Young Gyu Jeong

Multiwalled carbon nanotube/polydimethylsiloxane composite films as high performance flexible electric heating elements

High performance elastomeric electric heating elements were prepared by incorporating various contents of pristine multiwalled carbon nanotube (MWCNT) in polydimethylsiloxane (PDMS) matrix by using an efficient solution-casting and curing technique. The pristine MWCNTs were identified to be uniformly dispersed in the PDMS matrix and the electrical percolation of MWCNTs was evaluated to be at ∼0.27 wt. %, where the electrical resistivity of the MWCNT/PDMS composite films dropped remarkably. Accordingly, the composite films with higher MWCNT contents above 0.3 wt. % exhibit excellent electric heating performance in terms of temperature response rapidity and electric energy efficiency at constant applied voltages. In addition, the composite films, which were thermally stable up to 250 °C, showed excellent heating-cooling cyclic performance, which was associated with operational stability in actual electric heating applications.

A facile method for transparent carbon nanosheets heater based on polyimide

In this work, a novel film heater in nanometer-scale thickness based on catalyst-free and transfer-free carbon nanosheets (CNSs) with properties similar to graphene is fabricated. Here, poly(amic acid) (PAA), which is composed of several aromatic hydrocarbon rings, is used as the carbon precursor of CNS films. Altering the polymer concentration easily controls the morphological, optical, and electrical properties of the CNS films obtained by carbonization of PAA thin films. The CNS films with different thicknesses of 7.53–28.40 nm are simply prepared through spin-coating on a quartz substrate and post heat-treatment. Finally, their direct use as transparent film heaters is deeply investigated by considering electrical conductivity, temperature response rapidity, achievable maximum temperature, and electric power efficiency. For instance, an electrically conductive and optically transparent CNS film with 28.40 nm thickness exhibits excellent electric heating performance achieving well-defined steady-state maximum temperatures of 24–333 °C at low input electric power per unit film area of 0.027–1.005 W cm−2 in a relatively short time of ∼100 s.

Regenerated cellulose/multiwalled carbon nanotube composite films with efficient electric heating performance.

We have manufactured regenerated cellulose-based composite films reinforced with pristine multiwalled carbon nanotube (MWCNT) by a facile casting of cellulose/DMAc/LiCl solutions containing 0.2-10.0wt% MWCNT and have investigated their application as electric heating materials by examining microstructure, thermal stability, and electrical properties. TEM images showed that the pristine MWCNT was dispersed well in the regenerated cellulose matrix. The composite films were found to be stable thermally up to ∼275°C. The electrical resistivity of the regenerated cellulose/MWCNT composite films decreased significantly from ∼10(9)Ωcm to ∼10(1)Ωcm with increasing the MWCNT loading, particularly at a certain MWCNT content between 2.0 and 3.0wt%. Accordingly, the composite films with 5.0-10.0wt% MWCNT contents, which possessed low electrical resistivity of ∼10(2)-10(1)Ωcm, exhibited excellent electric heating performance in aspects of temperature responsiveness, steady-state maximum temperature, and electrical energy efficiency at constant applied voltages. For instance, the composite film with 10.0wt% MWCNT had well-controlled steady-state maximum temperatures of 40-189°C at 20-80V, characteristic temperature growth constant of ∼1s, and electric power efficiency of ∼5.4mW/°C, which performance remained unchanged under repeated experiments for several hours.

Preparation, structure and properties of poly(p-phenylene benzobisoxazole) composite fibers reinforced with graphene

Graphene-reinforced poly(p-phenylene benzobisoxazole) (PBO) composite fibers were manufactured via dry-jet wet-spinning of PBO/graphene mixtures in poly(phosphoric acid) (PPA), which were reaction products prepared by the in situ polymerization of 4,6-diaminoresorcinol dihydrochloride and terephthaloyl chloride in the presence of PPA solvent and exfoliated graphene sheets. The content of graphene sheets in the as-spun fibers was adjusted to 0.0∼2.0 wt%. The molecular structure, crystalline order, and morphology of the as-spun fibers of pristine PBO and PBO/graphene composites were identified using FTIR, X-ray diffraction, and electron/optical microscopy. The thermal stability and tensile mechanical properties of the composite fiber with 0.2 wt% graphene were found to be significantly improved compared to the pristine PBO fiber.

Electrical and dielectric properties of poly(1,3,4-oxdiazole) nanocomposite films with graphene sheets dispersed in layers

Electrical and dielectric measurements over a broad frequency range from 20 Hz to 2 MHz were carried out for a series of sulfonated poly(1,3,4-oxadiazole) (sPOD) nanocomposite films containing exfoliated graphene sheets of 0.1-10.0 wt%, which were manufactured via ultrasonication-based solution mixing and casting method. TEM and XRD data revealed that the graphene sheets were dispersed by forming a layered structure in the nanocomposite films with >2.0 wt% graphene. The frequency-dependent electrical conductivity and relative permittivity of the nanocomposite films were dependent on the graphene content. The neat sPOD and its nanocomposites with lower graphene contents of 0.1-2.0 wt% exhibited low electrical conductivity of ~10-13-10-12 S/cm and relative permittivity of 1.7-6.6 at 20 Hz. In cases of the nanocomposite films with high graphene contents of 5.0 and 10.0 wt%, highly improved relative permittivity of ~101 and ~560 at 20 Hz as well as electrical conductivity of ~10-9 S/cm and ~10-6 S/cm was attained at 20 Hz, respectively. In addition, the nanocomposite films with 5.0 and 10.0 wt% exhibited relatively high capacitance of ~39.7 pF and ~75.5 pF at 20 Hz, respectively. The highly enhanced relative permittivity and capacitance for the nanocomposite films with 5.0-10.0 wt% graphene was interpreted to be owing to the accumulation of electronic charges at the interfaces between insulating sPOD matrix and conductive graphene sheets dispersed in layers.

Carbon nanotube/polyimide bilayer thin films with high structural stability, optical transparency, and electric heating performance

Structurally stable and optically transparent multiwalled carbon nanotube/polyimide (MWCNT/PI) bilayer thin films with different MWCNT thicknesses of 56–155 nm are manufactured by a facile spin-coating of MWCNT aqueous solution and poly(amic acid) (PAA) solution on a glass substrate, followed by thermal treatment for imidization. For this purpose, PAA as a PI precursor is synthesized by the reaction of pyromellitic dianhydride and 4,4′-diaminodiphenylether. The MWCNT layer thickness in the bilayer films is controlled by the cycle number of the spin-coating process of the MWCNT aqueous solution on the glass substrate. SEM images of the bilayer films reveal that neat MWCNT layers are uniformly coated on glass substrates and they are covered well with a PI layer. Accordingly, the MWCNT/PI bilayer films are mechanically and structurally stable owing to the presence of a PI layer on the MWCNT layer, compared to neat MWCNT films on glass substrates. With the increase of the MWCNT layer thickness in the bilayer films from 56 nm to 155 nm, the sheet resistance decreases from ∼1.97 × 105 Ω sq−1 to ∼3.89 × 103 Ω sq−1 and the optical transparency at 550 nm wavelength also decreases from ∼78% to ∼52%. The MWCNT/PI bilayer thin films exhibit a high electric heating performance in view of the rapid heating/cooling response time of 13.8–16.8 s, high electric power efficiency of 11.5–14.8 mW per °C, and high steady-state maximum temperatures up to 219 °C as a function of the applied voltage of 20–100 V.

Microstructure and electrical property of epoxy/graphene/MWCNT hybrid composite films manufactured by UV-curing

UV-cured epoxy hybrid composite films were manufactured by efficient and facile cationic photochemical polymerization of 3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexane carboxylate mixtures including 5.0 wt% carbon nanofillers of different graphene/multi-walled carbon nanotube (MWCNT) compositions of 10/0, 9/1, 7/3, 5/5, 3/7, and 0/10 by weight ratio. TEM images confirmed that the mixed carbon nanofillers of graphene and MWCNT were well dispersed in the UV-cured epoxy matrix, while MWCNT as a single carbon nanofiller component was aggregated in the matrix. The electrical resistivity of the composite films was thus varied with the increment of the relative MWCNT content in 5.0 wt% carbon nanofillers, i.e., ∼160 Ωcm for the epoxy/graphene composite film, 30∼80 Ωcm for the epoxy/graphene/MWCNT composite films, and ∼16,200 Ωcm for the epoxy/MWCNT composite film. The decreased electrical resistivity of the epoxy/graphene/MWCNT composite films was associated with the interconnected network formation of graphene sheets and MWCNTs. Thus the UV-cured epoxy/graphene and epoxy/graphene/MWCNT composite films exhibited excellent electric heating performance in terms of rapid temperature response, stable maximum temperature, and high electric power efficiency. In addition, the UV-cured epoxy hybrid composite films as electric heating materials were found to be thermally stable up to ∼290 °C.

Synthesis and characterization of poly(2-cyano-1,4-phenylene terephthalamide) and its copolymers by phosphorylation-assisted polycondensation reaction

By using phosphorylation-based polycondensation reaction, poly(2-cyano-1,4-phenylene terephthalamide) (cyPPTA) and its copolymers with different cyano-monomer contents of 50–100 mol% were successfully synthesized from terephthalic acid, 2-cyano-1,4-phenylenediamine, and 1,4-phenylenediamine monomers in N-methyl-2-pyrrolidone/calcium chloride solvent system. The polymer concentration in the organic solvent after the polycondensation reaction was controlled to be 10 wt%. The cyPPTA-based polymers with higher cyano-monomer contents were found to have higher intrinsic viscosity and number-average molecular weight owing to their increased solubility in the polymerization solutions. The polarized optical microscopic images demonstrated that the polymers with high cyano-monomer contents of 100 and 80 mol% developed liquid crystalline structure in the polymerization solutions, while the polymers with relatively low cyano-monomer contents of 50–70 mol% formed highly condensed crystalline morphology. Consistently, the FT-IR spectra and X-ray diffraction patterns exhibited that the cyPPTA-based polymers with high cyano-monomer contents above 80 mol% have relatively week interchain hydrogen bond and thus form semi-ordered structures, in comparison with the polymer products with low cyano-monomer contents less than 70 mol%. In addition, the cyPPTAs with higher cyano-monomer contents were found to have higher thermal decomposition peak temperatures.

Transcrytalline structures and crystallization kinetics of Polyarylate/Nylon6 Islands-in-a-Sea conjugate fibers for high performance thermoplastic composite applications

We report the isothermal and non-isothermal crystallization kinetics and associated transcrystalline morphological features of polyarylate(PAR)/nylon6 islands-in-a-sea fibers, where 74 PAR islands serve as reinforcing fibers and nylon6 sea component acts as a semicrystalline matrix in final thermoplastic composites. The temperature-dependent polarized optical microscopic images obtained during a cooling process exhibit that the melt-crystallization is dominated by the interfacial crystallization of nylon6 on the surface of PAR fibers, leading to developing a transcrystalline structure. From the isothermal and non-isothermal melt-crystallization analyses of the islands-in-a sea fiber by using differential scanning calorimetry and the Avrami equation, the overall crystallization rates of the nylon6 sea component in the islands-in-a-sea fiber are found to be highly accelerated by the heterogeneous nucleating effect of the PAR island fibers. In addition, it is revealed that the isothermal and non-isothermal melt-crystallization kinetics of the nylon6 in the islands-in-a-sea fibers consists of two different mechanisms of the primary crystallization owing to the interfacial crystallization and the secondary crystallization due to the bulk crystallization.

Structure, electrical and mechanical properties of polyamide 66/acid-treated MWCNT composite films prepared by solution mixing in the presence of nonionic surfactant

Two different sets of polyamide 66(PA66)-based composite films containing 2.0-10.0 wt% acid-treated multiwalled carbon nanotubes (MWCNT) were manufactured by solution mixing and casting method in the presence or absence of a nonionic surfactant. For the improved dispersion and interfacial interaction of MWCNTs in the PA66 matrix, carboxylic acid-functionalized MWCNTs were prepared by the acid-treatment of pristine MWCNTs. The uniform dispersion of the acidtreated MWCNTs in the PA66 matrix was confirmed from FE-SEM images of the fractured composite film surfaces. DSC thermograms supported that the acid-treated MWCNTs served as nucleating agents for the melt-crystallization of PA66 in both composite films prepared with/without the addition of the surfactant. The electrical and tensile mechanical properties of the composite films prepared with the surfactant were ~20 % higher than those of the composite films manufactured without the surfactant. For both composite films, sheet resistivity and tensile mechanical properties were found to be highly decreased and increased, respectively, with the increment of the acid-treated MWCNT content.