Jae Whan Cho

Polyurethane‐Carbon Nanotube Nanocomposites Prepared by In‐Situ Polymerization with Electroactive Shape Memory

In‐situ polymerization was employed to achieve well‐dispersed carbon nanotube‐reinforced polyurethane composites. In‐situ polymerization showed predominant as primarily dispersal of carbon nanotubes in the matrix polymer according to scanning electron microscopy (SEM) observation and atomic force microscopy (AFM) images. Differential scanning calorimetry (DSC) results suggested that the addition of multi walled nanotubes (MWNTs) into polyurethane increased the rate of crystallization, this effect being more significant in polyurethane (PU)‐MWNT composite, which was prepared by an in‐situ polymerization process. The composites obtained by in‐situ polymerization showed enhanced mechanical properties as well as good electroactive shape memory. The original shape of the sample was almost recovered with bending mode when an electric field of 50 V was applied.

Electroactive Shape Memory Effect of Polyurethane Composites Filled with Carbon Nanotubes and Conducting Polymer

We report the electroactive shape memory composites obtained by shape memory polyurethane block copolymer (PU) and multi-walled carbon nanotubes (MWNTs), and polypyrrole (PPy). An addition of combined MWNTs and PPy contributed to an enhancement in conductivity of PU-MWNTs composites. PU containing 2.5% MWNTs showed better mechanical and thermal properties than other composites, but conductivity was not sufficient for showing the shape memory effect by applying electrical voltages. However, when the composite was lightly coated by PPy (2.5%), its conductivity was the highest than other composites. Such the conductivity of this composite was enough to show electroactive shape recovery by heating above transition temperature of 40–48°C due to melting of polycaprolactone soft segment domain. The good shape recovery of 90–96% could be obtained in the shape recovery test when an electric field of 25 V was applied.

Synthesis of multi-walled carbon nanotube/polyhedral oligomeric silsesquioxane nanohybrid by utilizing click chemistry

A new hybrid material consisting of a polyhedral oligomeric silsesquioxane (POSS) and carbon nanotube (CNT) was synthesized by a simple and versatile approach entailing click coupling between azide moiety-functionalized POSS and alkyne-functionalized multi-walled CNTs. This approach provides a simple and convenient route to efficiently functionalize a wide variety of nanoscale nanostructure materials on the surface of CNTs.

Highly stretchable, transparent and scalable elastomers with tunable dielectric permittivity

We report a fast and simple process for the large scale fabrication of highly flexible, optically transparent carbon nanotube composites with massive dielectric permittivity. Various concentrations of pristine SWNTs were dispersed in hyperbranched polyurethane to prepare uniformly dispersed nanocomposite materials. Our solution-based method does not require modification on the carbon nanotubes, thus preserving the intrinsic electronic and high mechanical properties of the composites.

Cycloaddition Reactions: A Controlled Approach for Carbon Nanotube Functionalization

Controlled functionalization of carbon nanotubes (CNTs) through the use of cycloaddition reactions is described. By employing various cycloaddition reactions, a wide range of molecules could be coupled onto CNTs without disruption of the structural integrity as well as with a statistical distribution of functional groups onto the surface of the CNTs. The cycloaddition reactions represent an effective and tailored approach for preparing CNT-based advanced hybrid materials that would be useful for a wide range of applications from nanobiotechnology to nanoelectronics.

The synergistic effect of the combined thin multi-walled carbon nanotubes and reduced graphene oxides on photothermally actuated shape memory polyurethane composites

We evaluated the synergistic effect of the hybrid-type nanocarbon, consisting of 1D thin-walled carbon nanotubes (TWNTs) and 2D reduced graphene oxide (RGO), on the shape memory performance of hyperbranched polyurethane composites. The shape recovery of the resulting composites was activated via a photothermal process using a near-infrared laser. The best laser-induced shape recovery performance was achieved for the composites with a 7/3 of TWNT/RGO ratio and a 1wt.% of nanocarbon content. Such result can be explained by good dispersion of TWNTs and RGO in the hyperbranched polymer as well as three-dimensionally enhanced interconnection between carbon nanotubes and graphenes. The optically active TWNTs with a high optical absorption section exhibited high ability of transferring laser-induced thermal energy to polymer matrix whereas RGO provided a high mechanical property to polymer matrix. The tensile modulus and electrical conductivity of the composites also showed a similar dependence on the TWNT/RGO composition ratio as the photothermal shape recovery. Our study demonstrated an effective conversion from light, thermal to mechanical work by irradiating shape memory polymer composite containing hybrid-type fillers using a near-infrared laser.

Graphene-crosslinked polyurethane block copolymer nanocomposites with enhanced mechanical, electrical, and shape memory properties

Highly flexible, conductive, and shape memory polyurethane nanocomposites were prepared using a robust and fast process. Functionalized graphene sheets were incorporated as crosslinkers in the prepolymer, prepared from a reaction of 4,4′-methylene bis(phenyl isocyanate) and poly(e-caprolactone)diol. The covalently bonded graphene sheets were homogeneously dispersed in the polymer matrix. In comparison to pristine polyurethane and carbon nanotube-crosslinked polyurethane, the graphene-crosslinked polyurethane composite exhibited higher modulus and breaking stress, and exceptional elongation-at-break. The resulting composite exhibited 97% shape recovery, 95% shape retention, enhanced shape recovery force, and fast electroactive shape recovery rate, thus it could be a promising material for the fabrication of graphene-based actuating devices.

Crystallization of poly(vinylidene fluoride)-SiO2 hybrid composites prepared by a Sol-gel process

Organic-inorganic hybrid composites consisting of poly(vinylidene fluoride) (PVDF) and SiO2 were prepared through a sol-gel process and the crystallization behavior of PVDF in the presence of SiO2 networks was investigated by spectroscopic, thermal and x-ray diffraction measurements. The hybrid composites obtained were relatively transparent, and brittleness increased with increasing content of tetraethoxysilane (TEOS). It was regarded from FT-IR and DSC thermal analyses that at least a certain interaction existed between PVDF molecules and the SiO2 networks. X-ray diffraction measurements showed that all of the hybrid samples had a crystal structure of PVDFγ-phase. Fresh gel prepared from the sol-gel reaction showed a very weak x-ray diffraction peak near 2θ=21° due to PVDF crystallization, and intensity increased gradually with time after gelation. The crystallization behavior of PVDF was strongly affected by the amount of SiO2 networks. That is, SiO2 content directly influenced preference and disturbance for crystallization. In polymer-rich hybrids, SiO2 networks had a favorable effect on the extent of PVDF crystallization. In particular, the maximum percent crystallinity of PVDF occurred at the content of 3.7 wt% SiO2 and was higher than that of pure PVDF. However, beyond about 10 wt% SiO2, the crystallization of PVDF was strongly confined.

Relationship between electrical resistance and strain of carbon fibers upon loading

The changes in electrical resistance of carbon fibers during a tensile elongation were investigated to understand the electromechanical mechanism in carbon fibers. The fractional electrical resistance of carbon fibers initially increased slightly with increasing elongation, however, increased abruptly beyond a certain strain where the rupture of fibers began to increase. Contribution to this change in electrical resistance was analyzed in terms of dimensional change of fibers, number of ruptured fibers, and degree of fiber contacts. The effect of the number of ruptured fibers was the most dominant, whereas the effect of the dimensional change of carbon fibers due to elongation was relatively small. The degree of contacts between fibers affected the change in electrical resistance dominantly at the large elongation. The residual electrical resistance appeared upon removal of the applied strain and increased with increasing elongation, regardless of the static and dynamic loading. Consequently, the smart characteristics of carbon fibers due to the existence of the residual electrical resistance are primarily ascribed to the number of ruptured fibers and contacts between fibers.

Soluble conducting polymer-functionalized graphene oxide for air-operable actuator fabrication

An effective route for the preparation of a processable, conducting polymer-functionalized graphene oxide for actuator applications is investigated. First, graphene oxide (GO) is covalently functionalized with a 3-thiophene acetic acid (TAA) monomer by an esterification reaction. Then, the TAA-functionalized GO is self-polymerized by chemical oxidative polymerization to yield poly(3-thiophene acetic acid)-functionalized GO (GO-f-PTAA). Further, the GO-f-TAA is also copolymerized with thiophene (Th) to yield GO-f-PTAA-co-PTh. The synthesis of GO-f-PTAA and GO-f-PTAA-co-PTh composites is confirmed by Fourier transform infrared, 1H-nuclear magnetic resonance, and X-ray photoelectron spectroscopies. The composites show better electrochemical properties than pure PTAA and superior solubility in organic solvents compared to pure GO. Using the soluble GO-f-PTAA and GO-f-PTAA-co-PTh composites, air-operable actuators are fabricated and their actuation performance is investigated. The copolymer-functionalized GO actuator exhibits good electroactive actuation behavior between 2 and 4 V, mainly because of the enhanced electrochemical performance of the composites, whereas the pure PTAA and GO-f-PTAA actuators do not show actuation under the applied voltage. The soluble conducting polymer-functionalized graphene composites developed in this study have potential applications in the fabrication of actuators that can be operated in air.