Gabriela Botelho

Studies on thermal and thermo-oxidative degradation of poly(ethylene terephthalate) and poly(butylene terephthalate)

Abstract A comparative study was conducted into the thermo-oxidative degradation of poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT). Degradation of the polymer films and model compounds, ethylene dibenzoate (EDB) and butylene dibenzoate (BDB), was carried out in an oxygen atmosphere at 160°C. On the basis of the compounds identified by GC–MS a mechanism is proposed for the degradation of the model compounds that involves the oxidation at the α-methylene carbon with formation of unstable peroxides and carboxylic acids. From the studies performed under N 2 at 160 °C it could be concluded that benzoic acid and esters are products of the thermal degradation, while benzoic and aliphatic acids, anhydride and alcohols are due to thermo-oxidative degradation. In contrast to the thermo-oxidative degradation of other polymers, for PET and PBT, especially at the beginning, thermal degradation plays an important role. The results clearly showed that PET is more stable towards degradation than PBT.

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.

Nanostructured Polymeric Coatings Based on Chitosan and Dopamine‐Modified Hyaluronic Acid for Biomedical Applications

In a marine environment, specific proteins are secreted by mussels and used as a bioglue to stick to a surface. These mussel proteins present an unusual amino acid 3,4-dihydroxyphenylalanine (known as DOPA). The outstanding adhesive properties of these materials in the sea harsh conditions have been attributed to the presence of the catechol groups present in DOPA. Inspired by the structure and composition of these adhesive proteins, dopamine-modified hyaluronic acid (HA-DN) prepared by carbodiimide chemistry is used to form thin and surface-adherent dopamine films. This conjugate was characterized by distinct techniques, such as nuclear magnetic resonance and ultraviolet spectrophotometry. Multilayer films are developed based on chitosan and HA-DN to form polymeric coatings using the layer-by-layer methodology. The nanostructured films formation is monitored by quartz crystal microbalance. The film surface is characterized by atomic force microscopy and scanning electron microscopy. Water contact angle measurements are also conducted. The adhesion properties are analyzed showing that the nanostructured films with dopamine promote an improved adhesion. In vitro tests show an enhanced cell adhesion, proliferation and viability for the biomimetic films with catechol groups, demonstrating their potential to be used in distinct biomedical applications.

Electrosprayed poly(vinylidene fluoride) microparticles for tissue engineering applications

Poly(vinylidene fluoride) (PVDF) microparticles have been produced by electrospraying as a suitable substrate for tissue engineering applications. The influence of the polymer solution concentration and processing parameters, such as electric field, flow rate and inner needle diameter, on microparticle size and distribution has been studied. Polymer concentration is the most influential parameter on PVDF microparticle formation. Higher concentrations promote the formation of fibers while dilute or semi dilute concentrations favor the formation of PVDF microparticles with average diameters ranging between 0.81 ± 0.34 and 5.55 ± 2.34 μm. Once the formation of microparticles is achieved, no significant differences were found with the variation of other electrospray processing parameters. The electroactive β-phase content, between 63 and 74%, and the crystalline phase content, between 45 and 55%, are mainly independent of the processing parameters. Finally, MC-3T3-E1 cell adhesion on the PVDF microparticles is assessed, indicating their potential use for biomedical applications.

Development of magnetoelectric CoFe2O4 /poly(vinylidene fluoride) microspheres

Magnetoelectric microspheres based on piezoelectric poly(vinylidene fluoride) (PVDF) and magnetostrictive CoFe2O4 (CFO), a novel morphology for polymer-based ME materials, have been developed by an electrospray process. The CFO nanoparticle content in the (3–7 μm diameter) microspheres reaches values up to 27 wt%, despite their concentration in the starting solution reaching values up to 70 wt%. Additionally, the inclusion of magnetostrictive nanoparticles into the polymer spheres has no relevant effect on the piezoelectric β-phase content (≈60%), crystallinity (40%) and the onset degradation temperature (460–465 °C) of the polymer matrix. The multiferroic microspheres show a maximum piezoelectric response |d33| ≈ 30 pC N−1, leading to a magnetoelectric response of Δ|d33| ≈ 5 pC N−1 obtained when a 220 mT DC magnetic field was applied. It is also shown that the interface between CFO nanoparticles and PVDF (from 0 to 55%) has a strong influence on the ME response of the microspheres. The simplicity and the scalability of the processing method suggest a large application potential of this novel magnetoelectric geometry in areas such as tissue engineering, sensors and actuators.

Thermooxidative studies of poly(ether-esters) 1. Copolymer of poly(butylene terephthalate) and poly(ethylene oxide)

To determine the thermo-oxidative degradation mechanism of a copolymer of poly(butylene terephthalate) and polybutylene oxide, degradation experiments were carried out on this polymer and a model compound (dibutyleneglycol dibenzoate) at 140°C in an oxygen atmosphere. The changes were analysed using various analytical techniques, such as oxygen uptake measurements, measurement of CO/CO2 formation, 1H and 13C NMR, FT-IR and GC–MS. On the basis of the results, an oxidation mechanism for the model compound was proposed and it was concluded that oxidation is initiated at the methylene carbon adjacent to the ether oxygen. The thermal oxidation mainly results in the formation of a stable ester together with a formate, aliphatic and aromatic acids and an alcohol. Based on the similar degradation products found for the model compound and the polymer, it was concluded that the thermo-oxidation mechanisms in these two compounds are similar.

Novel Anisotropic Magnetoelectric Effect on δ-FeO(OH)/P(VDF-TrFE) Multiferroic Composites

The past decade has witnessed increased research effort on multiphase magnetoelectric (ME) composites. In this scope, this paper presents the application of novel materials for the development of anisotropic magnetoelectric sensors based on δ-FeO(OH)/P(VDF-TrFE) composites. The composite is able to precisely determine the amplitude and direction of the magnetic field. A new ME effect is reported in this study, as it emerges from the magnetic rotation of the δ-FeO(OH) nanosheets inside the piezoelectric P(VDF-TrFE) polymer matrix. δ-FeO(OH)/P(VDF-TrFE) composites with 1, 5, 10, and 20 δ-FeO(OH) filler weight percentage in three δ-FeO(OH) alignment states (random, transversal, and longitudinal) have been developed. Results have shown that the modulus of the piezoelectric response (10-24 pC·N(-1)) is stable at least up to three months, the shape and magnetization maximum value (3 emu·g(-1)) is dependent on δ-FeO(OH) content, and the obtained ME voltage coefficient, with a maximum of ∼0.4 mV·cm(-1)·Oe(-1), is dependent on the incident magnetic field direction and intensity. In this way, the produced materials are suitable for innovative anisotropic sensor and actuator applications.

Nanoparticle size and concentration dependence of the electroactive phase content and electrical and optical properties of Ag/poly(vinylidene fluoride) composites.

This paper describes the processing of silver-nanoparticle-doped poly(vinylidene fluoride). The effects of the concentration and size of the filler on the electroactive phase of the polymer and the optical and electrical properties are discussed. Spherical silver nanoparticles incorporated into the poly(vinylidene fluoride) polymeric matrix induce nucleation of the electroactive γ phase. The electroactive phase content strongly depends on the content and size of the nanoparticles. In particular, there is a critical nanoparticle size, below which the filler losses its nucleation efficiency due to its small size relative to that of the polymer macromolecules. Furthermore, the presence of surface plasmon resonance absorption in the composites is observed, which once again shows a strong dependence on the concentration and size of the particles. The absorption is larger for higher concentrations, and for a given concentration increases with particle size. This behavior is correlated to the electrical response and is related to the extra bands and electrons provided by the nanoparticles in the large energy band gap of the polymer.

Magnetoelectric CoFe2O4/polyvinylidene fluoride electrospun nanofibres

Magnetoelectric 0-1 composites comprising CoFe2O4 (CFO) nanoparticles in a polyvinylidene fluoride (PVDF) polymer-fibre matrix have been prepared by electrospinning. The average diameter of the electrospun composite fibres is ∼325 nm, independent of the nanoparticle content, and the amount of the crystalline polar β phase is strongly enhanced when compared to pure PVDF polymer fibres. The piezoelectric response of these electroactive nanofibres is modified by an applied magnetic field, thus evidencing the magnetoelectric character of the CFO/PVDF 0-1 composites.

The Role of Solvent Evaporation in the Microstructure of Electroactive β-Poly(Vinylidene Fluoride) Membranes Obtained by Isothermal Crystallization

Electroactiveβ-poly(vinylidene fluoride) (PVDF) membranes were obtained by isothermal crystallization from the solution. Different morphologies and microstructures were obtained by crystallizing at different temperatures. The mechanism and kinetics of solvent evaporation from the polymeric solution were investigated using isothermal thermogravimetric analysis. The kinetic parameters and the activation energy were also calculated. The solvent evaporation is ruled by two steps, related with a metastable– unstable–metastable transition in the solution phase diagram. Scanning electron microscopy revealed the porous structure and the variations of the morphology with the variation of the isothermal evaporation temperature. Finally, the infrared spectroscopy measurements confirm that the polymer crystallizes in the electroactiveβ-phase of PVDF.