Vibrational analysis of phase III of poly (vinylidene fluoride)

Optical Properties of Composites Based on Poly(o-phenylenediamine), Poly(vinylenefluoride) and Double-Wall Carbon Nanotubes

In this work, synthesis and optical properties of a new composite based on poly(o-phenylenediamine) (POPD) fiber like structures, poly(vinylidene fluoride) (PVDF) spheres and double-walled carbon nanotubes (DWNTs) are reported. As increasing the PVDF weight in the mixture of the chemical polymerization reaction of o-phenylenediamine, the presence of the PVDF spheres onto the POPD fibers surface is highlighted by scanning electron microscopy (SEM). The down-shift of the Raman line from 1421 cm⁻¹ to 1415 cm⁻¹ proves the covalent functionalization of DWNTs with the POPD-PVDF blends. The changes in the absorbance of the IR bands peaked around 840, 881, 1240 and 1402 cm⁻¹ indicate hindrance steric effects induced of DWNTs to the POPD fiber like structures and the PVDF spheres, as a consequence of the functionalization process of carbon nanotubes with macromolecular compounds. The presence of the PVDF spheres onto the POPD fiber like structures surface induces a POPD photoluminescence (PL) quenching process. An additional PL quenching process of the POPD-PVDF blends is reported to be induced in the presence of DWNTs. The studies of anisotropic PL highlight a change of the angle of the binding of the PVDF spheres onto the POPD fiber like structures surface from 50.2° to 38° when the carbon nanotubes concentration increases in the POPD-PVDF/DWNTs composites mass up to 2 wt.%.

Influence of Different Solvents and High-Electric-Field Cycling on Morphology and Ferroelectric Behavior of Poly(Vinylidene Fluoride-Hexafluoropropylene) Films

P(VdF-HFP) films are fabricated via a solution casting doctor blade method using high (HVS) and low (LVS) volatile solvents, respectively. The structural properties and the ferroelectric behavior are investigated. The surface structure and crystal phase composition are found to be strongly dependent on the type of solvent. LVS leads to a rougher copolymer surface structure with large spherulites and a lower crystallinity in contrast with HVS. The crystalline phase of copolymer films fabricated with HVS consists almost exclusively of α-phase domains, whereas films from LVS solution show a large proportion of γ-phase domains, as concluded from Raman and X-ray diffraction spectra. Virgin films show no ferroelectric (FE) switching polarization at electric field amplitudes below 180 MV/m, independent of the solvent type, observed in bipolar dielectric displacement-electric field measurements. After applying electric fields of above 180 MV/m, a FE behavior emerges, which is significantly stronger for LVS films. In a repeated measurement, FE polarization switching already occurs at lower fields. A shielding effect may be related to this observation. Additionally, Raman bands of polar γ-phase increase by high-electric-field cycling for the LVS sample. The solvent used and the resulting crystal phase composition of the virgin sample is crucial for the copolymer behavior during bipolar electrical cycling.

Multifunctional Membrane for Thermal Management Applications

Space cooling and heating consume a large proportion of global energy, so passive thermal management materials (i.e., without energy input), especially dual-mode materials including cooling and heating bifunctions, are becoming more and more attractive in many areas. Herein, a function-switchable Janus membrane between cooling and heating consisting of a multilayer structure of polyvinylidene fluoride nanofiber/zinc oxide nanosheet/carbon nanotube/Ag nanowire/polydimethylsiloxane was fabricated for comprehensive thermal management applications. In the cooling mode, the high thermal radiation emissivity (89.2%) and sunlight reflectivity (90.6%) of the Janus membrane resulted in huge temperature drops of 8.2-12.6, 9.0-14.0, and 10.9 °C for a substrate, a closed space, and a semiclosed space, respectively. When switching to the heating mode, temperature rises of 3.8-4.6, 4.0-4.8, and 12.5 °C for the substrate, closed space, and semiclosed space, respectively, were achieved owing to the high thermal radiation reflectivity (89.5%) and sunlight absorptivity (74.1%) of the membrane. Besides, the Janus membrane has outstanding comprehensive properties of the membrane, including infrared camouflaging/disguising, electromagnetic shielding (53.1 dB), solvent tolerance, waterproof properties, and high flexibility, which endow the membrane with promising application prospects.

P(VDF-TrFE) nanofibers: structure of the ferroelectric and paraelectric phases through IR and Raman spectroscopies

This study elucidates the complex morphology and the related spectroscopic response of poly(vinylidene fluoride-co-trifluoroethylene) copolymer, with 80% molar VDF content, namely P(VDF-TrFE) (80/20). We investigate the molecular structure, the morphology and the thermal behaviour of P(VDF-TrFE) samples obtained as electrospun nanofibers; we discuss their thermal evolution crossing the Curie temperature and the structure resulting after annealing, giving a comparison with P(VDF-TrFE) films. The new experimental data here obtained, combined with previous spectroscopic studies carried out on piezoelectric fluorinated polymers and copolymers, allow identifying spectroscopic markers sensitive to the molecular structure, the molecular orientation, the conformational defects and the kind of crystalline phase. We assign the vibrational modes localized on TrFE units by combining experimental observation and density functional calculations carried out on suitable molecular models. This work provides a sound set of diagnostic tools, which can be exploited for the assessment of structure/property relationships aimed at clarifying the molecular mechanisms leading to the piezoelectric performance of fluorinated copolymers.

Bioinspired Multiresonant Acoustic Devices Based on Electrospun Piezoelectric Polymeric Nanofibers

Cochlear hair cells are critical for the conversion of acoustic into electrical signals and their dysfunction is a primary cause of acquired hearing impairments, which worsen with aging. Piezoelectric materials can reproduce the acoustic-electrical transduction properties of the cochlea and represent promising candidates for future cochlear prostheses. The majority of piezoelectric hearing devices so far developed are based on thin films, which have not managed to simultaneously provide the desired flexibility, high sensitivity, wide frequency selectivity, and biocompatibility. To overcome these issues, we hypothesized that fibrous membranes made up of polymeric piezoelectric biocompatible nanofibers could be employed to mimic the function of the basilar membrane, by selectively vibrating in response to different frequencies of sound and transmitting the resulting electrical impulses to the vestibulocochlear nerve. In this study, poly(vinylidene fluoride-trifluoroethylene) piezoelectric nanofiber-based acoustic circular sensors were designed and fabricated using the electrospinning technique. The performance of the sensors was investigated with particular focus on the identification of the resonance frequencies and acoustic-electrical conversion in fibrous membrane with different size and fiber orientation. The voltage output (1-17 mV) varied in the range of low resonance frequency (100-400 Hz) depending on the diameter of the macroscale sensors and alignment of the fibers. The devices developed can be regarded as a proof-of-concept demonstrating the possibility of using piezoelectric fibers to convert acoustic waves into electrical signals, through possible synergistic effects of piezoelectricity and triboelectricity. The study has paved the way for the development of self-powered nanofibrous implantable auditory sensors.

Lithium niobate nanoparticles as biofunctional interface material for inner ear devices

Sensorineural hearing loss (SNHL) affects the inner ear compartment and can be caused by different factors. Usually, the lack, death, or malfunction of sensory cells deputed to transduction of mechanic-into-electric signals leads to SNHL. To date, the therapeutic option for patients impaired by severe or profound SNHL is the cochlear implant (CI), a high-tech electronic device replacing the entire cochlear function. Piezoelectric materials have catalyzed attention to stimulate the auditory neurons by simply mimicking the function of the cochlear sensory epithelium. In this study, the authors investigated lithium niobate (LiNbO₃) as a potential candidate material for next generation CIs. LiNbO₃ nanoparticles resulted otocompatible with inner ear cells in vitro, had a pronounced immunomodulatory activity, enhanced human beta-defensin in epithelial cells, and showed direct antibacterial activity against P. aeruginosa. Moreover, LiNbO₃ nanoparticles were incorporated into poly(vinylidene fluoride-trifluoro ethylene) fibers via electrospinning, which enhanced the piezoelectric response. Finally, the resulting fibrous composite structures support human neural-like cell growth in vitro, thus showing promising features to be used in new inner ear devices.

Effect of Interfacial Interaction on the Conformational Variation of Poly(vinylidene fluoride) (PVDF) Chains in PVDF/Graphene Oxide (GO) Nanocomposite Fibers and Corresponding Mechanical Properties

Poly(vinylidene fluoride) (PVDF)/graphene oxide (GO) nanocomposite fibers were dry-jet wet spun at the GO concentrations of 0, 1, and 2 wt % with respect to the polymer. The as-spun fibers were drawn in the draw ratio (DR) range of 2-6.5, and the correlation between the PVDF chain conformation and the mechanical properties of the fibers upon drawing has been studied by two-dimensional correlation spectroscopy of Fourier-transformed infrared, wide-angle X-ray diffraction, differential scanning calorimetry, and tensile testing. The PVDF/GO nanocomposite fibers exhibited that the mobile PVDF crystals due to the conformational defects and kinks were nucleated because of the polar interaction between PVDF chains and functional groups of GO, whereas the control PVDF fiber showed the conventional conversion of crystal polymorphs (α and γ phases to β phase). As a result, the nanocomposite fiber showed dramatically improved toughness (enhanced by 1123% at a DR of 2 and 120% at a DR of 6.5) as compared to that of the control fiber. Furthermore, the tensile strength and modulus of the PVDF/GO (2 wt %) fiber were 394 MPa and 4.6 GPa, respectively, whereas those of the control PVDF fiber were 295 MPa and 3.9 GPa, respectively.

Magnetoelectric nanocomposite scaffold for high yield differentiation of mesenchymal stem cells to neural-like cells

While the differentiation factors have been widely used to differentiate mesenchymal stem cells (MSCs) into various cell types, they can cause harm at the same time. Therefore, it is beneficial to propose methods to differentiate MSCs without factors. Herein, magnetoelectric (ME) nanofibers were synthesized as the scaffold for the growth of MSCs and their differentiation into neural cells without factors. This nanocomposite takes the advantage of the synergies of the magnetostrictive filler, CoFe₂ O ₄ nanoparticles (CFO), and piezoelectric polymer, polyvinylidene difluoride (PVDF). Graphene oxide nanosheets were decorated with CFO nanoparticles for a proper dispersion in the polymer through a hydrothermal process. After that, the piezoelectric PVDF polymer, which contained the magnetic nanoparticles, underwent the electrospun process to form ME nanofibers, the ME property of which has the potential to be used in areas such as tissue engineering, biosensors, and actuators.

Molecular Orientation in Electrospun Poly(vinylidene fluoride) Fibers

Electrospun poly(vinylidene fluoride) (PVDF) nanofibers were collected on aluminum foil across a 10 mm gap. Scanning electron microscopy (SEM) images showed that fibers in the gap were macroscopically aligned and those off the gap were macroscopically randomly aligned. Polarized Fourier transform infrared (FTIR) spectra and single fiber selected area electron diffraction (SAED) patterns demonstrated that fibers deposited in the gap were highly aligned at the molecular level with the polymer backbones oriented along the fiber axis. SAED patterns of fibers deposited off the gap were also oriented at the molecular level, but the degree of orientation was lower.

Greatly enhanced energy density and patterned films induced by photo cross-linking of poly(vinylidene fluoride-chlorotrifluoroethylene)

Greatly enhanced energy density in poly(vinylidene fluoride-chlorotrifluoroethylene) [P(VDF-CTFE)] is realized through interface effects induced by a photo cross-linking method. Being different from nanocomposites with lowered dielectric strength, the cross-linked P(VDF-CTFE)s possess a high breakdown field as well as remarkably elevated polarization, both of which contribute to the enhanced energy density as high as 22.5 J · cm(-3). Moreover, patterned thin films with various shapes and sizes are fabricated by photolithography, which sheds new light on the integration of PVDF-based electroactive polymers into organic microelectronic devices such as flexible pyroelectric/piezoelectric sensor arrays or non-volatile ferroelectric memory devices.

Fourier transform infrared spectroscopic imaging of anisotropic poly(vinylidene fluoride) films with polarized radiation

The technique of Fourier transform infrared (FT-IR) spectroscopic imaging with focal plane array detectors has proved to be a powerful technique for rapid chemical visualization of samples with a lateral resolution up to about 10 mum. However, the potential of FT-IR imaging for the characterization of anisotropic materials can be significantly enhanced by using polarized radiation. This issue will be addressed in the present communication, which reports for the first time imaging investigations based on the FT-IR polarization spectra of poly(vinylidene fluoride) films that have been uniaxially elongated below and above the threshold temperature of the II(alpha) --> I(beta) phase transition.

Two-dimensional correlation infrared spectroscopic study on the crystallization and gelation of poly(vinylidene fluoride) in cyclohexanone

Poly(vinylidene fluoride) (PVDF) converts easily into a thermo-reversible gel through crystallization by standing at room temperature in cyclohexanone. In this study, the Fourier transform infrared (FT-IR) spectra were measured continuously at room temperature during the conversion of the solution into a gel. The IR difference spectra derived from these spectra by absorbance subtraction clearly indicate the presence of PVDF alpha-crystallites in the gel due to the presence of absorption bands corresponding to the TG+TG- conformation of the alpha-phase. In the time interval from 25 to 45 min after the beginning of the experiment, the IR bands of PVDF increased dramatically, indicating the conversion of polymer chains from random statistical coils to the ordered TG+TG- conformation (alpha-form). In the time interval from 45 to 90 min, the IR bands of PVDF increased slowly, reflecting no further crystallization. Using two-dimensional (2D) IR analysis, it could be shown that the nu(C=O) absorption band of cyclohexanone changed during the gelation process. During the conformational ordering process (25-45 min), the nu(C=O) absorption band of the cyclohexanone dimer (1707 cm(-1)) decreased while the corresponding band of the monomer at 1718 cm(-1) increased. Furthermore, a new band at 1695 cm(-1) increased, which could be assigned to C=O groups of the solvent interacting with the CF2 groups in the polymer chain. The bands of the crystalline PVDF share positive cross-peaks with the bands of cyclohexanone, which indicates that the chain of PVDF changed prior to the cyclohexanone molecules during the conformational ordering process. However, these positive cross-peaks disappeared during the crystallization process, which means that the chain of PVDF changed synchronously with the solvent molecules. As for the bands of PVDF chains, the band at 762 cm(-1) varied prior to the bands at 873 cm(-1) and 796 cm(-1) during the conformational ordering process. The 762 cm(-1) absorption is assigned to the CF2 group of PVDF, the 873 cm(-1) absorption involves the C-C group of PVDF, and the 796 cm(-1) band is attributed to the CH2 groups of PVDF. Thus, the CF2 functionalities change faster than the C-C and CH2 groups. However, the correlation cross-peaks between 762 cm(-1) and 873 cm(-1) and at 796 cm(-1) disappeared during the later state of the gelation process. At the same time, the bands of PVDF and solvent still varied, which suggests that it is a physical interaction process between PVDF chain and solvent.