Beta-phase enhancement in polyvinylidene fluoride through filler addition: comparing cellulose with carbon nanotubes and clay

Recent Advances in Materials, Devices and Algorithms Toward Wearable Continuous Blood Pressure Monitoring

Continuous blood pressure (BP) tracking provides valuable insights into the health condition and functionality of the heart, arteries, and overall circulatory system of humans. The rapid development in flexible and wearable electronics has significantly accelerated the advancement of wearable BP monitoring technologies. However, several persistent challenges, including limited sensing capabilities and stability of flexible sensors, poor interfacial stability between sensors and skin, and low accuracy in BP estimation, have hindered the progress in wearable BP monitoring. To address these challenges, comprehensive innovations in materials design, device development, system optimization, and modeling have been pursued to improve the overall performance of wearable BP monitoring systems. In this review, we highlight the latest advancements in flexible and wearable systems toward continuous noninvasive BP tracking with a primary focus on materials development, device design, system integration, and theoretical algorithms. Existing challenges, potential solutions, and further research directions are also discussed to provide theoretical and technical guidance for the development of future wearable systems in continuous ambulatory BP measurement with enhanced sensing capability, robustness, and long-term accuracy.

Unleashing the piezoelectric potential of PVDF: a study on phase transformation from gamma (γ) to beta (β) phase through thermal contact poling

Polyvinylidene fluoride (PVDF) is known for its piezoelectric properties. This material has different crystalline phases, alpha (α), beta (β) and gamma (γ), where the β-phase, in particular, is related to the piezoelectric behavior of PVDF. While the transformation from the α-phase to β-phase in PVDF is well-documented and widely studied, the transformation from γ- to β-phase has not yet been fully explored. However, when PVDF is produced by certain solution-based methods it can adopt its γ-form, which is not as piezoelectric as the β-phase. Hence, this study aims to bridge this gap by investigating the transformation from γ- to β-phase in PVDF nanocomposites films obtained from solution-based techniques. Our PVDF nanocomposite is made by solvent evaporation-assisted 3D printing of PVDF's nanocomposite with barium-titanate nanoparticles (BTO). To achieve the γ- to β-phase transformation, we first highlight the importance of annealing in the successful poling of PVDF samples. We then perform an in-depth analysis of the α-, β- and γ-crystallographic phases of PVDF-BTO using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). We observed that after annealing but before poling, the PVDF-BTO nanocomposite contains 76% of β + γ phases, the majority of which is the γ-phase. Poling of these samples resulted in the combination of the β + γ phases reaching 93% with the appearance of 40% of absolute fraction of the β-phase. We then demonstrated that the fraction of β-phase in the nanocomposite - as indicated by the 1275 cm-1 peak in PVDF's FTIR spectra - is not uniform on the surface area of the film. Additionally, the value of the absolute β-phase content also depends on the poling field's direction. Our work reveals that while considering PVDF's piezoelectric behavior, it is critical to be aware of these nuances and this article offers essential insights on how to address them. Overall, this study provides a step-by-step guideline to enhance the piezoelectricity of PVDF-based nanocomposites for sensing applications.

Bedolla D, D’Amico F, Koopmann AK, Vaccari L, Saccomano G, Kohns R, Huesing N. Polyvinylidene Fluoride Aerogels with Tailorable Crystalline Phase Composition

In this work, polyvinylidene fluoride (PVDF) aerogels with a tailorable phase composition were prepared by following the crystallization-induced gelation principle. A series of PVDF wet gels (5 to 12 wt.%) were prepared from either PVDF−DMF solutions or a mixture of DMF and ethanol as non-solvent. The effects of the non-solvent concentration on the crystalline composition of the PVDF aerogels were thoroughly investigated. It was found that the nucleating role of ethanol can be adjusted to produce low-density PVDF aerogels, whereas the changes in composition by the addition of small amounts of water to the solution promote the stabilization of the valuable β and γ phases. These phases of the aerogels were monitored by FTIR and Raman spectroscopies. Furthermore, the crystallization process was followed by in-time and in situ ATR−FTIR spectroscopy. The obtained aerogels displayed specific surface areas > 150 m2 g−1, with variable particle morphologies that are dependent on the non-solvent composition, as observed by using SEM and Synchrotron Radiation Computed micro-Tomography (SR-μCT).

Fabrication and Optical Properties of Transparent P(VDF-TrFE) Ultrathin Films

The films of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE)) are widely used in piezoelectric tactile sensors, vibration energy harvesters, optical frequency conversion materials and organic photo-voltaic devices because of high electroactive, good optical and nonlinear optical properties, respectively. In this work, the multilayer structured ultrathin films were fabricated by the Langmuir-Blodgett technique, and the thickness per layer can be controlled accurately. It was found that as the collapse pressure of P(VDF-TrFE) (25:75) and the optimal dipping value are 60~70 mN/m and 15 mN/m, respectively, a high-density film can be obtained due to the compression of molecules. The surface topography and optical properties of the LB films were characterized by X-ray diffraction, white light interferometer and variable-angle spectrum ellipsometer. It was observed that the films are transparent in the visible region and IR-band, but show a high absorption in the UV band. Besides, the transmittance of the films ranges from 50% to 85% in the visible region, and it linearly decreases with the number of monolayers. The average thickness of per deposition layer is 2.447 nm, 2.688 nm and 2.072 nm, respectively, under three measurement methods. The calculated refractive index ranged from 1.443 to 1.598 (600~650 nm) by the Cauchy-model.

A study on electroactive PVDF/mica nanosheet composites with an enhanced γ-phase for capacitive and piezoelectric force sensing

Herein, a multifunctional poly(vinylidene fluoride) (PVDF)/mica nanosheet composite (PMNC) thin film was developed for preparing a capacitive and piezoelectric force sensor. A high electroactive γ-phase content (89%) of PVDF was achieved through a facile rapid cooling process of PMNC films. The crystallinity of PVDF decreased upon the addition of mica nanosheets, while the dielectric constant increased significantly (∼300%). The capacitance-based PMNC pressure sensor was found to be sensitive to the applied pressure. On the other hand, piezoelectric voltages of 18 V (single layer) and 32 V (multi-layer) were generated for PMNCs loaded with 1% mica nanosheets. Furthermore, a PMNC based nanogenerator generated a power density of 8.8 μW cm^-2 and showed excellent durability (>60 000 cycles). High flexibility, lightweight and skin-friendly PMNCs could be a potential material in applications such as energy harvesting, energy storage, actuators, and self-powered and smart wearable electronic devices.

Piezoelectric Materials for Energy Harvesting and Sensing Applications: Roadmap for Future Smart Materials

Piezoelectric materials are widely referred to as "smart" materials because they can transduce mechanical pressure acting on them to electrical signals and vice versa. They are extensively utilized in harvesting mechanical energy from vibrations, human motion, mechanical loads, etc., and converting them into electrical energy for low power devices. Piezoelectric transduction offers high scalability, simple device designs, and high-power densities compared to electro-magnetic/static and triboelectric transducers. This review aims to give a holistic overview of recent developments in piezoelectric nanostructured materials, polymers, polymer nanocomposites, and piezoelectric films for implementation in energy harvesting. The progress in fabrication techniques, morphology, piezoelectric properties, energy harvesting performance, and underpinning fundamental mechanisms for each class of materials, including polymer nanocomposites using conducting, non-conducting, and hybrid fillers are discussed. The emergent application horizon of piezoelectric energy harvesters particularly for wireless devices and self-powered sensors is highlighted, and the current challenges and future prospects are critically discussed.

Enhanced Flexible Poly(vinylidene fluoride-trifluorethylene) Piezoelectric Nanogenerators by SnSe Nanosheet Doping and Solvent Treatment

Organic piezoelectric nanogenerators (PENGs) with light weight, good flexibility, and simple processing are promising in mechanical energy conversion. Here, a high-output polymer PENG composed of a poly(vinylidene fluoride-trifluorethylene)/SnSe nanosheet (NS) nanocomposite film has been fabricated and tested. After a treatment with mixed solvents of N,N-dimethylformamide and 1,1,2,2-tetrachloroethane (TCE), the optimal nanocomposite PENG exhibits high piezoelectric properties with a piezoelectric coefficient (d₃₃) of 25.06 pC·N^-1, a prominent open-circuit voltage (V_oc) of 15.36 V·cm^-2, and a short-circuit current (I_sc) of 1.02 μA·cm^-2. Moreover, the PENG can generate an output power density of up to 10.72 μW·cm^-2 under a vertical force of 50 N. The attractive piezoelectric performance results from the doping of SnSe NSs with a high piezoelectric coefficient and also the increased β-phase ascribed to the introduction of nucleating agent NSs and the TCE solvent. The PENGs reveal great application potentials in consumer electronics.

Ionic Liquid-Assisted 3D Printing of Self-Polarized β-PVDF for Flexible Piezoelectric Energy Harvesting

Three-dimensional (3D) printing technologies have unparalleled advantages in constructing piezoelectric devices with three-dimensional structures, which are conducive to improving the efficiency of energy harvesting. Among them, fused deposition modeling (FDM) is the most widely used thanks to its low cost and wide range of molding materials. However, as the best piezoelectric polymer, a high electroactive β-phase poly(vinylidene fluoride) (PVDF) piezoelectric device cannot be directly obtained by FDM printing because the β-crystal is unstable at the molten state. Herein, we develop for the first time ionic liquid (IL)-assisted FDM for direct printing of β-PVDF piezoelectric devices. An IL can induce and maintain β crystals during melt extrusion and FDM printing, ensuring that the β-crystal in the printed PVDF device is as high as 98.3%, which is the highest in 3D-printed PVDF as far as we know. Furthermore, the shearing force provided by the FDM facilitates the directional arrangement of the dipoles, resulting in the printed PVDF device having self-polarization characteristics without poling. Finally, the piezoelectric output voltage of the 3D-printed PVDF device is 4.7 times that of the flat PVDF device, and its area current density (17.5 nA cm^-2) is more than that of the reported 3D-printed PVDF piezoelectric device in the literature by two orders of magnitude. The one-step 3D printing strategy proposed in this paper can realize the rapid preparation of complex-shaped and lightweight self-polarized β-PVDF-based piezoelectric devices for energy harvesting.

Piezoelectric Nanogenerator Based on Lead-Free Flexible PVDF-Barium Titanate Composite Films for Driving Low Power Electronics

Self-powered sensor development is moving towards miniaturization and requires a suitable power source for its operation. The piezoelectric nanogenerator (PENG) is a potential candidate to act as a partial solution to suppress the burgeoning energy demand. The present work is focused on the development of the PENG based on flexible polymer-ceramic composite films. The X-ray spectra suggest that the BTO particles have tetragonal symmetry and the PVDF-BTO composite films (CF) have a mixed phase. The dielectric constant increases with the introduction of the particles in the PVDF polymer and the loss of the CF is much less for all compositions. The BTO particles have a wide structural diversity and are lead-free, which can be further employed to make a CF. An attempt was made to design a robust, scalable, and cost-effective piezoelectric nanogenerator based on the PVDF-BTO CFs. The solvent casting route was a facile approach, with respect to spin coating, electrospinning, or sonication routes. The introduction of the BTO particles into PVDF enhanced the dielectric constant and polarization of the composite film. Furthermore, the single-layered device output could be increased by strategies such as internal polarization amplification, which was confirmed with the help of the polarization-electric field loop of the PVDF-BTO composite film. The piezoelectric nanogenerator with 10 wt% BTO-PVDF CF gives a high electrical output of voltage 7.2 V, current 38 nA, and power density of 0.8 μW/cm2 at 100 MΩ. Finally, the energy harvesting using the fabricated PENG is done by various actives like coin dropping, under air blowing, and finger tapping. Finally, low-power electronics such as calculator is successfully powered by charging a 10 μF capacitor using the PENG device.

Characterization of Polyvinylidene Fluoride (PVDF) Electrospun Fibers Doped by Carbon Flakes

Polyvinylidene fluoride (PVDF) is a modern polymer material used in a wide variety of ways. Thanks to its excellent resistance to chemical or thermal degradation and low reactivity, it finds use in biology, chemistry, and electronics as well. By enriching the polymer with an easily accessible and cheap variant of graphite, it is possible to affect the ratio of crystalline phases. A correlation between the ratios of crystalline phases and different properties, like dielectric constant as well as piezo- and triboelectric properties, has been found, but the relationship between them is highly complex. These changes have been observed by a number of methods from structural, chemical and electrical points of view. Results of these methods have been documented to create a basis for further research and experimentation on the usability of this combined material in more complex structures and devices.

Electroactive γ-Phase, Enhanced Thermal and Mechanical Properties and High Ionic Conductivity Response of Poly (Vinylidene Fluoride)/Cellulose Nanocrystal Hybrid Nanocomposites

Cellulose nanocrystals (CNCs) were incorporated into poly (vinylidene fluoride) (PVDF) to tailor the mechanical and dielectric properties of this electroactive polymer. PVDF/CNC nanocomposites with concentrations up to 15 wt.% were prepared by solvent-casting followed by quick vacuum drying in order to ensure the formation of the electroactive γ-phase. The changes induced by the presence of CNCs on the morphology of PVDF and its crystalline structure, thermal properties, mechanical performance and dielectric behavior are explored. The results suggest a relevant role of the CNC surface -OH groups, which interact with PVDF fluorine atoms. The real dielectric constant ε' of nanocomposites at 200 Hz was found to increase by 3.6 times up to 47 for the 15 wt.% CNC nanocomposite due to an enhanced ionic conductivity provided by CNCs. The approach reported here in order to boost the formation of the γ-phase of PVDF upon the incorporation of CNCs serves to further develop cellulose-based multifunctional materials.

Development of self-poled PVDF/MWNT flexible nanocomposites with a boosted electroactive β-phase

In the present study, we report a simple fabrication method for poly(vinylidene fluoride) PVDF/MWCNT flexible nanocomposite films with a boosted electroactive phase that enhanced the dielectric and piezoelectric properties.

Effect of Cellulose Nanofibrils and TEMPO-mediated Oxidized Cellulose Nanofibrils on the Physical and Mechanical Properties of Poly(vinylidene fluoride)/Cellulose Nanofibril Composites

Cellulose nanofibrils (CNFs) are high aspect ratio, natural nanomaterials with high mechanical strength-to-weight ratio and promising reinforcing dopants in polymer nanocomposites. In this study, we used CNFs and oxidized CNFs (TOCNFs), prepared by a 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation process, as reinforcing agents in poly(vinylidene fluoride) (PVDF). Using high-shear mixing and doctor blade casting, we prepared free-standing composite films loaded with up to 5 wt % cellulose nanofibrils. For our processing conditions, all CNF/PVDF and TOCNF/PVDF films remain in the same crystalline phase as neat PVDF. In the as-prepared composites, the addition of CNFs on average increases crystallinity, whereas TOCNFs reduces it. Further, addition of CNFs and TOCNFs influences properties such as surface wettability, as well as thermal and mechanical behaviors of the composites. When compared to neat PVDF, the thermal stability of the composites is reduced. With regards to bulk mechanical properties, addition of CNFs or TOCNFs, generally reduces the tensile properties of the composites. However, a small increase (~18%) in the tensile modulus was observed for the 1 wt % TOCNF/PVDF composite. Surface mechanical properties, obtained from nanoindentation, show that the composites have enhanced performance. For the 5 wt % CNF/PVDF composite, the reduced modulus and hardness increased by ~52% and ~22%, whereas for the 3 wt % TOCNF/PVDF sample, the increase was ~23% and ~25% respectively.

PVDF-MWNT interactions control process induced β-lamellar morphology and orientation in the nanocomposites

The effect of methylene blue (MB) modified multiwall carbon nanotubes (MWNTs) on the nucleation and morphology of polyvinylidene fluoride (PVDF) in comparison with the effect of MWNTs was systematically assessed by DSC, 13C NMR, FT-IR, TEM, WAXS and SAXS analysis. TEM analysis of ultra-microtomed samples revealed that MB modification enhanced the dispersibility of MWNTs in PVDF. Further, the nanocomposites were subjected to mechanical rolling and the synergistic effect of processing and fillers on the PVDF morphology (before and after rolling) at different length scales was studied. Both FT-IR and WAXS analyses suggested that mechanical rolling transforms α-PVDF to β-PVDF (ca. 88%). TEM and two-dimensional WAXS analyses revealed that the MWNTs and β-crystallites are oriented preferentially along the rolling direction and the degree of orientation is not influenced by the fillers suggesting that crystallite orientation is fully controlled by mechanical rolling. On the other hand, β-lamellae showed perpendicular orientation with respect to the rolling direction. Unlike β-crystallites, the β-lamellar morphology and orientation are highly governed by the fillers as evident from SAXS analysis. Using MWNTs and the MWNT-MB π-complex, we demonstrate that the β-lamellar morphology and degree of orientation are controlled by the extent of interaction of fillers with PVDF. Interestingly, both β-lamellar morphology and degree of orientation correlate well with the mechanical properties of the rolled PVDF. More specifically, the dynamic storage modulus of the samples in the rolling direction increases with increasing β-lamellar morphology and degree of orientation. The present work demonstrates that the polymer-filler interaction plays a crucial role in regulating the processed polymer morphology and can be tuned by appropriately modifying the surface of fillers through either covalent or non-covalent interactions.

Crystal polymorphism in polydiacetylene-embedded electrospun polyvinylidene fluoride nanofibers

In this study, polydiacetylene (PDA) is embedded in electrospun polyvinylidene fluoride (PVDF) nanofibers for the preparation of mats with dual colorimetric and piezoelectric responses. The diacetylene monomers are self-assembled during the electrospinning process. The PDA-embedded PVDF nanofibers in the blue phase are obtained via photo-polymerization upon UV-light irradiation. The colorimetric transition of the nanofibers is studied as a function of temperature using a spectrophotometer. The morphology and crystal polymorphism of the nanofibers are investigated. The results show that the addition of PDA increases the diameter of the nanofibers due to the increase in the electrospinning solution viscosity. The results of Fourier transform infrared and wide angle X-ray diffraction demonstrate that PDA has the effect of inhibiting the growth of non-polar α-phase crystals, while promoting the growth of the polar β-phase. However, the red phase of PDA-embedded PVDF exhibits a lower intensity of the β-phase in comparison to that of the blue phase. In fact, the blue-to-red color transition of the PDA-embedded electrospun PVDF nanofibers is accompanied by the variation of piezoelectric signaling caused by variations in the β-phase. This phenomenon creates great potential in commercial detection sensors in addition to their colorimetric detection properties.

One-Step Solvent Evaporation-Assisted 3D Printing of Piezoelectric PVDF Nanocomposite Structures

Development of a 3D printable material system possessing inherent piezoelectric properties to fabricate integrable sensors in a single-step printing process without poling is of importance to the creation of a wide variety of smart structures. Here, we study the effect of addition of barium titanate nanoparticles in nucleating piezoelectric β-polymorph in 3D printable polyvinylidene fluoride (PVDF) and fabrication of the layer-by-layer and self-supporting piezoelectric structures on a micro- to millimeter scale by solvent evaporation-assisted 3D printing at room temperature. The nanocomposite formulation obtained after a comprehensive investigation of composition and processing techniques possesses a piezoelectric coefficient, d₃₁, of 18 pC N^-1, which is comparable to that of typical poled and stretched commercial PVDF film sensors. A 3D contact sensor that generates up to 4 V upon gentle finger taps demonstrates the efficacy of the fabrication technique. Our one-step 3D printing of piezoelectric nanocomposites can form ready-to-use, complex-shaped, flexible, and lightweight piezoelectric devices. When combined with other 3D printable materials, they could serve as stand-alone or embedded sensors in aerospace, biomedicine, and robotic applications.