Crystalline polymorphism in poly(vinylidenefluoride) membranes

Effect of Choline Chloride-Based DES on the Pore-Forming Ability and Properties of PVDF Membranes Prepared with Triethyl Phosphate as Green Solvent

This study explores the influence of various additives on the morphological, chemical, and thermal properties of poly(vinylidene fluoride) (PVDF) membranes prepared via the non-solvent induced phase separation (NIPS) technique. The use of a green solvent such as triethyl phosphate (TEP) was shown to be successful. A particular focus was dedicated to pore formers based on choline chloride-based deep eutectic solvents (DES) in combination with ethylene glycol and glycerol, i.e., ChCl/EG and ChCl/GLY, and its benchmark with traditional counterparts such as poly(ethylene glycol) (PEG) and glycerol (GLY). Comprehensive characterization was conducted using FESEM, FTIR, XRD, and DSC techniques to evaluate changes in membrane morphology, porosity, and crystallinity. PEG acted as a pore-forming agent, transitioning the internal structure from spherulitic to sponge-like with consistent pore sizes, while GLY produced a nodular morphology at higher concentrations due to increased dope solution viscosity. DES induced significant shifts in crystalline phase composition, decreasing α-phase fractions and promoting β-phase formation at higher concentrations. While the overall porosity remained unaffected by the addition of GLY or PEG, it was dependent on the DES concentration in the dope at lower values than those obtained by GLY and PEG. Membrane pore size with ChCl/GLY was lower than with ChCl/EG and GLY. All membranes showed performance at the hydrophobic regime. The findings demonstrate that ChCl/EG and ChCl/GLY can tailor the structural and thermal properties of TEP-driven PVDF membranes, providing a green and versatile approach to customize the membrane properties for specific applications.

Simultaneous Visualization of Dynamical and Static Tactile Perception Using Piezoelectric-Ultrasonic Bimodal Electronic Skin Based on In Situ Polarized PVDF-TrFE/2DBP Composites and the TFT Array

The key to realizing completed bionic tactile perception of human skin using electronic skin relies on simultaneously distinguishing dynamic and static stimuli and restoring their characteristic information, which is realized by integration of several individual sensors but remains certain limitations including large physical size and high energy consumption. In this study, a piezoelectric-ultrasonic bimodal electronic skin (PUVE) based on in situ polarized PVDF-TrFE/2DBP composites and a thin-film transistor (TFT) array is fabricated. The incorporation of 2DBP into the PVDF-TrFE film and the in situ polarization approach provide excellent piezoelectric and ultrasonic performances of PVDF-TrFE/2DBP composites. PUVE has an ultrahigh sensitivity of 3.2 mV kPa-1 over a wide pressure (0-310 kPa) range, with excellent spatial resolution (50 μm) and response time (40 ms). Meanwhile, the PUVE demonstrated outstanding repeatability and bending stability in 1500 cycles of cyclic pressure and 4000 cycles of 180° bending. The integrated piezoelectric and ultrasonic functions of PUVE can respond individually to dynamic and static tactile stimuli to ensure perceiving and decoupling of the dynamical and static mechanical signals with one single sensor. The PVDF-TrFE/2DBP composites is further integrated with the TFT array, realizing visualization function of contacting objects and restoring their characteristic information including the texture and location. Thus, the PUVE is expected to have a wide range of applications in intelligent robots and human prostheses.

Optimizing β-Phase Content in PVDF Membranes via Modification of Dope Solution with Citric Acid/Nano-TiO2 Using Nonsolvent-Induced Phase Separation Method

The morphology of Poly (vinylidene fluoride) (PVDF) membranes prepared via the nonsolvent-induced phase separation (NIPS) method was modulated by altering the dope solution with citric acid (CA) and titanium dioxide nanoparticles (nano-TiO2) to optimize the β-phase content. Three series of dope solutions were prepared in dimethyl acetamide (DMAc): (i) TONx series contained 0.0-10% citric acid, (ii) Mx series contained 0.0-0.4% nano-TiO2, and (iii) TAx series contained 5% CA and 0.0-0.40% nano-TiO2. A field emission scanning electron microscopy (FESEM) study revealed that CA enhances pore opening, and nano-TiO2 transforms the sponge-like uneven porous structures into a compact, relatively regular honeycomb structure in the PVDF membranes. The combined effect of CA and nano-TiO2 in the dope solution made the channels and chambers of the membrane well organized, and the walls of the channels transformed from solid fibrils to cross-woven nanofiber-like entities. Porosity initially peaked at 84% in the TAx series, gradually decreasing to 72% with increasing nano-TiO2 concentrations. X-ray diffraction (XRD), Fourier-Transformed Infrared Spectroscopy (FTIR), and Differential Scanning Calorimetry (DSC) revealed the presence of a combined relative amount of the β- and γ-polymorphs of 84% in a neat PVDF membrane, 88% in an Mx, and 96% in a TAx series membrane, with the β-PVDF constituting nearly the entire portion of the combined polymorphs. The presence of 96% electroactive polymorph content in the PVDF membrane is noteworthy, highlighting its potential biomedical and industrial applications.

Revealing Intrinsic Free Charge Generation: Promoting the Construction of Over 19% Efficient Planar p-n Heterojunction Organic Solar Cells

Due to high binding energy and extremely short diffusion distance of Frenkel excitons in common organic semiconductors at early stage, mechanism of interface charge transfer-mediated free carrier generation has dominated the development of bulk heterojunction (BHJ) organic solar cells (OSCs). However, considering the advancements in materials and device performance, it is necessary to reexamine the photoelectric conversion in current-stage efficient OSCs. Here, we propose that the conjugated materials with specific three-dimensional donor-acceptor conjugated packing potentially exhibit distinctive charge photogeneration mechanism, which spontaneously split Wannier-Mott excitons to free carriers in pure phases. Subsequently, the pure planar p-n heterojunction (PHJ) OSCs based on green orthogonal solvents were prepared and exhibited comparable even greater performance to that of BHJ OSCs. More interestingly, by introducing PVDF-TrFE as intrinsic region to regulate built-in electric field of the device, the planar p-i-n PHJ OSCs achieved much higher efficiency (>18%) and stability. Moreover, a prominent efficiency of over 19% has been obtained via ternary optimization, which is the new efficiency record for PHJ OSCs up to date. This study points towards the distinguishing intrinsic free charge generation mechanism, opens up a new avenue for OSCs to collectively realize high-efficiency, long-term duration, and simplified device engineering for future commercialization.

Multifunctionality and Processability of a Thermoplastic Based Gel Electrolyte Cell for the Realization of Structural Batteries

In this work, a battery layup consisting of a poorly flammable ionic liquid electrolyte and a poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) thermoplastic has been developed along with composite anode and cathode electrodes. The developed gel electrolyte exhibits feasible ionic conductivity of about 1 mS/cm at 30 °C. State-of-the-art active electrode materials, i.e., LiNi0.8Mn0.1Co0.1O2 (NMC811) and graphite, have been employed. Full cells were tested in coin and pouch cell format, obtaining capacities of about 120 and 100 mA h/gNMC811, respectively, at a C-rate of C/10. Thereby, it was observed that good contact between the individual cell layers is crucial. Recently, it was shown that the mechanical properties of structural batteries, realized by integrating battery cells into carbon fiber-reinforced polymer (CFRP) laminates, depend significantly on the mechanical properties of the cell itself. Hence, to promote the realization of such a structural battery concept, tensile tests were carried out to investigate the mechanical properties of cells as well as the individual components developed in this work. The full cell showed values of 10 GPa and 49 MPa for the Young's modulus and tensile strength, respectively. Thus, feasible multifunctionality could be verified on the cell level. However, regarding the contributions of the different components, it could be shown that mainly the current collector foils contribute to the mechanical properties, in contrast to the electrode loadings and the gel electrolyte. Additionally, the thermal and chemical stability of the developed system was evaluated, highlighting the importance of these secondary properties for the fabrication of structural batteries, i.e., the integration of cells into load-bearing CFRP laminates. Specifically, it was observed that the developed system is thermally stable up to 150 °C and no HF release was detected upon exposure to ambient conditions.

One-pot synthesis of crystalline polycarbonate-block-polyesters

We herein describe a simple and efficient one-pot synthesis approach to prepare crystalline polycarbonate-polyester diblock copolymers by copolymerizing tetrachlorophthalic anhydride, CO2, and ethylene oxide using a metal-free catalyst. The block copolymers possess a melting point as high as 169 °C and two distinct glass transition temperatures. It is also possible to control the length and composition of the copolymers, thereby customizing their crystallinity and physical performance.

Flexible piezoelectric materials and strain sensors for wearable electronics and artificial intelligence applications

With the rapid development of artificial intelligence, the applications of flexible piezoelectric sensors in health monitoring and human-machine interaction have attracted increasing attention. Recent advances in flexible materials and fabrication technologies have promoted practical applications of wearable devices, enabling their assembly in various forms such as ultra-thin films, electronic skins and electronic tattoos. These piezoelectric sensors meet the requirements of high integration, miniaturization and low power consumption, while simultaneously maintaining their unique sensing performance advantages. This review provides a comprehensive overview of cutting-edge research studies on enhanced wearable piezoelectric sensors. Promising piezoelectric polymer materials are highlighted, including polyvinylidene fluoride and conductive hydrogels. Material engineering strategies for improving sensitivity, cycle life, biocompatibility, and processability are summarized and discussed focusing on filler doping, fabrication techniques optimization, and microstructure engineering. Additionally, this review presents representative application cases of smart piezoelectric sensors in health monitoring and human-machine interaction. Finally, critical challenges and promising principles concerning advanced manufacture, biological safety and function integration are discussed to shed light on future directions in the field of piezoelectrics.

Piezoelectric Properties of As-Spun Poly(vinylidene Fluoride)/Multi-Walled Carbon Nanotube/Zinc Oxide Nanoparticle (PVDF/MWCNT/ZnO) Nanofibrous Films

Conductive multi-walled carbon nanotubes (MWCNTs) as well as piezoelectric zinc oxide (ZnO) nanoparticles are frequently used as a single additive and dispersed in polyvinylidene fluoride (PVDF) solutions for the fabrication of piezoelectric composite films. In this study, MWCNT/ZnO binary dispersions are used as spinning liquids to fabricate composite nanofibrous films by electrospinning. Binary additives are conducive to increasing the crystallinity, piezoelectric voltage coefficient, and consequent piezoelectricity of as-spun films owing to the stretch-enhanced polarization of the electrospinning process under an applied electric field. PCZ-1.5 film (10 wt. % PVDF/0.1 wt. % MWCNTs/1.5 wt. % ZnO nanoparticles) contains the maximum β-phase content of 79.0% and the highest crystallinity of 87.9% in nanofibers. A sensor using a PCZ-1.5 film as a functional layer generates an open-circuit voltage of 10 V as it is subjected to impact loads with an amplitude of 6 mm at 10 Hz. The piezoelectric sensor reaches a power density of 0.33 μW/cm2 and a force sensitivity of 582 mV/N. In addition, the sensor is successfully applied to test irregular motions of a bending finger and stepping foot. The result indicates that electrospun PVDF/MWCNT/ZnO nanofibrous films are suitable for wearable devices.

Preparation Method and Application of Porous Poly(lactic acid) Membranes: A Review

Porous membrane technology has garnered significant attention in the fields of separation and biology due to its remarkable contributions to green chemistry and sustainable development. The porous membranes fabricated from polylactic acid (PLA) possess numerous advantages, including a low relative density, a high specific surface area, biodegradability, and excellent biocompatibility. As a result, they exhibit promising prospects for various applications, such as oil-water separation, tissue engineering, and drug release. This paper provides an overview of recent research advancements in the fabrication of PLA membranes using electrospinning, the breath-figure method, and the phase separation method. Firstly, the principles of each method are elucidated from the perspective of pore formation. The correlation between the relevant parameters and pore structure is discussed and summarized, subsequently followed by a comparative analysis of the advantages and limitations of each method. Subsequently, this article presents the diverse applications of porous PLA membranes in tissue engineering, oil-water separation, and other fields. The current challenges faced by these membranes, however, encompass inadequate mechanical strength, limited production efficiency, and the complexity of pore structure control. Suggestions for enhancement, as well as future prospects, are provided accordingly.

Recent developments in solar-powered membrane distillation for sustainable desalination

The freshwater shortage continues to be one of the greatest challenges affecting our planet. Although traditional membrane distillation (MD) can produce clean water regardless of climatic conditions, the process wastes a lot of energy. The technique of solar-powered membrane distillation (SPMD) has received a lot of interest in the past decade, thanks to the development of photothermal materials. SPMD is a promising replacement for the traditional MD based on fossil fuels, as it can prevent the harmful effects of emissions on the environment. Integrating green solar energy with MD can reduce the cost of the water purification process and secure freshwater production in remote areas. At this point, it is important to consider the most current progress of the SPMD system and highlight the challenges and prospects of this technology. Based on this, the background, recent advances, and principles of MD and SPMD, their configurations and mechanisms, fabrication methods, advantages, and current limitations are discussed. Detailed comparisons between SPMD and traditional MD, assessments of various standards for incorporating photothermal materials with desirable properties, discussions of desalination and other applications of SPMD and MD, and energy consumption rates are also covered. The final section addresses the potential of SPMD to outperform traditional desalination technology while improving water production without requiring a significant amount of electrical or high-grade thermal energy.

Polymorph transformation in a mixed-stacking nickel-dithiolene complex with the derivative of 4,4'-bipyridinium

In this study, two polymorphs of the [1,1'-dibutyl-4,4'-bipyridinium][Ni(mnt)2] salt (1) were synthesized. The dark-green polymorph (designated as 1-g) was prepared under ambient conditions by the rapid precipitation method in aqueous solutions. Subsequently, the red polymorph (labeled as 1-r) was obtained by subjecting 1-g to ultrasonication in MeOH at room temperature. Microanalysis, infrared spectroscopy, thermogravimetry (TG), differential scanning calorimetry (DSC), and powder X-ray diffraction (PXRD) techniques were used to characterize the two polymorphs. Both 1-g and 1-r exhibit structural phase transitions: a reversible phase transition at ∼403 K (∼268 K) upon heating and 384 K (∼252 K) upon cooling for 1-g (1-r) within the temperature range below 473 K. Interestingly, on heating 1-r to 523 K, an irreversible phase transition occurred at about 494 K, resulting in the conversion of 1-r into 1-g. Relative to 1-r, 1-g represents a thermodynamically metastable phase wherein numerous high-energy conformations in butyl chains of cations are confined within the lattice owing to quick precipitation or rapid annealing from higher temperatures. Through variable-temperature single crystal and powder X-ray diffractions, UV-visible spectroscopy, dielectric spectroscopy, and DSC analyses, this study delves into the mechanism underlying phase transitions for each polymorph and the manual transformation between 1-g and 1-r as well.

Enhanced Electroactive Phases of Poly(vinylidene Fluoride) Fibers for Tissue Engineering Applications

Nanofibrous materials generated through electrospinning have gained significant attention in tissue regeneration, particularly in the domain of bone reconstruction. There is high interest in designing a material resembling bone tissue, and many scientists are trying to create materials applicable to bone tissue engineering with piezoelectricity similar to bone. One of the prospective candidates is highly piezoelectric poly(vinylidene fluoride) (PVDF), which was used for fibrous scaffold formation by electrospinning. In this study, we focused on the effect of PVDF molecular weight (180,000 g/mol and 530,000 g/mol) and process parameters, such as the rotational speed of the collector, applied voltage, and solution flow rate on the properties of the final scaffold. Fourier Transform Infrared Spectroscopy allows for determining the effect of molecular weight and processing parameters on the content of the electroactive phases. It can be concluded that the higher molecular weight of the PVDF and higher collector rotational speed increase nanofibers' diameter, electroactive phase content, and piezoelectric coefficient. Various electrospinning parameters showed changes in electroactive phase content with the maximum at the applied voltage of 22 kV and flow rate of 0.8 mL/h. Moreover, the cytocompatibility of the scaffolds was confirmed in the culture of human adipose-derived stromal cells with known potential for osteogenic differentiation. Based on the results obtained, it can be concluded that PVDF scaffolds may be taken into account as a tool in bone tissue engineering and are worth further investigation.

Preparation of N-MG-modified PVDF-CTFE substrate composite nanofiltration membrane and its selective separation of salt and dye

Poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) is considered an ideal membrane material for the treatment of complex environmental water due to its exceptional thermal stability and chemical resistance. Thus, to expand its application in the field of nanofiltration (NF) membranes, in this study, N-methylglucamine (N-MG) was used to hydrophilically modify PVDF-CTFE, overcoming the inherent hydrophobicity of PVDF-CTFE as a porous substrate membrane, which leads to difficulties in controlling the interfacial polymerization (IP) reaction and instability of the separation layer structure. The -OH present in N-MG could replace the C-Cl bond in the CTFE chain segment, thus enabling the hydrophilic graft modification of PVDF-CTFE. The influence of the addition of N-MG on the surface and pore structure, wettability, permeability, ultrafiltration separation, and mechanical properties of the PVDF-CTFE substrate membrane was studied. According to the comparison of the comprehensive capabilities of the prepared porous membranes, the M4 membrane with the addition of 1.5 wt% N-MG exhibited the best hydrophilicity and permeability, indicating that it is a desirable modified membrane for use as an NF substrate membrane. The experiments showed that the rejection of Na2SO4 by the NF membrane was 96.5% and greater than 94.0% for various dyes. In the test using dye/salt mixed solution, this membrane exhibited a good separation selectivity (CR/NaCl = 177.8) and long-term operational stability.

Poly(vinylidene fluoride) Aerogels with α, β, and γ Crystalline Forms: Correlating Physicochemical Properties with Polymorphic Structures

Strategic customization of crystalline forms of poly(vinylidene fluoride) (PVDF) aerogels is of great importance for a variety of applications, from energy harvesters to thermal and acoustic insulation. Here, we report sustainable strategies to prepare crystalline pure α, β, and γ forms of PVDF aerogels from their respective gels using a solvent exchange strategy with green solvents, followed by a freeze-drying technique. The crucial aspect of this process was the meticulous choice of appropriate solvents to enable the formation of thermoreversible gels of PVDF by crystallization-induced gelation. Depending on the polymer-solvent interactions, the chain conformation of PVDF can be modulated to obtain gels and aerogels with specific crystalline structures. The crystalline pure α-form and piezoelectric β-form aerogels were readily obtained by using cyclohexanone and γ-butyrolactone as gelation solvents. On the other hand, the γ-form aerogel was obtained using a binary solvent system consisting of dimethylacetamide and water. These aerogels with distinct crystalline structures exhibit different morphologies, mechanical properties, hydrophobicities, acoustic properties, and electrical properties. Measurement of thermal conductivity for these aerogels showed exceptionally low thermal conductivity values of ∼0.040 ± 0.003 W m-1 K-1 irrespective of their crystal structures. Our results showcase the fabrication approaches that enable PVDF aerogels with varied physicochemical properties for multifunctional applications.

Bioinspired Dielectric Nanocomposites with High Charge-Discharge Efficiency Enabled by Superspreading-Induced Alignment of Nanosheets

High-performance dielectric nanocomposites are promising candidates for thin-film dielectric capacitors for high-power pulse devices. However, the existing nanocomposites suffer from low charge-discharge efficiency (η), which results in severe generation and accumulation of Joule heat and subsequently the failure of the devices. In this work, we report nacre-inspired dielectric nanocomposites with outstanding η, which are enabled by superspreading shear flow-induced highly aligned two-dimensional (2D) nanofillers. Taking boron nitride nanosheets (BNNS) as an example, the highly aligned BNNS in the poly(vinylidene fluoride) (PVDF)-based nanocomposites contributes to a highly efficient Coulomb blockade effect for the injected charge carriers. Therefore, the bioinspired nanocomposites with highly aligned BNNS show significantly reduced dielectric loss (tan δ) (63.3%) and improved η (144.8%), compared to the ones with partially aligned nanosheets fabricated by solution casting. Furthermore, the optimized loading content of BNNS is as low as 3.6 wt %. The resulting nanocomposites exhibit reduced tan δ (0.018) and enhanced Eb (687 kV/mm), η (71%), and Ue (16.74 J/cm3). Our work demonstrates that the realization of high alignment of 2D nanofillers enabled by the superspreading shear flow is a promising way for the development of high-performance dielectric nanocomposites.

Piezoelectric dressings for advanced wound healing

The treatment of chronic refractory wounds poses significant challenges and threats to both human society and the economy. Existing research studies demonstrate that electrical stimulation fosters cell proliferation and migration and promotes the production of cytokines that expedites the wound healing process. Presently, clinical settings utilize electrical stimulation devices for wound treatment, but these devices often present issues such as limited portability and the necessity for frequent recharging. A cutting-edge wound dressing employing the piezoelectric effect could transform mechanical energy into electrical energy, thereby providing continuous electrical stimulation and accelerating wound healing, effectively addressing these concerns. This review primarily reviews the selection of piezoelectric materials and their application in wound dressing design, offering a succinct overview of these materials and their underlying mechanisms. This study also provides a perspective on the current limitations of piezoelectric wound dressings and the future development of multifunctional dressings harnessing the piezoelectric effect.

A Review of Polymer-Based Environment-Induced Nanogenerators: Power Generation Performance and Polymer Material Manipulations

Natural environment hosts a considerable amount of accessible energy, comprising mechanical, thermal, and chemical potentials. Environment-induced nanogenerators are nanomaterial-based electronic chips that capture environmental energy and convert it into electricity in an environmentally friendly way. Polymers, characterized by their superior flexibility, lightweight, and ease of processing, are considered viable materials. In this paper, a thorough review and comparison of various polymer-based nanogenerators were provided, focusing on their power generation principles, key materials, power density and stability, and performance modulation methods. The latest developed nanogenerators mainly include triboelectric nanogenerators (TriboENG), piezoelectric nanogenerators (PENG), thermoelectric nanogenerators (ThermoENG), osmotic power nanogenerator (OPNG), and moist-electric generators (MENG). Potential practical applications of polymer-based nanogenerator were also summarized. The review found that polymer nanogenerators can harness a variety of energy sources, with the basic power generation mechanism centered on displacement/conduction currents induced by dipole/ion polarization, due to the non-uniform distribution of physical fields within the polymers. The performance enhancement should mainly start from strengthening the ion mobility and positive/negative ion separation in polymer materials. The development of ionic hydrogel and hydrogel matrix composites is promising for future nanogenerators and can also enable multi-energy collaborative power generation. In addition, enhancing the uneven distribution of temperature, concentration, and pressure induced by surrounding environment within polymer materials can also effectively improve output performance. Finally, the challenges faced by polymer-based nanogenerators and directions for future development were prospected.

Development of multifunctional membranes via plasma-assisted nonsolvent induced phase separation

Demands on superhydrophobic, self-cleaning and piezoelectric membranes have gained significantly due to their potential to overcome global shortages in clean water and energy. In this study, we have discovered a novel plasma-assisted nonsolvent induced phase separation (PANIPS) method to prepare superhydrophobic, self-cleaning and piezoelectric poly(vinylidene difluoride) (PVDF) membranes without additional chemical modifications or post-treatments. The PANIPS membranes exhibit water contact angles ranging from 151.2° to 166.4° and sliding angles between 6.7° and 29.7°. They also show a high piezoelectric coefficient (d33) of 10.5 pC N-1 and can generate a high output voltage of 10 Vpp. The PANIPS membranes can effectively recover pure water from various waste solutions containing Rose Bengal dye, humic acid, or sodium dodecyl sulfate via direct contact membrane distillation (DCMD). This study may provide valuable insights to fabricate PANIPS membranes and open up new avenues to molecularly design advanced superhydrophobic, self-cleaning, and piezoelectric membranes in the fields of clean water production, motion sensor, and piezoelectric nanogenerator.

Crystal Polymorphism of Isodimorphic Polyesters Tuned by cis- and trans-C═C Comonomer Units

Polymorphism is ubiquitous in polymer crystallization due to the diversified chain conformations and interchain packings in polymer crystals. Controlling chain conformation is effective in tailoring the crystal polymorphism of polymers, which, however, is challenging at the molecular level. Herein, we have synthesized poly(butylene adipate) (PBA)-based copolymers containing C═C units and demonstrated the important role of trans/cis-C═C units in tuning the chain conformation and crystal polymorphism of polymers. Both PBA-based trans- and cis-copolymers show isodimorphic crystallization behavior with the partial inclusion of C═C units in PBA crystals. The presence of trans-C═C units favors the formation of metastable β-crystals of PBA and retards the β-to-α crystal transition upon heating due to the highly conformational matching between trans-C═C units and β-crystals. Conversely, the incorporation of cis-C═C units destroys the regularity of the trans conformation and favors the growth of α-crystals of PBA. This work has elucidated the crucial role of local chain conformation in the crystal polymorphism of polymers.

Microwave-Responsive Flexible Room-Temperature Phosphorescence Materials Based on Poly(vinylidene fluoride) Polymer

The development of flexible, room-temperature phosphorescence (RTP) materials remains challenging owing to the quenching of their unstable triplet excitons via molecular motion. Therefore, a polymer matrix with Tg higher than room temperature is required to prevent polymer segment movement. In this study, a RTP material was developed by incorporating a 4-biphenylboronic acid (BPBA) phosphor into a poly(vinylidene fluoride) (PVDF) matrix (Tg =-27.1 °C), which exhibits a remarkable UV-light-dependent oxygen consumption phosphorescence with a lifetime of 1275.7 ms. The adjustable RTP performance is influenced by the crystallinity and polymorph (α, β, and γ phases) fraction of PVDF, therefore, the low Tg of the PVDF matrix enables the polymeric segmental motion upon microwave irradiation. Consequently, a reduction in the crystallinity and an increase in the α phase fraction in PVDF film induces RTP after 2.45 GHz microwave irradiation. These findings open up new avenues for constructing crystalline and phase-dependent RTP materials while demonstrating a promising approach toward microwave detection.

Design and application of a flexible nano cardiac sound sensor based on P(VDF-TrFE)/KNN/GR composite piezoelectric film for heart disease diagnosis

The paper proposes a flexible micro-nano composite piezoelectric thin film. This flexible piezoelectric film is fabricated through electrospinning process, utilizing a combination of 12 wt% poly(vinylidene fluoride-co-trifluoroethylene)(P(VDF-TrFE)), 8 wt% potassium sodium niobate (KNN) nanoparticles, and 0.5 wt% graphene (GR). Under cyclic loading, the composite film demonstrates a remarkable increase in open-circuit voltage and short-circuit current, achieving values of 36.1 V and 163.7 uA, respectively. These values are 5.8 times and 3.6 times higher than those observed in the pure P(VDF-TrFE) film. The integration of this piezoelectric film into a wearable flexible heartbeat sensor, coupled with the RepMLP classification model, facilitates heartbeat acquisition and real-time automated diagnosis. After training and validation on a dataset containing 2000 heartbeat samples, the system achieved an accuracy of approximately 99% in two classification of heart sound signals (normal and abnormal). This research substantially enhances the output performance of the piezoelectric film, offering a novel and valuable solution for the application of flexible piezoelectric films in physiological signal detection.

Mechanism of PVDF Membrane Formation by NIPS Revisited: Effect of Precipitation Bath Nature and Polymer-Solvent Affinity

A new interpretation of the mechanism of the polyvinylidene fluoride (PVDF) membrane formation using the nonsolvent-induced phase separation (NIPS) method based on an analysis of the complete experimental phase diagram for the three-component mixture PVDF-dimethyl acetamide (DMAc)-water is proposed. The effects of the precipitation bath's harshness and thermodynamic affinity of the polymer's solvent on the morphology, crystalline structure, transport and physical-mechanical properties of the membranes are investigated. These characteristics were studied via scanning electron microscopy, wide-angle X-ray scattering, liquid-liquid porosimetry and standard methods of physico-mechanical analysis. It is established that an increase in DMAc concentration in the precipitation bath results in the growth of mean pore size from ~60 to ~150 nm and an increase in permeance from ~2.8 to ~8 L m-2 h-1 bar-1. It was observed that pore size transformations are accompanied by changes in the tensile strength of membranes from ~9 to ~11 and to 6 MPa, which were explained by the degeneration of finger-like pores and appearance of spherulitic structures in the samples. The addition of water to the dope solution decreased both the transport (mean pore size changed from ~55 to ~25 nm and permeance reduced from ~2.8 to ~0.5 L m-2 h-1 bar-1) and mechanical properties of the membranes (tensile strength decreased from ~9 to ~6 MPa). It is possible to conclude that the best membrane quality may be reached using pure DMAc as a solvent and a precipitation bath containing 10-30% wt. of DMAc, in addition to water.

Flexible Composites for Piezocatalysis

Despite piezoelectric materials have a long history of application, piezoelectric catalysis has continued to be a hot topic in recent years. Flexible piezoelectric materials have just emerged in recent years due to their versatility and designability. In this paper, we review the recent advances in flexible piezoelectric materials for catalysis, discuss the fundamentals of the catalytic properties of composite materials, and detail the typical structures of these materials. We pay special attention to the types of filler in flexible piezoelectric composites, their role and the interaction between the particles and the flexible substrate. Notable examples of flexible piezoelectric materials for organic pollutants degradation, enhanced piezo-photocatalysis and antibacterial applications are also presented. Finally, we present key issues and future prospects for the development of flexible piezoelectric catalysts.

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.

Ultrathin g-C3N4 composite Bi2WO6 embedded in PVDF UF membrane with enhanced permeability, anti-fouling performance and durability for efficient removal of atrazine

A novel Bi₂WO₆-g-C₃N₄/polyvinylidene fluoride (PVDF) composite ultrafiltration (UF) membrane (BWO-CN/PVDF) was prepared by microwave hydrothermal and immersion precipitation phase transformation method. The BWO-CN/PVDF-0.10 exhibited an outstanding photocatalytic removal rate of atrazine (ATZ) (97.65 %) under the simulated sunlight and enhanced permeate flux (1356.09 L·m^-2·h^-1). The multiple optical and electrochemical detection confirmed that combining ultrathin g-C₃N₄ and Bi₂WO₆ can increase carrier separation rate and prolong its lifetime. The quenching test revealed that h⁺ and ¹O₂ were the prominent reactive species. Additionally, after a 10-cycle photocatalytic process, the BWO-CN/PVDF membrane presented remarkable reusability and durability. And it showed excellent anti-fouling performance by filtering BSA, HA, SA, and Songhua River under simulated solar irradiation. The molecular dynamic (MD) simulation showed that the combination of g-C₃N₄ and Bi₂WO₆ can enhance the interaction between BWO-CN and PVDF. This work opens up a new idea for designing and constructing a highly efficient photocatalytic membrane for water treatment.

Poly(vinylidene fluoride) membrane with immobilized TiO2 for degradation of steroid hormone micropollutants in a photocatalytic membrane reactor

The lack of effective technologies to remove steroid hormones (SHs) from aquatic systems is a critical issue for both environment and public health. The performance of a flow-through photocatalytic membrane reactor (PMR) with TiO₂ immobilized on a photostable poly(vinylidene fluoride) membrane (PVDF-TiO₂) was evaluated in the context of SHs degradation at concentrations from 0.05 to 1000 µg/L under UV exposure (365 nm). A comprehensive investigation into the membrane preparation approach, including varying the surface Ti content and distribution, and membrane pore size, was conducted to gain insights on the rate-limiting steps for the SHs degradation. Increasing surface Ti content from 4 % to 6.5 % enhanced the 17β-estradiol (E2) degradation from 46 ± 12-81 ± 6 %. Apparent degradation kinetics were independent of both TiO₂ homogeneity and membrane pore size (0.1-0.45 µm). With optimized conditions, E2 removal was higher than 96 % at environmentally relevant feed concentration (100 ng/L), a flux of 60 L/m²h, 25 mW/cm², and 6.5 % Ti. These results indicated that the E2 degradation on the PVDF-TiO₂ membrane was limited by the catalyst content and light penetration depth. Further exploration of novel TiO₂ immobilization approach that can offer a larger catalyst content and light penetration is required to improve the micropollutant removal efficiency in PMR.

Formation of Porous Structures and Crystalline Phases in Poly(vinylidene fluoride) Membranes Prepared with Nonsolvent-Induced Phase Separation-Roles of Solvent Polarity

PVDF membranes were prepared with nonsolvent-induced phase separation, using solvents with various dipole moments, including HMPA, NMP, DMAc and TEP. Both the fraction of the polar crystalline phase and the water permeability of the prepared membrane increased monotonously with an increasing solvent dipole moment. FTIR/ATR analyses were conducted at the surfaces of the cast films during membrane formation to provide information on if the solvents were present as the PVDF crystallized. The results reveal that, with HMPA, NMP or DMAc being used to dissolve PVDF, a solvent with a higher dipole moment resulted in a lower solvent removal rate from the cast film, because the viscosity of the casting solution was higher. The lower solvent removal rate allowed a higher solvent concentration on the surface of the cast film, leading to a more porous surface and longer solvent-governed crystallization. Because of its low polarity, TEP induced non-polar crystals and had a low affinity for water, accounting for the low water permeability and the low fraction of polar crystals with TEP as the solvent. The results provide insight into how the membrane structure on a molecular scale (related to the crystalline phase) and nanoscale (related to water permeability) was related to and influenced by solvent polarity and its removal rate during membrane formation.

Preparation of Lateral Flow PVDF Membrane via Combined Vapor- and Non-Solvent-Induced Phase Separation (V-NIPS)

A large pore size Poly(vinylidene fluoride) (PVDF) membrane was prepared by the V-NIPS method using PVDF/N, N-dimethylacetamide (DMAc)/Polyvinyl pyrrolidone (PVP)/Polyethylene glycol (PEG) system. Firstly, the effect of different additive ratios on the membrane morphology and pore size was studied, and it was found that when the PVP:PEG ratio was 8:2, PVDF membranes with a relatively large pore size tend to be formed; the pore size is about 7.5 µm. Then, the effects of different exposure time on the membrane morphology and pore size were investigated, and it was found that as the vapor temperature increased, the pores on the surface of the membrane first became slightly smaller and then increased. Finally, the effects of different vapor temperatures on the membrane properties were discussed. The results showed that the as-prepared membrane exhibited suitable capillary flow rate and similar performance compared with a commercially available membrane in colloidal gold tests. The likely cause is that the amount of negative charge is less and the capillary migration rate is too fast. This paper provides a reference for the preparation of PVDF colloidal gold detection membrane.

Fluoropolymer Membranes for Membrane Distillation and Membrane Crystallization

Fluoropolymer membranes are applied in membrane operations such as membrane distillation and membrane crystallization where hydrophobic porous membranes act as a physical barrier separating two phases. Due to their hydrophobic nature, only gaseous molecules are allowed to pass through the membrane and are collected on the permeate side, while the aqueous solution cannot penetrate. However, these two processes suffer problems such as membrane wetting, fouling or scaling. Membrane wetting is a common and undesired phenomenon, which is caused by the loss of hydrophobicity of the porous membrane employed. This greatly affects the mass transfer efficiency and separation efficiency. Simultaneously, membrane fouling occurs, along with membrane wetting and scaling, which greatly reduces the lifespan of the membranes. Therefore, strategies to improve the hydrophobicity of membranes have been widely investigated by researchers. In this direction, hydrophobic fluoropolymer membrane materials are employed more and more for membrane distillation and membrane crystallization thanks to their high chemical and thermal resistance. This paper summarizes different preparation methods of these fluoropolymer membrane, such as non-solvent-induced phase separation (NIPS), thermally-induced phase separation (TIPS), vapor-induced phase separation (VIPS), etc. Hydrophobic modification methods, including surface coating, surface grafting and blending, etc., are also introduced. Moreover, the research advances on the application of less toxic solvents for preparing these membranes are herein reviewed. This review aims to provide guidance to researchers for their future membrane development in membrane distillation and membrane crystallization, using fluoropolymer materials.

Piezoelectric Polyvinylidene Fluoride Membranes with Self-Powered and Electrified Antifouling Performance in Pressure-Driven Ultrafiltration Processes

Electroactive membranes have the potential to address membrane fouling via electrokinetic phenomena. However, additional energy consumption and complex material design represent chief barriers to achieving sustainable and economically viable antifouling performance. Herein, we present a novel strategy for fabricating a piezoelectric antifouling polyvinylidene fluoride (PVDF) membrane (Pi-UFM) by integrating the ion-dipole interactions (NaCl coagulation bath) and mild poling (in situ electric field) into a one-step phase separation process. This Pi-UFM with an intact porous structure could be self-powered in a typical ultrafiltration (UF) process via the responsivity to pressure stimuli, where the dominant β-PVDF phase and the out-of-plane aligned dipoles were demonstrated to be critical to obtain piezoelectricity. By challenging with different feed solutions, the Pi-UFM achieved enhanced antifouling capacity for organic foulants even with high ionic strength, suggesting that electrostatic repulsion and hydration repulsion were behind the antifouling mechanism. Furthermore, the TMP-dependent output performance of the Pi-UFM in both air and water confirmed its ability for converting ambient mechanical energy to in situ surface potential (ζ), demonstrating that this antifouling performance was a result of the membrane electromechanical transducer actions. Therefore, this study provides useful insight and strategy to enable piezoelectric materials for membrane filtration applications with energy efficiency and extend functionalities.

Exploiting Partial Solubility in Partially Fluorinated Thermoplastic Blends to Improve Adhesion during Fused Deposition Modeling

This work studies the effect of interlayer adhesion on mechanical performance of fluorinated thermoplastics produced by fused deposition modeling (FDM). Here, we study the anisotropic mechanical response of 3D-printed binary blends of poly (vinylidene fluoride) (PVDF) and poly (methyl methacrylate) (PMMA) with the isotropic mechanical response of these blends fabricated via injection molding. Various PVDF/PMMA filament compositions were produced by twin-screw extrusion and, subsequently, injection-molded or 3D printed into dog-bone shapes. Specimen mechanical and thermal properties were evaluated by mode I tensile testing and differential scanning calorimetry, respectively. Results show that higher PMMA concentration not only improved the tensile strength and decreased ductility but reduced PVDF crystallization. As expected, injection-molded samples revealed better mechanical properties compared to 3D printed specimens. Interestingly, 3D printed blends with lower PMMA content demonstrated better diffusion (adhesion) across interfaces than those with a higher amount of PMMA. The present study provides new findings that may be used to tune mechanical response in 3D printed fluorinated thermoplastics, particularly for energy applications.

Solvent Evaporation Rate as a Tool for Tuning the Performance of a Solid Polymer Electrolyte Gas Sensor

Solid polymer electrolytes show their potential to partially replace conventional electrolytes in electrochemical devices. The solvent evaporation rate represents one of many options for modifying the electrode–electrolyte interface by affecting the structural and electrical properties of polymer electrolytes used in batteries. This paper evaluates the effect of solvent evaporation during the preparation of solid polymer electrolytes on the overall performance of an amperometric gas sensor. A mixture of the polymer host, solvent and an ionic liquid was thermally treated under different evaporation rates to prepare four polymer electrolytes. A carbon nanotube-based working electrode deposited by spray-coating the polymer electrolyte layer allowed the preparation of the electrode–electrolyte interface with different morphologies, which were then investigated using scanning electron microscopy and Raman spectroscopy. All prepared sensors were exposed to nitrogen dioxide concentration of 0–10 ppm, and the current responses and their fluctuations were analyzed. Electrochemical impedance spectroscopy was used to describe the sensor with an equivalent electric circuit. Experimental results showed that a higher solvent evaporation rate leads to lower sensor sensitivity, affects associated parameters (such as the detection/quantification limit) and increases the limit of the maximum current flowing through the sensor, while the other properties (hysteresis, repeatability, response time, recovery time) change insignificantly.

Two-Stage Evolution of Gamma-Phase Spherulites of Poly (Vinylidene Fluoride) Induced by Alkylammonium Salt

We investigated the evolution of the γ-phase spherulites of poly(vinylidene fluoride) (PVDF) added to 1 wt% of tetrabutylammonium hydrogen sulfate during the isothermal crystallization at 165 °C through polarized optical microscopy and light scattering measurements. Optically isotropic domains grew, and then optical anisotropy started to increase in the domain to yield spherulite. Double peaks were seen in the time variation of the Vv light scattering intensity caused by the density fluctuation and optical anisotropy, and the Hv light scattering intensity caused by the optical anisotropy started to increase during the second increase in the Vv light scattering intensity. These results suggest the two-stage evolution of the γ-phase spherulites, i.e., the disordered domain grows in the first stage and ordering in the spherulite increases due to the increase in the fraction of the lamellar stacks in the spherulite without a change in the spherulite size in the second stage. Owing to the characteristic crystallization behavior, the birefringence in the γ-phase spherulites of the PVDF/TBAHS was much smaller than that in the α-phase spherulites of the neat PVDF.

Effect of the TrFE Content on the Crystallization and SSA Thermal Fractionation of P(VDF-co-TrFE) Copolymers

In this contribution, we study the effect of trifluoro ethylene (TrFE) comonomer content (samples with 80/20, 75/25, and 70/30 VDF/TrFE molar ratios were used) on the crystallization in P(VDF-co-TrFE) in comparison with a PVDF (Poly(vinylidene fluoride)) homopolymer. Employing Polarized Light Optical Microscopy (PLOM), the growth rates of spherulites or axialites were determined. Differential Scanning Calorimetry (DSC) was used to determine overall crystallization rates, self-nucleation, and Successive Self-nucleation and Annealing (SSA) thermal fractionation. The ferroelectric character of the samples was explored by polarization measurements. The results indicate that TrFE inclusion can limit the overall crystallization of the copolymer samples, especially for the ones with 20 and 25% TrFE. Self-nucleation measurements in PVDF indicate that the homopolymer can be self-nucleated, exhibiting the classic three Domains. However, the increased nucleation capacity in the copolymers provokes the absence of the self-nucleation Domain II. The PVDF displays a monomodal distribution of thermal fractions after SSA, but the P(VDF-co-TrFE) copolymers do not experience thermal fractionation, apparently due to TrFE incorporation in the PVDF crystals. Finally, the maximum and remnant polarization increases with increasing TrFE content up to a maximum of 25% TrFE content, after which it starts to decrease due to the lower dipole moment of the TrFE defect inclusion within the PVDF crystals.

High Conduction Band Inorganic Layers for Distinct Enhancement of Electrical Energy Storage in Polymer Nanocomposites

Dielectric polymer nanocomposites are considered as one of the most promising candidates for high-power-density electrical energy storage applications. Inorganic nanofillers with high insulation property are frequently introduced into fluoropolymer to improve its breakdown strength and energy storage capability. Normally, inorganic nanofillers are thought to introducing traps into polymer matrix to suppress leakage current. However, how these nanofillers effect the leakage current is still unclear. Meanwhile, high dopant (> 5 vol%) is prerequisite for distinctly improved energy storage performance, which severely deteriorates the processing and mechanical property of polymer nanocomposites, hence brings high technical complication and cost. Herein, boron nitride nanosheet (BNNS) layers are utilized for substantially improving the electrical energy storage capability of polyvinylidene fluoride (PVDF) nanocomposite. Results reveal that the high conduction band minimum of BNNS produces energy barrier at the interface of adjacent layers, preventing the electron in PVDF from passing through inorganic layers, leading to suppressed leakage current and superior breakdown strength. Accompanied by improved Young's modulus (from 1.2 GPa of PVDF to 1.6 GPa of nanocomposite), significantly boosted discharged energy density (14.3 J cm-3) and charge-discharge efficiency (75%) are realized in multilayered nanocomposites, which are 340 and 300% of PVDF (4.2 J cm-3, 25%). More importantly, thus remarkably boosted energy storage performance is accomplished by marginal BNNS. This work offers a new paradigm for developing dielectric nanocomposites with advanced energy storage performance.

Effects of Molecular Weight on the Electrochemical Properties of Poly(vinylidene difluoride)-Based Polymer Electrolytes

Polymer-based electrolytes have attracted ever-increasing attention for solid-state batteries due to their excellent flexibility and processability. Among them, poly(vinylidene difluoride) (PVDF)-based electrolytes with high ionic conductivity, wide electrochemical stability window, and good mechanical properties show great potential and have been widely investigated by using different Li salts, solvents, and inorganic fillers. Here, we report the influence of the molecular weight of PVDF itself on the electrochemical properties of the electrolytes by using two kinds of common PVDF polymers, i.e., PVDF 761 and 5130. Our results demonstrate that the electrolyte with a larger molecular weight (PVDF 5130) has a denser structure and lower crystallinity, and thus much better electrochemical performance, than one with a smaller molecular weight (PVDF 761). With PVDF 5130, the LiFePO₄-based solid-state cells present a steady cycling performance with a capacity retention of 85% after 1000 cycles at 1 C and 30 °C. The cycle life of the LiCoO₂-based solid-state cells is also extended by using PVDF 5130.

An Optical/Ferroelectric Multiplexing Multidimensional Nonvolatile Memory from Ferroelectric Polymer

Multiplexing physical dimensions to realize multidimensional storage in a single material has been a goal to increase storage density and data security. Multidimensional storage is only achieved in optical storage material (OSM) by far. Poly(vinylidene fluoride) (PVDF), a semicrystalline polymer, is widely studied as a candidate for ferroelectric random access (FeRAM). Herein, the atomic force microscopy (AFM)-based infrared spectroscopy techniqueis used to induce multilevel phase transformations in PVDF ultrathin film on nanometric scales and for writing/readout of IR signals. An optical/ferroelectric multiplexing PVDF memory, where information can be coded with independent four-level optical IR and bilevel ferroelectric signals, is demonstrated. High data security and a storage density up to 180 GBit in.^-2 are achieved simultaneously. Owing to the different critical temperature for phase transformation (optical data, <167 °C) and polarization switching (ferroelectric data, <100 °C), the multiplexing memory can function both as optical read-only memory and FeRAM. This work expands material supporting physical dimensions multiplexing beyond OSM for the first time, opening up new opportunities for future high-capacity, multifunctional nano-memory. The strategy proposed here enables on-demand and tunable programming on IR waves, offering prospects for fabrication of active nano-optical devices.