With the continuous advancement of electronic devices, lightweight, flexible, and easily processable materials have gained substantial techno-commercial importance. Most electronic devices must possess a lightweight, high conductivity, high dielectric permittivity, low dielectric loss, and high breakdown strength. Hence, polymer-based piezoelectric materials are in great demand for design and development in energy storage, electromagnetic interference (EMI) shielding, and ultrafiltration applications. Among the piezoelectric polymers, poly­(vinylidene fluoride) (PVDF) with a predominantly polar β-phase is the most important. However, the main drawbacks of the PVDF matrix are its relatively low electrical conductivity and dielectric permittivity, and poor energy harvesting and EMI shielding performance. In this context, the incorporation of conductive nanofillers such as reduced graphene oxides, graphene quantum dots, and carbon nanotubes in the PVDF matrix has attracted considerable interest owing to their extraordinary properties. The final properties of these piezoelectric composites depend on the preparation methods, structural conformation, processing conditions, dispersion of nanofillers in the matrix, surface modification of fillers, and specific or nonspecific interaction of the fillers with the PVDF matrix. Herein, we have critically reviewed the formation mechanism of the electroactive β-phase in PVDF, the effects of nanofillers on the phase transformation of PVDF (dispersion and specific interaction), and the correlation of β-phase PVDF piezoelectric and dielectric properties with energy harvesting, EMI shielding, and membrane applications.

Electroactive polymers are ideal candidates for dielectric and piezoelectric applications due to their very high breakdown strength, good flexibility, and ease of processing. Among the various polymers, poly­(vinylidenefluoride) (PVDF) is of special interest due to potential applications in sensors, capacitors, actuators, etc. PVDF exists in four different crystalline phases (α-, β-, γ-, and δ-phases). Among these, the electroactive β-phase is very important for dielectric and piezoelectric applications. In the recent past, research has been focused on obtaining the maximum amount of electroactive β-polymorph in PVDF. The piezoelectric and dielectric properties of PVDF are influenced by the amount and the possible orientation of the polar β-phase. However, the effect of orientation on the electroactive β-phase in the field direction toward improving dielectric and piezoelectric applications is still not clear. This review attempts to bring in mechanistic insights into various processes that induce the electroactive β-phase, orientation, and correlation with dielectric and piezoelectric properties in PVDF-based composites. Some of the challenges and possible solutions for the future fabrication of piezoelectric materials are also emphasized.

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Poly­(vinylidene fluoride) (PVDF) is a semicrystalline polymer that exists in four crystalline phases (α, β, γ, δ). Among these, the β-phase has received tremendous techno-commercial importance due to its higher dipole moment as compared to the other phases, and thus many strategies have been explored in the recent past to obtain the β-polymorph of PVDF. In this study, the effect of shear history on crystalline morphology and behavior of PVDF has been investigated systematically by polarized optical microscopy coupled to a hot stage, Fourier transform infrared spectroscopy, differential thermal analysis, rheometry, and dielectric relaxation spectroscopy. Thin films of PVDF (120–150 μm) were sheared at different temperatures ranging from 155 to 220 °C and were allowed to isothermally crystallize at 155 °C. When the samples were isothermally crystallized at 155 °C, a remarkable increase in β-phase content was observed. More interestingly, this phenomenon was observed to be shear history dependent. For instance, the samples which were sheared at high temperature (220 °C) reflected in higher β fraction as compared to samples which were sheared at lower temperature (155 °C). It is envisaged that the distance between T shear (temperature at which the samples were sheared) and T cry (crystallization temperature) significantly influences the content of the β-phase in PVDF. This study clearly demonstrates the fact that both shear history and the depth from T c influence the conformational changes in PVDF.