Comprehensive Three-Dimensional Dielectric Characterization of Anisotropic PVDF Films

The dielectric properties of piezoelectric polymers such as polyvinylidene fluoride (PVDF) are highly anisotropic due to molecular orientation induced by mechanical stretching and electrical poling. In this work, the TA Instruments™ Rheo-Impedance Spectroscopy Accessory is used to measure the complex permittivity (ε* = ε′ + iε″) of PVDF films along three orthogonal axes: the stretching direction (ε*₂), the polarization direction (ε*₃), and perpendicular to stretching (ε*₁). Measurements under film tension, combined with a controlled temperature ramp, reveal the influence of orientation-induced asymmetry and thermal transitions on dielectric response. Results demonstrate a 35–38% higher ε*₂ compared to ε*₁ at room temperature, decreasing with temperature. These findings illustrate the value of three-dimensional impedance characterization for understanding structure–property relationships in piezoelectric polymers.

Polyvinylidene fluoride (PVDF) is a semicrystalline piezoelectric polymer exhibiting dielectric anisotropy when mechanically stretched and electrically poled [1, 2]. Mechanical stretching aligns polymer chains and dipoles, leading to direction-dependent mobility and permittivity. The dielectric constant in the stretching direction is typically lower than in the polarization axis, reflecting distinct dipolar dynamics [3]. Three-dimensional dielectric measurements are essential to fully characterize anisotropy and guide device design for sensors and actuators. The Rheo-Impedance Spectroscopy Accessory (Rheo-IS) enables impedance-based permittivity measurements under controlled mechanical and thermal conditions without upper electrodes, allowing measurements in-plane and along the thickness direction [4]. Here, the Rheo-IS Accessory and a complementary film tension accessory was used to quantify PVDF anisotropy under temperature ramp and tensile load.

Materials and Sample Preparation

Commercial β-phase-rich PVDF films (25 μm thickness, PolyK Technologies) were uniaxially stretched 200% at 80 °C using a mechanical draw frame. Samples were electrically poled at 100 MV/m and 70 °C for 20 minutes in silicon oil to prevent corona discharge. Post-poling annealing at 90 °C for 1 hour stabilized the β-phase.

All the measurements were made with a TA Instruments™ Discovery™ Hybrid Rheometer (HR) 20. The Rheo-IS Accessory or the tension clamp were installed easily depending on the measurement done.

The IS measurements were made in plane (without the Upper geometry in place (Figure 1).

And in the Z direction, as shown in Figure 2.

Tension oscillatory measurements were performed in the stretching (2) and perpendicular to the stretching (1) directions.

Three-Dimensional Dielectric Anisotropy

Average room-temperature permittivity values show ε*₂ = 11.2 ± 0.2 and ε*₁ = 8.3 ± 0.1, yielding an anisotropy factor of 35% (Figure 4) consistent with film tension results (Figure 5).

The polarization direction exhibits ε*₃ = 12.4 ± 0.3, slightly higher than ε*₂ due to oriented dipole alignment along the poling field [5].

Temperature Dependence

As temperature increases from 25 to 100 °C, all ε* components increase, reflecting enhanced dipole mobility (Figure 6).

The change in slope around 65 °C corresponds to the PVDF beta relaxation Above 70 °C, the difference (ε*₂–ε*₁)/ε*₁ decreases from 38% to 15%, indicating reduced orientation effects near melting (Figure 7).

Regarding E*, both moduli in the stretching direction and perpendicular to the stretching direction decrease with temperature (Figure 8).

The values of Figures 7 and 9 match quite well, indicating that orientation produces a similar effect in the mechanical and dielectric properties.