Relaxation dynamics of poly(vinylidene fluoride) studied by dynamical mechanical measurements and dielectric spectroscopy

β Phase Optimization of Solvent Cast PVDF as a Function of the Processing Method and Additive Content

A semicrystalline polymer with high piezo-, pyro-, and ferroelectric characteristics, poly(vinylidene fluoride) (PVDF) offers exciting possibilities in various applications. The semicrystalline structure of PVDF is composed of several phases including α, β, θ, γ, and ε phases. β phase polymorphs of PVDF exhibit the highest piezoelectric properties, which can be enhanced through different processing methods. This study aims to investigate the β phase transformation of PVDF through different processes/treatment methods and the processing of a PVDF polymer composite containing 0.2 wt % multiwalled carbon nanotubes and/or 20 wt % modified/unmodified barium titanate. The effects of annealing, uniaxial stretching, rolling, atmospheric plasma treatment, UV treatment, and their combinations were investigated. The transformation of α to β phase was determined by Fourier transform infrared spectrometer, X-ray diffractometer and differential scanning calorimeter. The most remarkable β phase transformation of PVDF films was obtained by stretching following solvent casting and hot pressing. It was observed that various process combinations, as well as the incorporation of additives, influence the β phase content of PVDF. Alongside studying β phase content of PVDF, the investigation extends to analyzing the tan δ and elastic and loss modulus values of rolled PVDF polymer composite films.

Smart and Multifunctional Materials Based on Electroactive Poly(vinylidene fluoride): Recent Advances and Opportunities in Sensors, Actuators, Energy, Environmental, and Biomedical Applications

From scientific and technological points of view, poly(vinylidene fluoride), PVDF, is one of the most exciting polymers due to its overall physicochemical characteristics. This polymer can crystalize into five crystalline phases and can be processed in the form of films, fibers, membranes, and specific microstructures, being the physical properties controllable over a wide range through appropriate chemical modifications. Moreover, PVDF-based materials are characterized by excellent chemical, mechanical, thermal, and radiation resistance, and for their outstanding electroactive properties, including high dielectric, piezoelectric, pyroelectric, and ferroelectric response, being the best among polymer systems and thus noteworthy for an increasing number of technologies. This review summarizes and critically discusses the latest advances in PVDF and its copolymers, composites, and blends, including their main characteristics and processability, together with their tailorability and implementation in areas including sensors, actuators, energy harvesting and storage devices, environmental membranes, microfluidic, tissue engineering, and antimicrobial applications. The main conclusions, challenges and future trends concerning materials and application areas are also presented.

Nuomici-Inspired Universal Strategy for Boosting Piezoresistive Sensitivity and Elasticity of Polymer Nanocomposite-Based Strain Sensors

Electrically conductive polymer composites (CPCs) are potential alternatives to conventional strain gauges due to their tunable sensitivity and strain ranges. Currently, to achieve very high piezoresistive sensitivity in thermoplastic-based CPCs with Gauge factors G_F above 20 at low tensile strains (ε ≤ 5%) is a big challenge, but critical for structural health monitoring application in infrastructures. Here, inspired by the unique structures of a famous Chinese food, nuomici, we coat carbon nanotubes (CNTs) onto sticky acrylic rubber (AR) granules (ARG) to form nuomici-like CNT@ARG composite granules, which are employed as unique conductive filler to fabricate highly piezoresistive and flexible CPCs based on poly(vinylidene fluoride) (PVDF). This strategy of localizing CNTs densely on the surface of touching rubbery particles resulted in a much more sensitive elastic conductive network built by the CNT@AR composite and showed a big gain effect. The resultant PVDF/CNT@AR nanocomposites (AR content ranging from 0 to 10 wt %) show extremely high piezoresistive sensitivity at low strain, depending on the AR content. In particular, the G_F value of PVDF with 1.5 wt % CNT@10 wt % AR is 41 at 5% strain, which is more than one magnitude higher than that (ca. 3) of traditional PVDF/CNT nanocomposite sensors. Moreover, the elongation at break increases by about 60% with the addition of 1.5 wt CNT@10 wt % AR. This study introduces a universal effective strategy for tailoring the mechanical properties and strain sensitivity of conductive network in CPCs, which is critical for the fabrication of high-performance strain sensors.

Polymer matrix ferroelectric composites under pressure: Negative electric capacitance and glassy dynamics

This report presents the results of high-pressure and broadband dielectric spectroscopy studies in polyvinylidene difluoride (PVDF) and barium strontium titanate (BST) microparticles composites (BST/PVDF). It shows that the Arrhenius behaviour for the temperature-related dynamics under atmospheric pressure is coupled to Super-Arrhenius/Super-Barus isothermal pressure changes of the primary relaxation time. Following these results, an explanation of the unique behaviour of the BST/PVDF composite is proposed. Subsequently, it is shown that when approaching the GPa domain the negative electric capacitance phenomenon occurs.

Development of a superhydrophobic electrospun poly(vinylidene fluoride) web via plasma etching and water immersion for energy harvesting applications

Smart textiles have been enormously developed recently, but attachment of batteries and low washing resistance are the major challenges in the development of wearable smart textiles. However, piezoelectric materials harvesting energy from mechanical action can be readily integrated with smart textiles and can replace conventional batteries. Therefore, energy harvesters with poly(vinylidene fluoride) (PVDF) were fabricated by the electrospinning process. In addition, simple CF4 plasma etching followed by water immersion of the electrospun PVDF webs resulted in superhydrophobicity, with a water contact angle of 169.8 ± 1.5°, a water shedding angle of 4.7 ± 1.8°, and self-cleaning properties. This would decrease the number of washing cycles during use and increase the durability of the smart textile. X-ray photoelectron spectroscopy indicated that metals were co-deposited as etch-resisting masks to fabricate a nanostructure during plasma etching and were removed by water immersion. The piezoelectric performance of the superhydrophobic electrospun PVDF web showed a higher peak-to-peak output voltage of 3.50 V than the untreated electrospun PVDF web (2.86 V). Furthermore, the breathability of the superhydrophobic PVDF web was remarkably higher than those of the PVDF film. Therefore, the new flexible electrospun PVDF web with superhydrophobicity and piezoelectricity has significant potential as energy harvesters in wearable smart textiles.

Metallic Glass/PVDF Magnetoelectric Laminates for Resonant Sensors and Actuators: A Review

Among magnetoelectric (ME) heterostructures, ME laminates of the type Metglas-like/PVDF (magnetostrictive+piezoelectric constituents) have shown the highest induced ME voltages, usually detected at the magnetoelastic resonance of the magnetostrictive constituent. This ME coupling happens because of the high cross-correlation coupling between magnetostrictive and piezoelectric material, and is usually associated with a promising application scenario for sensors or actuators. In this work we detail the basis of the operation of such devices, as well as some arising questions (as size effects) concerning their best performance. Also, some examples of their use as very sensitive magnetic fields sensors or innovative energy harvesting devices will be reviewed. At the end, the challenges, future perspectives and technical difficulties that will determine the success of ME composites for sensor applications are discussed.

Improvement of electroactive β phase nucleation and dielectric properties of WO3·H2O nanoparticle loaded poly(vinylidene fluoride) thin films

Electroactive β phase nucleation mechanism and promising dielectric properties of WO₃·H₂O nanoparticle loaded PVDF thin films.

Optimization of the magnetoelectric response of poly(vinylidene fluoride)/epoxy/Vitrovac laminates

The effect of the bonding layer type and piezoelectric layer thickness on the magnetoelectric (ME) response of layered poly(vinylidene fluoride) (PVDF)/epoxy/Vitrovac composites is reported. Three distinct epoxy types were tested, commercially known as M-Bond, Devcon, and Stycast. The main differences among them are their different mechanical characteristics, in particular the value of the Young modulus, and the coupling with the polymer and Vitrovac (Fe39Ni39Mo4Si6B12) layers of the laminate. The laminated composites prepared with M-Bond epoxy exhibit the highest ME coupling. Experimental results also show that the ME response increases with increasing PVDF thickness, the highest ME response of 53 V·cm(-1)·Oe(-1) being obtained for a 110 μm thick PVDF/M-Bond epoxy/Vitrovac laminate. The behavior of the ME laminates with increasing temperatures up to 90 °C shows a decrease of more than 80% in the ME response of the laminate, explained by the deteriorated coupling between the different layers. A two-dimensional numerical model of the ME laminate composite based on the finite element method was used to evaluate the experimental results. A comparison between numerical and experimental data allows us to select the appropriate epoxy and to optimize the piezoelectric PVDF layer width to maximize the induced magnetoelectric voltage. The obtained results show the critical role of the bonding layer and piezoelectric layer thickness in the ME performance of laminate composites.