoelectric β polymorph in particular have been well explored in the case of PVDF films 6–9. Although the nonpolar α phase is usually formed during crystallization from the melt or in solution‐cast films, the polar β phase can be created by mechanical deformation of α‐phase material.…”

“…As mentioned previously, the β phase can be formed by mechanical deformation of α‐phase material, and several researchers have attempted to find the optimal drawing parameters for producing PVDF films with a high content of β‐phase crystallites. It has been found that the formation of a necking region during drawing is advantageous, and the draw ratio (λ) as well as the draw rate should be as high as possible for maximum α‐to‐β transformation to take place 7, 8. The drawing temperature for maximum α‐to‐β transformation is in the range of 80–90°C.…”