Crystallinity and β Phase Fraction of PVDF in Biaxially Stretched PVDF/PMMA Films

Tuning Dielectric Properties with Nanofiller Dimensionality in Polymer Nanocomposites

Polymer nanocomposites hold great potential as dielectrics for energy storage devices and flexible electronics. The structural architecture of the nanofillers is expected to play a crucial role in the fundamental mechanisms governing the electrical breakdown and dielectric properties of the nanocomposites. However, the effect of nanofiller structure and dimensionality on these properties has not been studied thoroughly to date. This study explores the critical relationship between nanofiller dimensionality and dielectric properties in polymer nanocomposites. We fabricated polyvinylidene fluoride (PVDF) nanocomposites by incorporating a range of carbon-based nanofillers separately, including zero-dimensional (0D) carbon black (CB), one-dimensional (1D) multiwalled carbon nanotubes (MWCNT), 1D single-walled carbon nanotubes (SWCNT), two-dimensional (2D) reduced graphene oxide (rGO), and three-dimensional (3D) graphite. The frequency-dependent (1 kHz to 1 MHz) dielectric permittivity (k) of the nanocomposites at the same concentration of nanofillers demonstrated a hierarchical order, with MWCNT showing the highest permittivity (∼400%), succeeded by rGO (∼360%), CB (∼290%), SWCNT (∼230%), and graphite (∼70%), respectively. The temperature-dependent (50-150 °C) dielectric spectroscopy revealed high k with increasing temperature due to the enhanced dipole movement. However, their dielectric breakdown strength and energy densities were not correlated to k and exhibited the following order: SWCNT > MWCNT > CB > rGO > graphite. As the electrical breakdown depends upon the nanocomposites' mechanical strength, we correlated the mechanical properties with the nanofiller dimensionality, and Young's modulus followed the 1D ≈ 2D > 0D > 3D order. These findings will provide fundamental insights into designing tunable, conducive nanofiller-based nanocomposites in next-generation flexible electronics and capacitive energy storage devices.

Recent Progress in the Auxiliary Phase Enhanced Flexible Piezocomposites

Piezocomposites with both flexibility and electromechanical conversion characteristics have been widely applied in various fields, including sensors, energy harvesting, catalysis, and biomedical treatment. In the composition of piezocomposites or their preparation process, a category of materials is commonly employed that do not possess piezoelectric properties themselves but play a crucial role in performance enhancement. In this review, the concept of auxiliary phase is first proposed to define these materials, aiming to provide a new perspective for designing high‐performance piezocomposites. Three different categories of modulation forms of auxiliary phase in piezocomposites are systematically summarized, including the modification of piezo‐matrix, the modification of piezo‐fillers, and the construction of special structures. Each category emphasizes the role of the auxiliary phase and systematically discusses the latest advancements and the physical mechanisms of the auxiliary phase enhanced flexible piezocomposites. Finally, a summary and future outlook of piezocomposites based on the auxiliary phase are provided.

Graphene-Doped Piezoelectric Transducers by Kriging Optimal Model for Detecting Various Types of Laryngeal Movements

This study fabricated piezoelectric fibers of polyvinylidene fluoride (PVDF) with graphene using near-field electrospinning (NFES) technology. A uniform experimental design table U*774 was applied, considering weight percentage (1-13 wt%), the distance between needle and disk collector (2.1-3.9 mm), and applied voltage (14.5-17.5 kV). We optimized the parameters using electrical property measurements and the Kriging response surface method. Adding 13 wt% graphene significantly improved electrical conductivity, increasing from 17.7 µS/cm for pure PVDF to 187.5 µS/cm. The fiber diameter decreased from 21.4 µm in PVDF/1% graphene to 9.1 µm in PVDF/13% graphene. Adding 5 wt% graphene increased the β-phase content by 6.9%, reaching 65.4% compared to pure PVDF fibers. Material characteristics were investigated using scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction analysis (XRD), contact angle measurements, and tensile testing. Optimal parameters included 3.47 wt% graphene, yielding 15.82 mV voltage at 5 Hz and 5 N force (2.04 times pure PVDF). Force testing showed a sensitivity (S) of 7.67 log(mV/N). Fibers were attached to electrodes for piezoelectric sensor applications. The results affirmed enhanced electrical conductivity, piezoelectric performance, and mechanical strength. The optimized piezoelectric sensor could be applied to measure physiological signals, such as attaching it to the throat under different conditions to measure the output voltage. The force-to-voltage conversion facilitated subsequent analysis.

Alloying One-Dimensional Coordination Polymers To Create Ductile Materials

The preparation of coordination polymer (CP) alloys is demonstrated by the use of two meltable, one-dimensional crystal structures via melt-kneading. The polymer structures of the alloys are studied by synchrotron X-ray absorption and scattering, solid-state NMR spectroscopy, DSC, and viscoelastic measurements. Crystalline and amorphous domains and thermal properties (melting and glass transition) in the alloys depend on the ratio of the two constituent CPs. The glassy alloy composed of an equivalent amount of two CPs shows high plastic deformation properties, and the fracture point reaches 128% without a filler or compatibilizing agent, hence behaving as ductile materials.

High-Resolution X-ray Image from Copper-Based Perovskite Hybrid Polymer

Cs3Cu2I5 nanocrystals (NCs) are considered to be promising materials due to their high photoluminescence efficiency, lack of lead toxicity, and X-ray responsiveness. However, during the crystallization process, NCs are prone to agglomeration and exhibit uneven size distribution, resulting in several light scattering that severely affect their imaging resolution. Herein, we successfully developed a high-resolution scintillator film by growing copper-based perovskite NCs within a hybrid polymer matrix. By leveraging the ingenious integration of polyvinylidene fluoride (PVDF) and polymethyl methacrylate (PMMA), the size and distribution uniformity of Cs3Cu2I5 NCs can be effectively controlled. Consequently, a high spatial resolution of 14.3 lp mm-1 and a low detection limit of 105 nGy s-1 are achieved, and the scintillator film has excellent flexibility and stability. These results highlight the promising application of Cs3Cu2I5 scintillator films in low-cost, flexible, and high-performance medical imaging.

β-Phase Crystallinity, Printability, and Piezoelectric Characteristics of Polyvinylidene Fluoride (PVDF)/Poly(methyl methacrylate) (PMMA)/Cyclopentyl-Polyhedral Oligomeric Silsesquioxane (Cp-POSS) Melt-Compounded Blends

Poly(vinylidene fluoride) (PVDF) is a semicrystalline polymer that exhibits unique piezoelectric characteristics along with good chemical resistance and high thermal stability. Layer-based material extrusion (MEX) 3D printing of PVDF is desired to create complex structures with piezoelectric properties; however, the melt processing of PVDF typically directs the formation of the α crystalline allomorph, which does not contribute to the piezoelectric response. In this work, PVDF was compounded with poly(methyl methacrylate) (PMMA) and cyclopentyl-polyhedral oligomeric silsesquioxane (Cp-POSS) nanostructured additives in binary and ternary blends to improve MEX printability while maintaining piezoelectric performance. Overall crystallinity and β phase content were evaluated and quantified using a combination of attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and differential scanning calorimetry (DSC). Enhancement of MEX printability was measured by quantifying the interlayer adhesion and warpage of printed parts. All blends studied contained a significant percentage of β allomorph, but it could be detected by ATR-FTIR only after the removal of a thin surface layer. Inclusion of 1% Cp-POSS and up to 10% PMMA in blends with PVDF improved interlayer adhesion (2.3-3.6x) and lowered warpage of MEX printed parts compared to neat PVDF. The blend of 1% Cp-POSS/1% PMMA/PVDF was demonstrated to significantly improve the quality of MEX printed parts while showing similar piezoelectric performance to that of neat PVDF (average piezoelectric coefficient 24 pC/N). MEX printing of PVDF blends directly into usable parts with significant piezoelectric performance while reducing the challenges of printing the semicrystalline polymer opens the potential for application in a number of high value sectors.

MXene-Based Nanocomposites for Piezoelectric and Triboelectric Energy Harvesting Applications

Due to its superior advantages in terms of electronegativity, metallic conductivity, mechanical flexibility, customizable surface chemistry, etc., 2D MXenes for nanogenerators have demonstrated significant progress. In order to push scientific design strategies for the practical application of nanogenerators from the viewpoints of the basic aspect and recent advancements, this systematic review covers the most recent developments of MXenes for nanogenerators in its first section. In the second section, the importance of renewable energy and an introduction to nanogenerators, major classifications, and their working principles are discussed. At the end of this section, various materials used for energy harvesting and frequent combos of MXene with other active materials are described in detail together with the essential framework of nanogenerators. In the third, fourth, and fifth sections, the materials used for nanogenerators, MXene synthesis along with its properties, and MXene nanocomposites with polymeric materials are discussed in detail with the recent progress and challenges for their use in nanogenerator applications. In the sixth section, a thorough discussion of the design strategies and internal improvement mechanisms of MXenes and the composite materials for nanogenerators with 3D printing technologies are presented. Finally, we summarize the key points discussed throughout this review and discuss some thoughts on potential approaches for nanocomposite materials based on MXenes that could be used in nanogenerators for better performance.

Interfacial Engineering of PVDF-TrFE toward Higher Piezoelectric, Ferroelectric, and Dielectric Performance for Sensing and Energy Harvesting Applications

The electrical properties of pristine fluoropolymers are inferior due to their low polar crystalline phase content and rigid dipoles that tend to retain their fixed moment and orientation. Several strategies, such as electrospinning, electrohydrodynamic pulling, and template-assisted growing, have been proven to enhance the electrical properties of fluoropolymers; however, these techniques are mostly very hard to scale-up and expensive. Here, a facile interfacial engineering approach based on amine-functionalized graphene oxide (AGO) is proposed to manipulate the intermolecular interactions in poly(vinylidenefluoride-trifluoroethylene) (PVDF-TrFE) to induce β-phase formation, enlarge the lamellae dimensions, and align the micro-dipoles. The coexistence of primary amine and hydroxyl groups on AGO nanosheets offers strong hydrogen bonding with fluorine atoms, which facilitates domain alignment, resulting in an exceptional remnant polarization of 11.3 µC cm^-2 . PVDF-TrFE films with 0.1 wt.% AGO demonstrate voltage coefficient, energy density, and energy-harvesting figure of merit values of 0.30 Vm N^-1 , 4.75 J cm^-3 , and 14 pm³  J^-1 , respectively, making it outstanding compared with state-of-the-art ceramic-free ferroelectric films. It is believed that this work can open-up new insights toward structural and morphological tailoring of fluoropolymers to enhance their electrical and electromechanical performance and pave the way for their industrial deployment in next-generation wearables and human-machine interfaces.

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).

A study of degradation mechanisms in PVDF-based photovoltaic backsheets

Commercial backsheets based on polyvinylidene fluoride (PVDF) can experience premature field failures in the form of outer layer cracking. This work seeks to provide a better understanding of the changes in material properties that lead to crack formation and find appropriate accelerated tests to replicate them. The PVDF-based backsheet outer layer can have a different structure and composition, and is often blended with a poly(methyl methacrylate) (PMMA) polymer. We observed depletion of PMMA upon aging with sequential (MAST) and combined (C-AST) accelerated stress testing. In field-aged samples from Arizona and India, where PVDF crystallizes in its predominant α-phase, the degree of crystallinity greatly increased. MAST and C-AST protocols were, to some extent, able to replicate the increase in crystallinity seen in PVDF after ~ 7 years in the field, but no single-stress test condition (UV, damp heat, thermal cycling) resulted in significant changes in the material properties. The MAST regimen used here was too extreme to produce realistic degradation, but the test was useful in discovering weaknesses of the particular PVDF-based outer layer structure studied. No excessive β-phase formation was observed after aging with any test condition; however, the presence of β-phase was identified locally by Fourier transform infrared spectroscopy (FTIR). We conclude that both MAST and C-AST are relevant tests for screening outdoor failure mechanisms in PVDF backsheets, as they were successful in producing material degradation that led to cracking.

Data Mining of Polymer Phase Transitions upon Temperature Changes by Small and Wide-Angle X-ray Scattering Combined with Raman Spectroscopy

The complex physical transformations of polymers upon external thermodynamic changes are related to the molecular length of the polymer and its associated multifaceted energetic balance. The understanding of subtle transitions or multistep phase transformation requires real-time phenomenological studies using a multi-technique approach that covers several length-scales and chemical states. A combination of X-ray scattering techniques with Raman spectroscopy and Differential Scanning Calorimetry was conducted to correlate the structural changes from the conformational chain to the polymer crystal and mesoscale organization. Current research applications and the experimental combination of Raman spectroscopy with simultaneous SAXS/WAXS measurements coupled to a DSC is discussed. In particular, we show that in order to obtain the maximum benefit from simultaneously obtained high-quality data sets from different techniques, one should look beyond traditional analysis techniques and instead apply multivariate analysis. Data mining strategies can be applied to develop methods to control polymer processing in an industrial context. Crystallization studies of a PVDF blend with a fluoroelastomer, known to feature complex phase transitions, were used to validate the combined approach and further analyzed by MVA.