Effect of annealing on the structure and properties of poly(vinylidene fluoride) β‐form films

Abstract

Oriented poly(vinylidene fluoride) (PVDF) films consisting of ␤ crystals were prepared by the solid-state coextrusion (SC) of a gel film near the melting temperature (T m ) and by conventional cold tensile drawing (TD) of a melt-quenched film. These films were annealed over the temperature range of 75-190°C (below and above the static T m ) while the sample length was kept constant or constant loads were applied. After annealing with the sample length kept constant, the dynamic Young's modulus markedly decreased because of the relaxation of oriented amorphous chains, as shown by infrared spectroscopy. In contrast, annealing under a constant load improved the chain orientation in both the crystalline and amorphous regions, resulting in an increase in the modulus from an initial 10.5 to 14.3 GPa for the SC and from an initial 3.3 to 4.8 GPa for the TD. The SC, annealed at 190°C with a constant load corresponding to an initial tension of 200 MPa, exhibited an extreme crystalline-chain orientation of 0.998 and a modulus of 14.3 GPa, among the highest values ever reported for PVDF. Although the remanent polarization (P r ) of the TD increased slightly from the initial 62 to 68 mC/m 2 , P r of the SC stayed constant at 100 mC/m 2 independently of the annealing conditions. This suggests that the P r value of 100 mC/m 2 approached the equilibrium value for this PVDF sample containing 3.5 mol % structural defects. Therefore, although the modulus and P r of the TD increased slightly with annealing, the maximum values achieved by annealing were markedly lower than those of the SC and annealed SC.

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

“…This was in agreement with results reported by other workers. 7,22 The piezoelectric properties of the polarized PVDF films significantly depended on the b-phase content. 10,14 The d 33 values of the films prepared with both dies after cold drawing to two different draw ratios are reported in Table III.…”

“…[3][4][5] These crystalline forms can transform to each other under specific conditions, such as under the application of mechanical deformation and a high-temperature electrical field. [6][7][8] The a-crystalline form does not show a net lattice polarization due to the antiparallel TG þ TG À chain arrangement in its unit cell. Therefore, the chain dipoles oppose each other.…”

PVDF Phase Formation and Its Influence on Electrical and Structural Properties of PZT-PVDF Composites

“…Among the five polymorphs; β-phase is most intriguing due to its piezo-, pyro-and ferroelectric properties, which can be attributed to the all trans conformation of the polymer chains [23,24]. The orthorhombic alpha phase is usually obtained during crystallization of PVDF from melt, and special processing methods are required to obtain pseudo-hexagonal polar beta phase [25]. Typical methods to produce β-phase are drawing [26] and poling.…”

Microscopic origin of the high-strain mechanical response of poled and non-poled poly(vinylidene fluoride) in the β-phase

“…The electroactive properties depend on the b-phase content, orientation and structural characteristics. Thus, the increment of the b-phase content has always attracted great interest [3][4][5][6][7][8][9].…”