Modification and processing of poly (vinylidene fluoride) (PVDF)-based Polymers towards ferroelectric thin films

In this work, thin films of poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) were prepared via two routes, spin coating and electrospinning. It was found that electrospinning of PVDF from its N,N-dimethylformamide/acetone solutions can induce molecular orientation of PVDF chains and hence promote the formation of ?-phase. The addition of 3 wt.% of tetrabutylammonium chloride (TBAC) into the polymer solutions can effectively improve the morphology of the electrospun fibers due to the great increase in the conductivity of the solutions, and lead to almost pure ?-phase in the fibers. TBAC, a hygroscopic salt, could retain water in the fibers, which may led to hydrogen bonding between the water molecules and the fluorine atoms of PVDF and induce more trans conformers, while electrospinning induces chain orientation, which promotes inter-chain registration and hence stabilizes ?-nuclei. Modification of the rotating disk collector allowed the collection of thin films of aligned electrospun PVDF and P(VDF-TrFE) fibers, enabling the measurement of the hysteresis loops of these thin films. Electrospun PVDF performed significantly better than P(VDF-TrFE) which differs from the trend observed from spin-coated films. The degree of crystallinity and the contribution of other polar phases in electrospun PVDF fibers were believed to be the main reason for this phenomenon. The blending of PVDF and P(VDF-TrFE) across various compositions did not give rise to significant co-crystallization, regardless of the processing method used. Preliminary results of treatment of PVDF with hydrogen peroxide yielded spin-coated films with high polar phases (both ?- and ?-phase) with impressive a-phase suppression, allowing a one step method to produce films with high polar phase content.Master of Engineering (SME

Morphologies, structures and properties of electrospun poly(vinylidene difluoride)-based materials

Electrospinning is a technique that can be used to produce nanofibers, and it is discovered that it is able to enhance the β-phase of poly(vinylidene difluoride), PVDF. Owing to its highest polarization per unit cell, the β-phase of PVDF exhibits the maximum electroactive properties, i.e., ferro-, pyro- and piezo-electric properties, in comparison with other phases of PVDF. Although researches have shown that the β-phase of PVDF can be improved by (1) alteration of the chemical structure, (2) usage of additives, (3) application of a stretching force and (4) the use of high temperature and pressure treatments, the synergistic effects among (2), (3) and (4) on polymorphism behaviors and electro-active of PVDF have not been studied. Thus, this work was undertaken to study polymorphism behaviors, morphologies and electro-active properties of electrospun poly(vinylidene fluoride-co-hexafluoropropene) (P(VDF-HFP)) nanofibers and P(VDF-HFP)/carbon nanotube (CNT) composite nanofibers, which experienced high extensional forces in the electrospinning and collection processes, as well as high temperature and pressure treatments in supercritical carbon dioxide (SCCO2). Firstly, a modified rotating disk collector is used in electrospinning to provide stronger stretching force to the electrospun P(VDF-HFP) nanofibers. By attaching seperate, parallel electrodes onto a rotating disk collector, well aligned nanofibers of P(VDF-HFP)) and P(VDF-HFP)/CNT nanocomposites can be directly deposited onto flat substrates forming uniform and compact nanofibrous mats. The attachment alters the electric-field distribution on the rotating disk while providing an additional stretching force during the fanning of the fibers. This additional stretching force exerted by the modified rotating disk prevents the relaxation of the polymer chains during solvent evaporation, and hence results in the formation of extended-chain β-chain crystallites with a slightly reduced inter-chain distance, as evidenced by wide angle X-ray diffraction (WAXD) patterns and Fourier transform infrared (FTIR) spectra of the samples.DOCTOR OF PHILOSOPHY (MSE

Polymorphism of electrospun polyvinylidene difluoride/carbon nanotube (CNT) nanocomposites : synergistic effects of CNT surface chemistry, extensional force and supercritical carbon dioxide treatment

In this work, the effects of single-walled carbon nanotubes (SWCNT) surface chemistry on the polymorphism behaviors of electrospun poly(vinylidene difluoride) (PVDF) nanofibers collected under high extensional force and subsequently treated with supercritical carbon dioxide (SCCO2) are investigated via X-ray diffraction, infrared and Raman spectroscopy. It is found that the SWCNTs with hydroxyl groups (h-SWCNTs) interact with PVDF chains more intensively than the ones with ester groups, and the interaction in couple with the high extensional force promotes the formation of β-form extended-chain crystallites (ECCs). With the SCCO2 treatment, the growth of the β-form ECCs is further promoted in the PVDF/h-SWCNT nanofibers owing to the synergistic effect of the strong confinement effect of h-SWCNT for stabilizing the nuclei of the β-form ECCs and the high pressure applied

Designing calcium phosphate-based bifunctional nanocapsules with bone-targeting properties

Using sodium dodecyl sulphate micelles as template, hollow-cored calcium phosphate nanocapsules were produced. The surfaces of the nanocapsule were subsequently silanised by a polyethylene glycol (PEG)-based silane with an N-hydroxysuccinimide ester end groups which permits for further attachment with bisphosphonates (BP). Characterisations of these nanocapsules were investigated using Field Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy, Fourier Transform Infra-Red Spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Dynamic Light Scattering. To further validate the bone-targeting potential, dentine discs were incubated with these functionalised nanocapsules. FESEM analysis showed that these surface-modified nanocapsules would bind strongly to dentine surfaces compared to non-functionalised nanocapsules. We envisage that respective components would give this construct a bifunctional attribute, whereby (1) the shell of the calcium phosphate nanocapsule would serve as biocompatible coating aiding in gradual osteoconduction, while (2) surface BP moieties, acting as targeting ligands, would provide the bone-targeting potential of these calcium phosphate nanocapsules

Enhanced functional and structural characteristics of poly(vinylidene-trifluoroethylene) copolymer thin films by corona poling

Electrical poling is of much interest for the enhancement of polarization properties of ferroelectric materials such as poly(vinylidene-trifluoroethylene) [P(VDF-TrFE)] in applications ranging from memory devices to piezoelectric actuation. Here, we report the changes in dielectric and optical properties correlated with structural modification of P(VDF-TrFE) thin films (<1 μm thickness) subjected to corona poling at low voltages (6 kV) under various thermal conditions. Enhanced crystallinity of hot-poled samples was observed independent of thermal history and crystalline structure, indicating the absence of recrystallization during poling. Absorption measurements confirmed the ordered structure in hot-poled samples due to the permanent orientation of the dipoles and higher crystallinity, thus resulting in increased and long-retained capacitance of the ferroelectric P(VDFTrFE) capacitor device from high frequency C-V characterization. X-ray photoelectron spectroscopy further confirmed the shifting of C and F core orbitals to higher binding energy is related to formation of more ferroelectric phases by corona poling.Published versio

A high throughput method for preparation of highly conductive functionalized graphene and conductive polymer nanocomposites

Highly conductive graphene sheets were prepared by coating graphene oxide with polydopamine (PDA) followed by reduction with hydrazine. Polyacrylonitrile/graphene nanocomposites prepared via solution blending exhibit high electrical conductivities at very low graphene loadings owing to the good exfoliation and relatively planar conformation of the PDA-coated graphene in the polymer matrix

Highly conductive graphene by low-temperature thermal reduction and in situ preparation of conductive polymer nanocomposites

Polydopamine-coated graphene oxide (DGO) films exhibit electrical conductivities of 11 000 S m−1 and 30 000 S m−1 upon vacuum annealing at 130 °C and 180 °C, respectively. Conductive poly(vinyl alcohol)/graphene and epoxy/graphene nanocomposites show low percolation thresholds due to the excellent dispersibility of the DGO sheets and their effective in situ reduction

Electrospinning-derived “Hairy Seaweed” and its photoelectrochemical properties

Highly porous three-dimensional (3D) hierarchical nanostructures suspended in aqueous media were facilely prepared via electrospinning of polyacrylonitrile (PAN)/indium tin oxide (ITO) nanofibers and collection of the hybrid nanofibers by water, followed by hydrothermally growing ZnO nanorods from the nanofibers. The large inter-fiber distances facilitated the uniform growth of the ZnO nanorods throughout the whole system. The suspended PAN/ITO nanofibers process excellent light trapping capability due to their centimeter-sized dimensions and hence large light penetration path. This significantly increases the probability of multiple-reflections, leading to high absorption with almost zero transmission when the size of the sample reaches 10 mm in the direction parallel to incident light. High photocurrent was generated when the nanorods-on-nanofibers was used as a photoanode. The high photocurrent density generated by the anode can be attributed to its excellent light-trapping capability brought by the large amount of interaction sites between the ZnO nanorods and light, its large contact area with electrolyte, as well as the conduction path constructed by high-content ITO nanoparticles.Accepted versio

Transition-metal-ion-mediated polymerization of dopamine : Mussel-inspired approach for the facile synthesis of robust transition-metal nanoparticle-graphene hybrids

Inspired by the high transition-metal-ion content in mussel glues, and the cross-linking and mechanical reinforcement effects of some transition-metal ions in mussel threads, high concentrations of nickel(II), cobalt(II), and manganese(II) ions have been purposely introduced into the reaction system for dopamine polymerization. Kinetics studies were conducted for the Ni2+–dopamine system to investigate the polymerization mechanism. The results show that the Ni2+ ions could accelerate the assembly of dopamine oligomers in the polymerization process. Spectroscopic and electron microscopic studies reveal that the Ni2+ ions are chelated with polydopamine (PDA) units, forming homogeneous Ni2+–PDA complexes. This facile one-pot approach is utilized to construct transition-metal-ion–PDA complex thin coatings on graphene oxide, which can be carbonized to produce robust hybrid nanosheets with well-dispersed metallic nickel/metallic cobalt/manganese(II) oxide nanoparticles embedded in PDA-derived thin graphitic carbon layers. The nickel–graphene hybrid prepared by using this approach shows good catalytic properties and recyclability for the reduction of p-nitrophenol

Stress-induced structural changes in electrospun polyvinylidene difluoride nanofibers collected using a modified rotating disk

By attaching separate, parallel electrodes onto a rotating disk collector, well aligned electrospun polyvinylidene difluoride (PVDF), PVDF/carbon nanotube nanocomposite and vinylidene fluoride–trifluoroethylene copolymer nanofibers are directly deposited onto flat substrates forming relatively large, uniform and compact fibrous thin films. The attachments alter the electric-field distribution on the rotating disk, which fosters the fanning of the nanofibers, while the electric field between the separate electrodes and the mechanical force exerted by the rotational disk facilitate the alignment. X-ray diffraction and infrared spectroscopic studies show that the specific environment and force fields created on the modified rotating disk cause the electrospun fibers being effectively stretched to form highly oriented β-form crystallites with slightly reduced inter-chain distance. They also lead to slight increases in crystallinity and crystal size. A mechanism is proposed to account for the structural alteration induced by the modified rotating disk collector. Ferroelectricity of the aligned electrospun PVDF fibrous thin films is also demonstrated.Accepted versio