Kap Jin Kim

Origin of Piezoelectricity in an Electrospun Poly(vinylidene fluoride-trifluoroethylene) Nanofiber Web-Based Nanogenerator and Nano-Pressure Sensor

A single stage electrospinning process can give rise to preferentially oriented induced dipoles in poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] nanofibers. The piezoelectricity of as-electrospun P(VDF-TrFE) nanofiber webs opens up new possibilities for their use as a flexible nanogenerators and nano-pressure sensors. In this work, the origin of the piezoelectricity has been spotlighted by randomization of the induced dipoles at the Curie temperature and analyzed by polarized FT-IR spectroscopic techniques as well as by detecting the piezoelectric signal from a nano-pressure sensor.

Spin cast ferroelectric beta poly(vinylidene fluoride) thin films via rapid thermal annealing

We describe a method of fabricating ferroelectric beta-type poly(vinylidene fluoride) (PVDF) thin films on Au substrate by the humidity controlled spin casting combined with rapid thermal treatment. Our method produces thin uniform ferroelectric PVDF film with ordered beta crystals consisting of characteristic needlelike microdomains. A capacitor with a 160nm thick ferroelectric PVDF film exhibits the remanent polarization and coercive voltage of ∼7.0μC∕cm2 and 8V, respectively, with the temperature stability of up to 160°C. A ferroelectric field effect transistor also shows a drain current bistablility of 100 at zero gate voltage with ±20V gate voltage sweep.

Release of albumin from chitosan-coated pectin beads in vitro.

The release behavior of albumin from chitosan-coated pectin beads in vitro was investigated. The factors, such as concentration of CaCl(2), molecular weight of chitosan, pH of chitosan solution, and pH of release medium, which can have a significant effect on the release behavior from the beads, were discussed in this study. The loading efficiency (LE) of albumin showed maximum value when the concentration of CaCl(2) and the weight ratio of pectin to albumin were 2 wt.% and 2, respectively. The release of albumin from pectin beads could be retarded by coating with chitosan at various pH medium. The increase of the concentration of CaCl(2) induced the decrease of albumin release for uncoated-pectin beads, but not much difference of release for coated-pectin ones. The higher molecular weight of chitosan showed less albumin release than the lower one. The release of albumin from the chitosan-coated pectin beads was dependent on pH of coating solution and release medium, which might affect the degree of swelling of pectin beads.

Irreversible extinction of ferroelectric polarization in P(VDF-TrFE) thin films upon melting and recrystallization

We observed the irreversible extinction of ferroelectric polarization in spun coated poly(vinylidene fluoride-co-trifluoroethylene) thin films upon melting and recrystallization. We investigate the alteration of the ferroelectric properties correlated with the preferred polymer crystal orientation with respect to the electrodes using grazing incident scattering, spectroscopy, and electron microscopes. Heat treatment above melting point gave rise to the significant reduction of the ferroelectric performance mainly caused by the modification of molecular orientation of polymer crystals whose c and b axes are perpendicular and parallel to the electrode surface, respectively, leading to almost zero effective electric field.

Simple synthesis of palladium nanoparticles, β-phase formation, and the control of chain and dipole orientations in palladium-doped poly(vinylidene fluoride) thin films.

Palladium nanoparticles (Pd-NPs) are prepared by a simple one-step procedure when poly(vinylidene fluoride) (PVDF) is used as a polymer stabilizer. High-quality Pd-NP-doped PVDF thin films are fabricated where the heat-controlled spin-coating technique is adopted. The effect of Pd-NPs on the crystal modifications and lamellae orientation in PVDF films is investigated using Fourier transform infrared-grazing incidence reflection absorption spectroscopy. The electroactive β phase and edge-on crystalline lamellae are found to be formed preferentially in Pd-NP-doped PVDF films. As a result, Pd-NP-doped PVDF ultrathin films gave a very good discernible contrast between the written and erased data bits, which suggests that they can be used as a scanning-probe-microscopy-based ferroelectric memory device or a ferroelectric gate field-effect transistor memory device in the future.

Factors determining the formation of the β crystalline phase of poly(vinylidene fluoride) in poly(vinylidene fluoride)-poly(methyl methacrylate) blends

Abstract Factor analysis of infrared spectra of poly(vinylidene fluoride)-poly(methyl methacrylate) blends indicates the presence of specific interactions in the melt state. The presence of a band at 882 cm−1 in the interaction spectrum of the melted blend indicates that trans sequences of poly(vinylidene fluoride) (PVDF) in the melt state can increase upon addition of poly(methyl methacrylate) (PMMA), which is highly responsible for the preferential formation of the β phase upon quenching the melted blends. The drastic reduction of the rate of crystallization upon addition of PMMA is another cause which favours the formation of the β crystalline phase. The critical quenching temperature above which the formation of the β phase was rapidly reduced is 0° C for neat PVDF, whereas it was found to be 30 or 55° C for the PVDF-PMMA blends. The percentage of PMMA which gives the maximum β-phase content was found to be around 20 wt.%. It was not PMMA, but the solvent dimethylformamide which affected the crystalline form of PVDF in the case of the samples solvent-cast and annealed. A new method to determine the contents of α- and β-phases in the PVDF-PMMA blends was proposed and used to estimate more quantitatively the effects of quenching temperature and composition on the crystalline structure of PVDF in the blends.

Nanofiber Web Textile Dry Electrodes for Long-Term Biopotential Recording

Electrode properties are key to the quality of measured biopotential signals. Ubiquitous health care systems require long-term monitoring of biopotential signals from normal volunteers and patients in home or hospital environments. In these settings it is appropriate to use dry textile electrode networks for monitoring purposes, rather than the gel or saline-sponge skin interfaces used with Ag/AgCl electrodes. In this study, we report performance test results of two different electrospun conductive nanofiber webs, and three metal plated fabrics. We evaluated contact impedance, step response, noise and signal fidelity performance indices for all five dry electrodes, and compared them to those of conventional Ag/AgCl electrodes. Overall, we found nanofiber web electrodes matched Ag/AgCl electrode performance more closely than metal plated fabric electrodes, with the contact resistance and capacitance of Ag plated PVDF nanofiber web electrodes being most similar to Ag/AgCl over the 10 Hz to 500 kHz frequency range. We also observed that step responses of all three metal-plated fabrics were poorer than those for nanofiber web electrodes and Ag/AgCl. Further, noise standard deviation and noise power spectral densities were generally lower in nanofiber web electrodes than metal plated fabrics; and waveform fidelity of ECG-like traces recorded from nanofiber web electrodes was higher than for metal plated fabrics. We recommend textile nanofiber web electrodes in applications where flexibility, comfort and durability are required in addition to good electrical characteristics.

Curie transition, ferroelectric crystal structure and ferroelectricity of a VDF/TrFE (7525) copolymer: 2. The effect of poling on Curie transition and ferroelectric crystal structure

Abstract Three typical 75 25 VDF/TrFE copolymer samples, differing in the amounts of the three kinds of ferroelectric phases of different thermodynamic stability in the ferroelectric state, were prepared by consecutive annealing below the Curie transition point and were then poled at a field strength of 0.9 MV cm−1. New findings were made based on d.s.c., X-ray diffraction, i.r. spectroscopy and Raman scattering results of the poled samples. D.s.c. results gave indirect evidence that, upon poling, the most stable ferroelectric phase with nearly perfect all-trans conformation is transformed to the unstable ferroelectric phase with some gauche defects resulting from the imperfect propagation of dipole rotation along the chain due to the large steric hindrance against dipole rotation, whereas the less stable ferroelectric phase with considerable gauche defects along the chain shows a reduction of gauche defects through structural reorganization by dipole rotation. The total enthalphy of the Curie transition was greater for all samples after poling. From X-ray diffraction, the average ferroelectric domain size was found to increase in the direction perpendicular to the chain axis upon poling. These results suggest that all samples show increases in the average packing density, the total amount of trans conformers and the degree of ferroelectric crystallinity, irrespective of the initial content of ferroelectric phases of different thermodynamic stability present in the sample prior to poling. From vibrational spectroscopies, however, clear evidence that the total amount of trans conformers increases and the gauche defects are reduced upon poling was not found. No significant differences in the average degree of dipole orientation after poling were observed between the samples from i.r. absorbance spectra in the CH2 stretching vibration region. This indicates that, contrary to expectation, the most important factor in obtaining the highest degree of dipole orientation, i.e. the highest piezoelectric constant, is not the thermodynamic stability of the ferroelectric phase present in the sample.

Fabrication of micropatterned ferroelectric gamma poly(vinylidene fluoride) film for non-volatile polymer memory

We describe a facile and robust method for fabricating ferroelectric γ-type poly(vinylidene fluoride) (PVDF) thin films useful for non-volatile polymer memory. Our method is based on heating and cooling rate-independent melt-recrystallization of a thin PVDF film confined under a surface-energy-controlled top layer that strictly forbids paraelectric α crystals. Thin and uniform PVDF films with ferroelectric γ crystals consisting of characteristic twisted lamellae are formed with versatile top layers including metals, oxides, and even polymers. Micropatterns of ferroelectric γ PVDF domains isolated by paraelectric α domains are readily developed when pre-patterned top layers are employed. Our ferroelectric films are conveniently incorporated into arrays of either capacitor or transistor-type non-volatile memory units. Arrays of ferroelectric transistors with vacuum deposited pentacene channels are fabricated with micropatterned γ PVDF films. Furthermore, arrays of bottom-gate ferroelectric transistor memories are demonstrated, in which our ferroelectric PVDF film is directly micropatterned during crystallization under the patterned poly(3-hexyl thiophene) active channels.

Effect of P(MMA-co-MAA) compatibilizer on the miscibility of nylon 6/PVDF blends

Abstract With the ultimate objective of enhancing the impact strength and weatherability of nylon 6 engineering plastic, blending with poly(vinylidene fluoride) (PVDF) was studied. In the absence of a compatibilizer the two polymers phase separate, resulting in a deterioration of the properties. Since poly(methyl methacrylate) is known to be miscible with PVDF, we evaluated poly(methyl methacrylate-co-methacrylic acid) (P(MMA-co-MAA)) of low methacrylic acid content as the compatibilizer. The carboxylic acid groups in the MAA units were expected to react with the end amino groups of nylon 6 forming block or graft copolymers, P(MMA-co-MAA)–g-nylon 6, in situ, which will function as the actual compatibilizer. The amount of P(MMA-co-MAA) added, the MMA/MAA composition and heat treatment time were varied to study their effects on the miscibility, morphology, and mechanical properties of nylon 6/PVDF blends. The enhancement of the compatibility of nylon 6 and PVDF by addition of P(MMA-co-MAA) and the partial miscibility of nylon 6 and PVDF has been confirmed through DSC, dynamic mechanical testing, SEM of fracture surfaces, and tensile testing. The decrease in the crystallization temperatures on addition of compatibilizer in DSC experiments suggests that the compatibilizer enhances the interaction between the two components and retards the crystallization. The dynamic mechanical thermal analysis experiments suggest that the compatibility in the amorphous regions of nylon 6 and PVDF in particular has been enhanced. The increase in the heat treatment time in the molten state resulted in further enhancement of the miscibility. The enhancement of compatibility by addition of a reactive compatibilizer and heat treatment resulted in a significant increase in the energy of rupture in tensile testing.