Daehwan Cho

Metal nanofibers with highly tunable electrical and magnetic properties via highly loaded water-based electrospinning.

Nanofibers are synthesized by electrospinning highly loaded water-based precursor-polymer hybrid solutions followed by thermal treatment to control crystal structure. Electrical conductivity and magnetic coercivity, as shown, are tested displaying independent magnetic and electrical property control from coercive to superparamagnetic and resistive to near-bulk conductivity at room temperature.

Characterizing zeta potential of functional nanofibers in a microfluidic device.

The measurement of surface charge on nanofibers was achieved by characterizing zeta potential of the nanofibers via a newly developed device for streaming current measurement. Low flow rates were sufficient to generate detectable streaming currents in the absence of an externally applied voltage without damaging nanofiber samples. Zeta potential was calculated by using the Helmholtz-Smoluchowski equation and the measured streaming currents. Two acrylic plates were machined and assembled to form a microfluidic channel that is 150 μm high, 2.0mm wide, and 30 mm long. Two electrodes for the measurement of streaming currents were housed in the top plate. Two nanofibers of pure polyacrylonitrile (PAN) fibers and charged (TiO(2) incorporated) PAN fibers were prepared and characterized in the device. Monobasic sodium phosphate and dibasic sodium phosphate were used to prepare four different pH buffer solutions ranging from pH 5 to pH 8 in order to characterize the zeta potentials. The pure PAN nanofibers had negatively-charged surfaces regardless of pH. However, the zeta potentials of PAN/TiO(2) nanofibers changed from positive to negative at pH 6.5. The zeta potential measurements made on the nanofibers in this new microfluidic device matched with those of the powdered raw materials using a commercial Zetasizer.

Electrohydrodynamic quenching in polymer melt electrospinning

Infrared thermal measurements on polymer melt jets in electrospinning have revealed rapid quenching by ambient air, an order of magnitude faster than predicted by the classical Kase and Matsuo correlation. This drastic heat transfer enhancement can be linked to electrohydrodynamic (EHD) effects. Analysis of EHD-driven air flow was performed and included into a comprehensive model for polymer melt electrospinning. The analysis was validated by excellent agreement of both predicted jet radius and temperature profiles with experimental results for electrospinning of Nylon-6 (N6), polypropylene (PP), and polylactic acid (PLA) melts. Based on this analysis, several methods that can be used to inhibit or enhance the quenching are described.

Enhanced spinnability of carbon nanotube fibers by surfactant addition

A surfactant is used to enhance spinnability of carbon nanotube (CNT) fibers during direct spinning via chemical vapor deposition (CVD). In this study, the non-ionic surfactant, polysorbate, is used due to its good solubility in the CNT synthesis solution. The addition of the surfactant increased the specific strength and electrical conductivity of CNT fibers. Due to these enhanced properties, CNT fibers can be spun at higher speeds which results in lower linear density. These enhancements are due to the reduced agglomeration of iron catalysts during the synthesis of CNT fibers via CVD. This simple approach may create new applications for CNT fibers, such as for artificial muscles and power cables.

Preparation and Characterization of Amphiphilic Triblock Terpolymer-Based Nanofibers as Antifouling Biomaterials

Antifouling surfaces are critical for the good performance of functional materials in various applications including water filtration, medical implants, and biosensors. In this study, we synthesized amphiphilic triblock terpolymers (tri-BCPs, coded as KB) and fabricated amphiphilic nanofibers by electrospinning of solutions prepared by mixing the KB with poly(lactic acid) (PLA) polymer. The resulting fibers with amphiphilic polymer groups exhibited superior antifouling performance to the fibers without such groups. The adsorption of bovine serum albumin (BSA) on the amphiphilic fibers was about 10-fold less than that on the control surfaces from PLA and PET fibers. With the increase of the KB content in the amphiphilic fibers, the resistance to adsorption of BSA was increased. BSA was released more easily from the surface of the amphiphilic fibers than from the surface of hydrophobic PLA or PET fibers. We have also investigated the structural conformation of KB in fibers before and after annealing by contact angle measurements, transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and coarse-grained molecular dynamics (CGMD) simulation to probe the effect of amphiphilic chain conformation on antifouling. The results reveal that the amphiphilic KB was evenly distributed within as-spun hybrid fibers, while migrated toward the core from the fiber surface during thermal treatment, leading to the reduction in antifouling. This suggests that the antifouling effect of the amphiphilic fibers is greatly influenced by the arrangement of amphiphilic groups in the fibers.

Facile Synthesis of Porous Silicon Nanofibers by Magnesium Reduction for Application in Lithium Ion Batteries

We report a facile fabrication of porous silicon nanofibers by a simple three-stage procedure. Polymer/silicon precursor composite nanofibers are first fabricated by electrospinning, a water-based spinning dope, which undergoes subsequent heat treatment and then reduction using magnesium to be converted into porous silicon nanofibers. The porous silicon nanofibers are coated with a graphene by using a plasma-enhanced chemical vapor deposition for use as an anode material of lithium ion batteries. The porous silicon nanofibers can be mass-produced by a simple and solvent-free method, which uses an environmental-friendly polymer solution. The graphene-coated silicon nanofibers show an improved cycling performance of a capacity retention than the pure silicon nanofibers due to the suppression of the volume change and the increase of electric conductivity by the graphene.

Effects of post-treated carbon nanotube films on electrochemical performance for bendable lithium-ion batteries

The different post treatments of carbon nanotube (CNT) films were studied to investigate the effects of their crystalline perfection on lithium-ion battery performance. The crystal structures of the CNT films were differently developed by heat and acid-heat treatments. The treated CNT films were prepared as an anode electrode for use in flexible lithium ion batteries. This free-standing electrode did not contain a polymeric binder, conducting carbon powder and current collector. The electrochemical performance of the electrodes made with differently treated CNT films was evaluated via charge-discharge test, cyclic voltammetry and impedance measurement. The heat treated CNT films showed a higher capacity (250 mAh/g at a 0.5C rate) due to the increase of their crystalline perfection compared to the raw CNT films. In addition, the CNT film electrodes exhibited a good capacity retention at a high charging rate. This result suggested that the CNT film electrodes performed well without some components within cells. Eventually, the CNT films might be utilized for bendable lithium-ion batteries due to their flexibility and high electrical conductivity. The CNT films would be a potential electrode material for a specific thin/bendable lithium-ion battery that does not need a high capacity.

Surface hydro-properties of electrospun fiber mats

Hydrophobic and hydrophilic properties (hydro-properties) on material surfaces have been an active research area due to their numerous practical applications. Various fiber mats were prepared to investigate the effect of fiber morphology on the surface properties. Four polymers with intrinsically different hydro-properties are used to fabricate the electrospun fibers with diameter ranging from 0.1 μm to 10 μm by both methods of melt and solution. The pore size, pore size distribution, porosity, and the surface roughness of electrospun fiber mats are evaluated by a Porometry and an image processing technique. The contact angles are measured to characterize the surface properties of fiber mats using the mixture solutions of water and ethanol. As a result, the pore size and surface roughness are closely related to the contact angles. The contact angle is highly increased with the large deviation of fiber diameter and the high surface roughness of fiber mats. It is noted that the designed surface property is achieved by modifying fiber morphology without any complex treatment of material surface.