P. Martins

On the origin of the electroactive poly(vinylidene fluoride) β-phase nucleation by ferrite nanoparticles via surface electrostatic interactions

Flexible multiferroic 0–3 composite films, comprising NiFe2O4 and CoFe2O4 ferrite nanoparticles in a polyvinylidene fluoride (PVDF) matrix, have been prepared by solvent casting and melt crystallization to investigate the polymer β-phase nucleation mechanism. Infrared spectroscopy confirms the nucleation of the polymeric electroactive β-phase with the addition of both ferrites, although the loading of ferrite nanoparticles needed to obtain the highest amount of β-phase was found to be one order of magnitude higher in the NiFe2O4/PVDF nanocomposites. Transmission electron microscopy imaging and thermogravimetric analyses indicate the formation of an interface in the nanocomposites with the β-phase nucleation. It is shown that the essential factor for the nucleation of the β-phase in the ferrites/PVDF nanocomposites is the static electric interaction between the magnetic particles with a negative zeta potential and the CH2 groups having a positive charge density.

Influence of ferrite nanoparticle type and content on the crystallization kinetics and electroactive phase nucleation of poly(vinylidene fluoride)

This work reports on the nucleation of the β-phase of poly(vinylidene fluoride) (PVDF) by incorporating CoFe(2)O(4) and NiFe(2)O(4) nanoparticles, leading in this way to the preparation of magnetoelectric composites. The fraction of filler nanoparticles needed to produce the same β- to α-phase ratio in crystallized PVDF is 1 order of magnitude lower in the cobalt ferrite nanoparticles. The interaction between nanoparticles and PVDF chains induce the all-trans conformation in PVDF segments, and this structure then propagates in crystal growth. The nucleation kinetics is enhanced by the presence of nanoparticles, as corroborated by the increasing number of spherulites with increasing nanoparticle content and by the variations of the Avramis exponent. Further, the decrease of the crystalline fraction of PVDF with increasing nanoparticle content indicates that an important fraction of polymer chains are confined in interphases with the filler particle.

Optimizing piezoelectric and magnetoelectric responses on CoFe2O4/P(VDF-TrFE) nanocomposites

Magnetoelectric (ME) nanocomposite films composed of magnetostrictive CoFe2O4 nanoparticles with sizes between 35 and 55xa0nm embedded in P(VDF-TrFE) have been successfully prepared by a solvent casting method. The ferroelectric, piezoelectric, magnetic and ME properties of the nanocomposite and their variation with the wt% of the ferrite filler, thickness of the composite and direction of the applied magnetic field have been investigated.Ferroelectric and piezoelectric properties were improved when a small amount of ferrite nanoparticles was added to the polymeric matrix. Magnetic properties vary linearity with ferrite content. The highest ME response of 41.3xa0mVxa0cm−1xa0Oe−1 was found in the composite with 72xa0wt% when a 2.5xa0kOe DC field was transversely applied to the sample surface. This value is among the highest reported in two phase particulate polymer nanocomposites. Thickness of the composite has no influence in the ME response, allowing tailoring sensor thickness for specific applications. The good value of the ME coefficient and the flexibility of the films make these composites suitable for applications in ME smart devices.

Proving the suitability of magnetoelectric stimuli for tissue engineering applications

A novel approach for tissue engineering applications based on the use of magnetoelectric materials is presented. This work proves that magnetoelectric Terfenol-D/poly(vinylidene fluoride-co-trifluoroethylene) composites are able to provide mechanical and electrical stimuli to MC3T3-E1 pre-osteoblast cells and that those stimuli can be remotely triggered by an applied magnetic field. Cell proliferation is enhanced up to ≈ 25% when cells are cultured under mechanical (up to 110 ppm) and electrical stimulation (up to 0.115 mV), showing that magnetoelectric cell stimulation is a novel and suitable approach for tissue engineering allowing magnetic, mechanical and electrical stimuli.

Interface characterization and thermal degradation of ferrite/poly(vinylidene fluoride) multiferroic nanocomposites

Flexible multiferroic 0–3 composite films, with CoFe2O4, Ni0.5Zn0.5Fe2O4 or NiFe2O4 ferrite nanoparticles as filler and polyvinylidene fluoride (PVDF) as the polymer matrix, have been prepared by solvent casting and melt crystallization. The inclusion of ferrite nanoparticles in the polymer allows to obtain magnetoelectric nanocomposites through the nucleation of the piezoelectric β-phase of the polymer by the ferrite fillers. Since the interface between PVDF and the nanoparticles has an important role in the nucleation of the polymer phase, thermogravimetric analysis was used in order to identify and quantify the interface region and to correlate it with the β-phase content. It is found that an intimate relation exists between the size of the interface region and the piezoelectric β-phase formation that depends on the content and type of ferrite nanoparticles. The interface value and the β-phase content increase with increasing ferrite loading and they are higher for CoFe2O4 and Ni0.5Zn0.5Fe2O4 ferrite nanoparticles. The composites shows lower thermal stability than the pure polymer due to the existence of mass loss processes at lower temperature than the main degradation of the polymer. The main degradation of the polymer matrix, nevertheless, shows increased degradation temperature with increasing ferrite content.

Osteoblast, fibroblast and in vivo biological response to poly(vinylidene fluoride) based composite materials

Electroactive materials can be taken to advantage for the development of sensors and actuators as well as for novel tissue engineering strategies. Composites based on poly(vinylidene fluoride), PVDF, have been evaluated with respect to their biological response. Cell viability and proliferation were performed in vitro both with Mesenchymal Stem Cells differentiated to osteoblasts and Human Fibroblast Foreskin 1. In vivo tests were also performed using 6-week-old C57Bl/6 mice. It was concluded that zeolite and clay composites are biocompatible materials promoting cell response and not showing in vivo pro-inflammatory effects which renders both of them attractive for biological applications and tissue engineering, opening interesting perspectives to development of scaffolds from these composites. Ferrite and silver nanoparticle composites decrease osteoblast cell viability and carbon nanotubes decrease fibroblast viability. Further, carbon nanotube composites result in a significant increase in local vascularization accompanied an increase of inflammatory markers after implantation.

TiO2/graphene oxide immobilized in P(VDF-TrFE) electrospun membranes with enhanced visible-light-induced photocatalytic performance

Here, we report on the electrospinning of poly(vinylidene difluoride-co-trifluoroethylene) (P(VDF-TrFE)) copolymer fibrous membranes decorated with titanium dioxide/graphene oxide (TiO2/GO). The presence of the TiO2/GO increases the photocatalytic efficiency of the nanocomposite membrane towards the degradation of methylene blue (MB) when compared with the membranes prepared with naked TiO2, in UV and particularly in the visible range. Even a low content (3xa0%, w/w) of TiO2/GO in the fibers yields excellent photocatalytic performance by degrading ~100xa0% of a MB solution after 90xa0min of visible light exposure. This may be attributed to a rapid electron transport and the delayed recombination of electron–hole pairs due to improved ionic interaction between titanium and carbon combined with the advantageous electric properties of the polymer, such as high polarization and dielectric constant combined with low dielectric loss. Thus, a promising system to degrade organic pollutants in aqueous or gaseous systems under visible light irradiation has been developed.

Comparative efficiency of TiO2 nanoparticles in suspension vs. immobilization into P(VDF–TrFE) porous membranes

Photocatalytic processes based on titanium dioxide (TiO2) nanoparticles have attracted increasing attention in the last decades. However, approaches based on nanoparticles show some drawbacks, in particular due to the need for expensive and time consuming post-treatment of nanoparticles filtration/separation. This hindrance demands the development of immobilized configurations with tailored properties, as an alternative to allow simple recovery of the photocatalytic particles. Thus, this work reports on the development of photocatalytic membranes based on TiO2 nanoparticles immobilized into a poly(vinylidenefluoride–trifluoroethylene) (P(VDF–TrFE)) membrane and the comparative study of their performance with dispersed TiO2 nanoparticles. Photocatalytic nanocomposite membranes with a highly porous structure (∼75%) and controlled wettability by NaY addition were successfully produced. These properties were paramount to achieve a methylene blue degradation efficiency of 96% in 40 min under ultraviolet (UV) irradiation, corresponding to an efficiency loss of just 3% regarding the TiO2 nanoparticle assays.

Electronic optimization for an energy harvesting system based on magnetoelectric Metglas/poly(vinylidene fluoride)/Metglas composites

Harvesting magnetic energy from the environment is becoming increasingly attractive for being a renewable and inexhaustible power source, ubiquitous and accessible in remote locations. In particular, magnetic harvesting with polymer-based magnetoelectric (ME) materials meet the industry demands of being flexible, showing large area potential, lightweight and biocompatibility. In order to get the best energy harvesting process, the extraction circuit needs to be optimized in order to be useful for powering devices. This paper discusses the design and performance of five interface circuits, a full-wave bridge rectifier, two Cockcroft–Walton voltage multipliers (with 1 and 2 stages) and two Dickson voltage multipliers (with 2 and 3 stages), for the energy harvesting from a Fe61.6Co16.4Si10.8B11.2 (Metglas)/polyvinylidene fluoride/Metglas ME composite. Maximum power and power density values of 12 μW and 0.9 mW cm−3 were obtained, respectively, with the Dickson voltage multiplier with two stages, for a load resistance of 180 kΩ, at 7 Oe DC magnetic field and a 54.5 kHz resonance frequency. Such performance is useful for microdevice applications in hard-to-reach locations and for traditional devices such as electric windows, door locking, and tire pressure monitoring.

Understanding nucleation of the electroactive β-phase of poly(vinylidene fluoride) by nanostructures

β-Poly(vinylidene fluoride) (PVDF) is of large technological relevance due to its piezoelectric, pyroelectric and/ferroelectric properties. In this way, a variety of methods have been developed to obtain such electroactive β-phase, being the addition of fillers one of the most popular, upscalable and innovative methods. The electrostatic interaction between negative charged fillers with the CH2 groups having a positive charge density has been the most widely accepted mechanism for the direct formation of polar β-phase on nanocomposites. Nevertheless some controversy remains in this matter as the dominating crystallization into the β-phase within PVDF is sometimes attributed to the interaction between the positively charged surfaces of the fillers and the CF2 dipoles in PVDF. In order to clarify such a controversial issue, this work uses two types of nanostructures, Fe3O4 nanorods and Fe3O4 nanoparticles, with distinct sizes and surface charges to study, isolate and evaluate the effects of the different ion–dipole interactions and shapes on the crystalline structures of PVDF. As a result it is shown that in the case of positive ion–CF2 dipole based β-phase nucleation, and beyond the effect of the intermolecular interactions, the rod-shape optimizes the crystallization in the electroactive conformation, thus promoting current development in PVDF-based electroactive devices.