Hoik Lee

High thermal stability and high tensile strength terpolyester nanofibers containing biobased monomer: fabrication and characterization

This research fabricated novel nanofibers with a terpolyester of isosorbide, ethylene glycol, 1,4-cyclohexane dimethanol, and terephthalic acid (PEICT) using electrospinning and characterized their properties. The nanofibers have higher glass transition temperature (Tg) than other polyester-type polymers, and a smaller diameter nanofiber has higher Tg than a larger diameter nanofiber. This is due to the orientation of polymer chains inside nanofibers, which was verified by DSC and polarized ATR-FTIR. The morphology and diameter of the nanofibers affected by concentration of PEICT solution were studied by SEM. It demonstrated smooth and well-formed nanofibers, and showed an increase of the diameter with increasing concentration. In addition, the tensile property, which was confirmed by UTM, was enhanced with increasing diameter because molecular orientation existed in finer nanofibers. They show a better tensile property than general biobased nanofibers such as silk, chitosan, and gelatin. Finally, fabrication of PEICT nanofibers was optimized and characterized. They can be utilized in various industrial applications such as tissue engineering, wound dressings, and health care devices.

Graphene oxide/poly(acrylic acid) hydrogel by γ-ray pre-irradiation on graphene oxide surface

AbstractGraphene oxide/poly(acrylic acid) (GO/PAA) hybrid hydrogel was prepared using a γ-ray pre-irradiation technique. The functional groups in graphene oxide were modified to peroxide in an O2 environment with γ-ray radiation. Radical species from the thermal decomposition of peroxides initiated radical polymerization of the acrylic acid monomers. Modified GO and GO/PAA hydrogels were investigated using scanning electron microscopy and Fourier transform infrared (FTIR), Raman, and electron spin resonance spectroscopy. The thermal, mechanical, and swelling properties of GO/PAA hydrogel were studied by a tensile stress-strain curve and thermal gravimetric analysis. A genuine binary hybrid hydrogel of graphene oxide and PAA was obtained from a simple synthetic procedure based on γ-ray pre-irradiation without further additives.

Preparation of an imogolite/poly(acrylic acid) hybrid gel

Many efforts in the field of hydrogels have been focused toward increasing the mechanical strength of the gel using inorganic materials. In this study, we synthesized a hydrogel that has excellent mechanical properties using surface-modified inorganic nanofibers composed of imogolite (Al2SiO3(OH)4), which is a hydrated aluminum silicate that has a hollow tube structure. Gamma ray radiation generates peroxide radicals on the nanofibers (imogolite), resulting in an additive free hybrid hydrogel. Structural optimization was carried out by changing the composition of imogolite and poly(acrylic acid). Chemical bonding between the nanofiber and the polymer was simulated by a cluster model and characterized by wide area Raman spectroscopy. The results indicate that imogolite embedded in a polymer matrix can align along the direction of an elongational force, as confirmed by small angle X-ray scattering (SAXS).

Electrospun tungsten trioxide nanofibers decorated with palladium oxide nanoparticles exhibiting enhanced photocatalytic activity

Tungsten trioxide (WO3) based nanofibers have many advantages as photocatalysts due to its band gap which fits with readily accessible light sources. We successfully fabricated novel palladium oxide (PdO) particles decorated WO3 nanofibers by electrospinning combined with chemical deposition processes, leading to improved photocatalytic efficiency for organic dye degradation up to 86.4%. Morphologies, elemental compositions and structural analyses confirmed the successful uniform decoration of PdO particles along WO3 nanofibers. Photodegradation of methylene blue as a model pollutant in water media was performed under UV and visible light in the presence of fabricated nanofibers as a photocatalyst. As a result, improved photocatalytic activity by PdO decoration was observed compared to commercially available WO3 NFs without PdO, attributed to its ability to hold excited electrons and increase surface area of NFs. This fibrous hybrid catalytic materials platform will open up a new and practical route and stimulate further research to improve photocatalytic performance.

Handspinning Enabled Highly Concentrated Carbon Nanotubes with Controlled Orientation in Nanofibers.

The novel method, handspinning (HS), was invented by mimicking commonly observed methods in our daily lives. The use of HS allows us to fabricate carbon nanotube-reinforced nanofibers (CNT-reinforced nanofibers) by addressing three significant challenges: (i) the difficulty of forming nanofibers at high concentrations of CNTs, (ii) aggregation of the CNTs, and (iii) control of the orientation of the CNTs. The handspun nanofibers showed better physical properties than fibers fabricated by conventional methods, such as electrospinning. Handspun nanofibers retain a larger amount of CNTs than electrospun nanofibers, and the CNTs are easily aligned uniaxially. We attributed these improvements provided by the HS process to simple mechanical stretching force, which allows for orienting the nanofillers along with the force direction without agglomeration, leading to increased contact area between the CNTs and the polymer matrix, thereby providing enhanced interactions. HS is a simple and straightforward method as it does not require an electric field, and, hence, any kinds of polymers and solvents can be applicable. Furthermore, it is feasible to retain a large amount of various nanofillers in the fibers to enhance their physical and chemical properties. Therefore, HS provides an effective pathway to create new types of reinforced nanofibers with outstanding properties.

Formation and characterization of poly(acrylic acid) on silica particles irradiated by γ-ray radiation

Organic/inorganic hybrid gels were directly prepared by polymerization on the peroxide surface of silica particles where the particle surface was irradiated by a 60Co γ-ray. These hydrogels have no residues of initiators or cross-linkers, so they can be used in biocompatible gel applications. Wide Raman spectroscopy was used to verify the interaction between the particles and poly(acrylic acid) (PAA). We observed that covalent bonds existing between the peroxide particles and acrylic acid, and the hydrogen bonds between the acrylic acids. For these studies, we prepared hydrogels by varying the particles’ concentration and the size of the silica particles to classify the number of reaction sites, which are the dominant factor for the chemical reaction between the silica particles and PAA.

Effect of graphene incorporation in carbon nanofiber decorated with TiO2 for photoanode applications

The photovoltaic performance of dye-sensitized solar cells (DSSCs) using a photoanode fabricated with graphene incorporated carbon nanofibers with a TiO2 layer on their surfaces is reported. The composite nanofibers were prepared through a process consisting of electrospinning and sol–gel process. The distribution of TiO2 particles on the surface of the graphene-incorporated carbon nanofiber was characterized by scanning electron microscopy (SEM), and transmission electron microscopy (TEM). From the microscopic studies, we found that graphene plays a significant role to form TiO2 particles on carbonaceous materials. Further studies on chemical composition using X-ray photoelectron spectroscopy reveal that different oxidation states of Ti in the synthesized titanium oxide were achieved by incorporating graphene in the nanofibers. Furthermore, TiO2 synthetic conditions in the sol–gel process largely affected photovoltaic performance, particularly short circuit current; acidic conditions for the sol–gel process were more effective compared to neutral conditions. The incorporated graphene facilitates conducting charge carriers in TiO2 by coordination with carbon, and increasing the adsorption of dye molecules owing to homogeneous distribution of TiO2 along the nanofibers. This study further highlights the advantages of hybridizing different materials with 1D nanofiber geometry, offering a promising route to improving the resulting efficiency in light harvesting applications.

Electrospun tri-layered zein/PVP-GO/zein nanofiber mats for providing biphasic drug release profiles

Simple sequential electrospinning was utilized to create a functional tri-layered nanofiber mesh that achieves time-regulated biphasic drug release behavior. A tri-layered nanofiber mesh -composed of zein and poly(vinylpyrrolidone) (PVP) as the top/bottom and middle layers, respectively - was constructed through sequential electrospinning with ketoprofen (KET) as the model drug. PVP was blended with graphene oxide (GO) to improve the drug release functionality of PVP nanofiber as well as its mechanical properties. Scanning electron microscopy confirmed that the resultant nanofibers had a linear morphology, smooth surface, and tri-layered structure. In addition, X-ray diffraction patterns, differential scanning calorimetric analyses, and Fourier transform infrared spectra verified that the drugs were uniformly dispersed throughout the nanofiber due to good compatibility between the polymer and KET induced by hydrogen interaction. In vitro release test of the tri-layered structure, each component of which had distinct release features, successfully demonstrated time-regulated biphasic drug release. Also, it was confirmed that the drug release rate and duration can be controlled by designing a morphological feature - namely, mesh thickness - which was achieved by simply regulating the spinning time of the first and third layer. This multilayered electrospun nanofiber mesh fabricated by sequential electrospinning could provide a useful method of controlling drug release behavior over time, which will open new routes for practical applications and stimulate further research in the development of effective drug release carriers.

Enhanced Wettability and Thermal Stability of a Novel Polyethylene Terephthalate-Based Poly(Vinylidene Fluoride) Nanofiber Hybrid Membrane for the Separator of Lithium-Ion Batteries

In this study, a novel membrane for the separator in a lithium-ion (Li-ion) battery was proposed via a mechanically pressed process with a poly(vinylidene fluoride) (PVDF) nanofiber subject and polyethylene terephthalate (PET) microfiber support. Important physical properties, such as surface morphology, wettability, and heat stability were considered for the PET-reinforced PVDF nanofiber (PRPN) hybrid separator. Images of scanning electron microscopy (SEM) showed that the PRPN hybrid separator had a homogeneous pore size and high porosity. It can wet out in battery electrolytes completely and quickly, satisfying wettability requirements. Moreover, the electrolyte uptake was higher than that of dry-laid and wet-laid nonwovens. For heat stability, no shrink occurred even when the heating temperature reached 135 °C, demonstrating thermal and dimensional stability. Moreover, differential scanning calorimetry (DSC) showed that the PRPN hybrid separator possessed a shutdown temperature of 131 °C, which is the same as conventional separators. Also, the meltdown temperature reached 252 °C, which is higher than the shutdown temperature, and thus can protect against internal cell shorts. The proposed PRPN hybrid separator is a strong candidate material for utilization in Li-ion batteries.

Antibacterial efficacy of poly(vinyl alcohol) composite nanofibers embedded with silver-anchored silica nanoparticles

Silver has been widely used as an effective antibacterial agent especially for treating burns and wounds. However, release of silver from materials often arouse side effects due to toxicity of silver towards mammalian cells. Argyria and argyrosis are well known problems of acute toxicity of silver towards human body. Immobilization of silver is an effective approach to reduce silver release. Herein, we present poly(vinyl alcohol) (PVA) composite nanofibers embedded with silver-anchored silica nanoparticles (SSNs) as a novel antibacterial material. Silver nanoparticles anchored on silica nanoparticles were prepared and incorporated into PVA nanofibers to fabricate silver-silica embedded PVA nanofibers (SSN-PVA) by electrospinning. Incorporation of SSNs into PVA was confirmed by TEM and SEM results revealed regular nanofibers whose diameter increased with successive addition of SSNs. The SSN-PVA nanofibers showed significant antibacterial efficacy against both Gram-negative and Gram-positive bacteria. Our research results demonstrated SSN-embedded polymeric nanofibers can open up a promising prospect for the prevention of bacterial infection in diverse biomedical fields including wound dressing.