Nanostructures of polyvinyledenedifluoride-tetrafluoroethylene (PVDF-TrFE), a semicrystalline polymer with high piezoelectricity, results in significant enhancement of crystallinity and better device performance as sensors, actuators, and energy harvesters. Using electrospinning of PVDF to manufacture nanofibers, we demonstrate a new method to pattern high-density, highly aligned nanofibers. To further boost the charge transfer from such a bundle of nanofibers, we fabricated novel core-shell structures. Finally, we developed pressure sensors utilizing these fiber structures for endovascular applications. The sensors were tested in vitro under simulated physiological conditions. We observed significant improvements using core-shell electrospun fibers (4.5 times gain in signal intensity, 4000 μV/mmHg sensitivity) over PVDF nanofibers (280 μV/mmHg). The preliminary results showed that core-shell fiber-based devices exhibit nearly 40-fold higher sensitivity, compared to the thin-film structures demonstrated earlier.
The exploitation of soft conducting polymer-based actuators suffers from two main shortcomings: their short life cycle and the reproducibility of the fabrication techniques. The short life cycle usually results from the delamination of the components due to stresses at the interface during the actuation. In this work, to achieve strong adhesion to poly(3,4- ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) electrodes, the wetting properties of the surface of a polyvinylidene fluoride (PVDF) membrane are improved using argon-plasma-induced surface polymerization of poly(ethylene glycol) monomethyl ether methacrylate (PEGMA). Hybrid membranes are created with hydrophilic PVDF-graft-PEGMA outer surfaces and hydrophobic bulk. The width of each layer is controlled by spray coating, as it allows for the deposition of the reaction precursor to a certain depth. Subsequently, a PEDOT:PSS water solution fills the pores of the functionalized part of the membrane and a mixing layer between PEDOT:PSS and PVDF is created. We also show that PVDF-graft-PEGMA copolymers play an important role in binding the membrane to the electrodes and that direct mechanical interlocking in the pores can further improve the adhesion. Finally, PEDOT:PSS/PVDF-graft-PEGMA/PEDOT:PSS actuators are made by simple solution casting. They are capable of producing high strains of 0.6% and show no signs of delamination after more than 150 h or 104 actuation cycles. Furthermore, the preservation of the hydrophobic membrane in between two PEDOT:PSS layers increases the resistance between them from 0.36 Ω to 0.16 MΩ, thus drastically modifying the power dissipation of the actuators.
It is well known that the ferroelectric performance of poly (vinylidene fluoride) (PVDF) is caused by its β-crystal structure, which can be efficiently induced through a stretching process applied to the PVDF. Though numerous PVDF nanocomposites have been reported on, there is still a lack of studies on how the stretching process affects the phase transformation in PVDF nanocomposites. In this study, the effects of stretching on the crystalline structures and alternating current (AC) conductivity of PVDF nanocomposites with different concentrations (up to 5.0 wt.%) of CNFs were investigated. Results revealed that the stretching process is not only an effective approach to produce β-crystal from pure PVDF, but also for CNF/PVDF composites. The extremely high phase transformation from α- to β-crystal (⩾96%) is maintained for the nanocomposites with above 1.0 wt.% CNFs. The AC conductivity of CNF/PVDF composites remarkably decreases when the resultant percolation threshold is raised from 1.0 to 4.2 wt.% CNFs after stretching. This is attributed to the reduced crystallinity induced by the phase transformation from α- to β-PVDF as well as the CNF re-orientation.
► Bulk graphite oxide was exfoliated into graphene oxide nanosheets in DMF solvent. ► PVDF/graphene oxide nanocomposite films were prepared via solution coating method. ► Graphene oxide was homogeneously dispersed and distributed within PVDF matrix. ► PVDF nanocomposite films exhibit a purely β-phase at very low graphene oxide content. ► Tensile properties of PVDF nanocomposites were significantly improved. Graphene oxide nanosheets (GOn)/PVDF nanocomposite films were prepared by solution casting method with various GOn contents. GOn were obtained via sonication of bulk graphite oxide in dimethylformamide (DMF). Due to the strong and specific interaction between carbonyl group ( C O) in GOn surface and fluorine group ( CF ) in PVDF, the GOn were homogeneously dispersed and distributed within the matrix. The chosen approach for preparation and the high compatibility between GOn and PVDF result in the formation of purely piezoelectric β-polymorph at only 0.1 wt.% GOn content. Below that content a mixture of β and α-polymorph is observed. The Young's modulus and tensile strength of PVDF were respectively increased by 192% and 92% with the addition of 2 wt.% GOn. The thermal stability of PVDF polymer was also significantly increased with increasing of GOn loading. The as-obtained flexible nanocomposite films with such low GOn content can be used as active materials in the field of piezoelectric applications.
The fabrication of spiropyran-doped poly(vinylidene fluoride- co -hexafluoropropylene) electrospun fibers and the investigation of their optochemosensing properties when exposed to organic acid vapors are reported. The system is first activated through the conversion of the spiropyran molecules dispersed within the polymer to their fluorescent merocyanine photoisomers upon UV irradiation. The latter undergo protonation in the presence of acid vapors, giving rise to an evident colorimetric transition of the composite from purple to yellow and to a concurrent quenching in the spectral emission. Upon gas depletion under ambient conditions, the reversible interactions with the acid molecules lead to a spontaneous recovery of the free merocyanine form and, thus, of the initial materials' color and emission signal. Consistent with their higher surface to volume ratio, the nanofibrous mats exhibit a reduction in the response and recovery times by 25% and by more than 90% compared to composite fibers and films of the same material. The efficient vapor permeation/desorption confers a fast optical detection, which is linear with the acid concentration, and an outstanding reusability for over 40 protonation/recovery cycles. Therefore, the photoswitchable acidochromism of spiropyrans combined with the high specific surface area of the nanofibrous polymeric support provides a photoactivated fast optical and visual acid recognition suitable for integration within portable and reusable platforms targeting the real-time detection of acidic vapors for industrial and environmental applications. Electrospun spiropyran-embedded nanofibers for fast photocontrollable and reversible optochemosensing of acidic vapors.
High energy density Li–S batteries are highly attractive. However, their use in practical applications has been greatly affected by their poor cycle life and low rate performance, which can be partly attributed to the dissolution of polysulfides from the S cathode and their migration to the Li anode through the separator. While much effort has been devoted to designing the structure of the S cathodes for suppressing the dissolution of polysulfides, relatively little emphasis has been placed on modifying the separator. Herein, we demonstrate a new approach for modifying the separator with a polyvinylidene fluoride-carbon (PVDF-C) layer, where the polysulfides generated in the Li–S cells can be localized on the cathode side. Li–S batteries based on the novel separator and a cathode prepared by the simple mixing of a S powder and super P have delivered discharge capacities of 918.6 mAh g–1, 827.2 mAh g–1, and 669.1 mAh g–1 after 100, 200, and 500 cycles, respectively, at a discharge rate of 0.5 C. Even under current densities of up to 5 C, the cells were able to retain a discharge capacity of 393 mAh g–1, thereby demonstrating an excellent high rate performance and stability. The exceptional electrochemical performance could be attributed to the intense adsorption capability of the micropores, presence of C–C double bonds, and conductivity of the C network in the PVDF-C layer. This economical and simple strategy to overcome the polysulfide dissolution issues provides a commercially feasible method for the construction of Li–S batteries.
Novel electrospun nanofibrous microfiltration membranes (ENMs) are fabricated by using sulfonated polyvinylidene fluoride (S-PVDF)/PVDF and S-PVDF/PVDF/graphene oxide (GO) with negative charge which was prepared by electrospinning technique. The GO with different weight percentages (0, 0.1, 0.5 and 1.0 wt%) was used as additive in membrane and its effect on hydrophilicity, pure water flux, mean fiber and pore diameter was studied. The results showed that the most appropriate amount of GO is 0.5 wt% (low contact angle (77.08°) with high pure water flux (1222 L/m h) was obtained). Therefore, in the fabrication of the rest of the membranes 0.5 wt% GO was utilized as optimal amount of GO. In addition, the effects of sulfonation on morphology, oil removal, and fouling parameters were investigated. Finally, data analysis indicated that among the novel prepared ENMs, M (S-PVDF/PVDF/GO-50/50=volume ratio) displayed the lower irreversible fouling (41%), the highest flux recovery ratio (59%), and the superior antifouling properties. This higher antifouling performance of the M compared with other novel ENMs was attributed to hydrophilicity and enhanced strong electrostatic repulsion force between the membrane matrix and the oily foulant. The low degree of total fouling (70%) for the intact PVDF nanofibrous compared with novel ENMs (91–97%) was interpreted by the bigger pore diameter (300 nm) of the intact PVDF nanofibrous.
To study the effects of nano-TiO2 particles on membrane performance and structure and to explore possible interactions between nano-TiO2 particles and polymer, polymer/TiO2 embedded hybrid membranes and neat polymer membranes were prepared using the phase inversion method. Poly(vinylidene difluoride) (PVDF), poly(vinylidene difluoride)-g-(maleic anhydride) (PVDF-g-MA), and poly(vinylidene difluoride)-g-poly(acryl amide) (PVDF-g-PAM) were selected as the membrane materials. SEM images showed that the hybrid membranes had a thinner skin layer and a larger number of pores in the sublayer than the neat membranes, which was the main cause of the increase in water flux of the hybrid membranes. They also exhibited a better antifouling property than the neat ones in the continuous BSA solution filtration process. In the 48-h-long pure-water experiment, the hybrid membranes underwent a water flux decline and an increase in contact angle. The loss of nano-TiO2 particles, revealed by EDS analysis, influenced the stability of hybrid membrane performance. The XPS analysis suggested that nano-TiO2 particles were immobilized in the membrane surface layer through the formation of a stable chemical structure resulting from its reaction with polymer and/or through intertwining with polymer chains.