Sung Cik Mun

Highly Stretchable and Wearable Graphene Strain Sensors with Controllable Sensitivity for Human Motion Monitoring

Because of their outstanding electrical and mechanical properties, graphene strain sensors have attracted extensive attention for electronic applications in virtual reality, robotics, medical diagnostics, and healthcare. Although several strain sensors based on graphene have been reported, the stretchability and sensitivity of these sensors remain limited, and also there is a pressing need to develop a practical fabrication process. This paper reports the fabrication and characterization of new types of graphene strain sensors based on stretchable yarns. Highly stretchable, sensitive, and wearable sensors are realized by a layer-by-layer assembly method that is simple, low-cost, scalable, and solution-processable. Because of the yarn structures, these sensors exhibit high stretchability (up to 150%) and versatility, and can detect both large- and small-scale human motions. For this study, wearable electronics are fabricated with implanted sensors that can monitor diverse human motions, including joint movement, phonation, swallowing, and breathing.

Enhanced Sensitivity of Patterned Graphene Strain Sensors Used for Monitoring Subtle Human Body Motions

With the growth of the wearable electronics industry, structural modifications of sensing materials have been widely attempted to improve the sensitivity of sensors. Herein, we demonstrate patterned graphene strain sensors, which can monitor small-scale motions by using the simple, scalable, and solution-processable method. The electrical properties of the sensors are easily tuned via repetition of the layer-by-layer assembly, leading to increment of thickness of the conducting layers. In contrast to nonpatterned sensors, the patterned sensors show enhanced sensitivity and the ability to distinguish subtle motions, such as similar phonations and 81 beats per minute of pulse rate.

Enhancing the dielectric properties of highly compatible new polyimide/γ-ray irradiated MWCNT nanocomposites

Novel polyimide/γ-ray irradiated MWCNT (PI/γ-MWCNT) nanocomposites with improved dielectric properties were fabricated by casting and curing processes. The interfacial interactions between the two domains, i.e. PI and MWCNTs, were enhanced by hydrogen bonding between the hydroxyl groups present on PI and modified CNTs. A PI matrix having pendant phenolic hydroxyl groups was derived from pyromellitic dianhydride (PMDA) and diamine monomer 4,4′-diamino-4′′-hydroxytriphenylmethane. MWCNTs (5–20 wt%) were dispersed in the synthesized PI matrix. Before addition to PI, the surface of MWCNTs was equipped with hydroxyl and carboxylic groups by irradiating with γ-rays under a dry oxygen environment. Surface examination of PI/γ-MWCNTs composite films by scanning electron microscopy (SEM) revealed that MWCNTs are uniformly dispersed and completely wrapped by the PI matrix, most likely due to the hydrogen bonding. The influence of greater adhesion of MWCNTs with PI matrix on the dielectric, visco-elastic, and mechanical properties of final PI/γ-MWCNTs nanocomposites was explored using appropriate analytical techniques. The composite films exhibited high dielectric constant, a 7.6 fold improvement as compared to pristine PI. The storage modulus (E′) and glass transition temperature (Tg) demonstrated an improvement of 1.4 and 1.2 fold, respectively. Similarly, mechanical and thermal properties were also found to be improved remarkably. We believe that significant property enhancement of PI/γ-MWCNTs nanocomposites is the direct consequence of increased interface compatibility via hydrogen bonding between the polymer matrix and the carbon nano-filler.

Preferential positioning of γ-ray treated multi-walled carbon nanotubes in polyamide 6,6/poly( p -phenylene ether) blends

AbstractMorphological characteristics and electrical conductivity of polyamide 6,6/poly(p-phenylene ether)/multi-walled carbon nanotube (PA66/PPE/MWCNT) ternary nanocomposites were investigated. The MWCNTs were modified by 60Co gamma ray (γ-ray) irradiation under a dry condition and O2 atmosphere, which introduces oxygen-containing functional groups on the surfaces of the MWCNTs and thereby provides better compatibility with the hydrophilic PA66 phase. It was observed that the MWCNTs are preferentially positioned in the continuous PA66 matrix, whereas PPE domains are almost free of MWCNTs. Since PA66 consists of a continuous phase and the MWCNTs are preferentially positioned in the PA66 phase, electrical conductivity of PA66/PPE/MWCNT ternary composites is higher than that of PA66/MWCNT binary composites at the same MWCNT content. It was observed that raising the processing temperature and increasing the mixing time were effective means of improving the electrical conductivity of the composites, via enhancement of MWCNT dispersion.

Development of novel coatable compatibilized polyimide-modified silica nanocomposites

A series of novel coatable polyimide silica (PI-SiO2) nanocomposites have been synthesized. A new PI matrix, containing pendant hydroxyl groups, was prepared reacting diamine monomers (4,4’-diamino-4”-hydroxytriphenylmethane, and 4,4’-oxydianiline) and pyromellitic dianhydride (PMDA). Whereas, silica reinforcement was generated using TEOS. A coupling oligomeric species 2,6-bis(3-(triethoxysilyl)propyl)pyrrolo[3,4-f]isoindole-1,3,5,7(2H,6H)-tetraone (APA) was used to furnish silica nanoparticles with imide linkages and hydroxyl groups. As these groups are already present in PI matrix, so their presence in nanoparticles brought structural similarity, and hence enhanced phase connectivity among two phases. The resulting PI-SiO2 hybrids, with improved interfacial interactions through hydrogen bonding and like-like chemical interactions, displayed much enhanced morphological, thermomechanical, and thermal properties. The properties of resulting hybrids were studied by various advanced techniques and compared with PI-SiO2 hybrid system which was prepared from same polyimide and unmodified silica network.

Preparation of functional composite materials based on chemically derived graphene using solution process

Chemically derived graphenes were assembled into functional composite materials using solution process from stable solvent dispersion. We have developed foldable electronic circuits on paper substrates using vacuum filtration of graphene nanoplates dispersion and a selective transfer process without need for special equipment. The electronic circuits on paper substrates revealed only a small change in conductance under various folding angles and maintained an electronic path after repetitive folding and unfolding. We also prepared flexible. binder-free graphene paper-like materials by addition of graphene oxide as a film stabilizer. This graphene papers showed outstanding electrical conductivity up to 26,000 S/m and high charge capacity as an anode in lithium-ion battery without any post-treatments. For last case, multi-functional thin film structures of graphene nanoplates were fabricated by using layer-by-layer assembly technique, showing optical transparency, electrical conductivity and enhanced gas barrier property.

A new method to produce cellulose nanofibrils from microalgae and the measurement of their mechanical strength

Despite the enormous potential of cellulose nanofibrils (CNFs) as a reinforcing filler in various fields, the use of them has been limited by high-energy mechanical treatments that require a lot of energy and time consumption. To reduce the demands of energy and time required for mechanical treatments, microalgae, in particular, Nannochloropsis oceanica, which has small size, rapid growth rate, and high productivity was used as a CNFs source. This study obtains the CNFs by lipid/protein extraction, purification, and TEMPO-mediated oxidation processes under gentle mixing without high-energy mechanical treatments. Furthermore, to evaluate the applicability of microalgal CNFs as a reinforcing filler, this study estimated the mechanical strength of the fibrils by the sonication-induced scission method. To achieve a precise estimation, an effective method to distinguish straight fibrils from buckled fibrils was also developed, and subsequently, only straight fibrils were used to calculate the mechanical strength in the sonication-induced scission method. Consequently, the tensile strength of the N. oceanica CNFs is around 3-4GPa on average which is comparable with the mechanical strength of general reinforcing fillers and even higher than that of wood CNFs. Thus, this study has shown that the newly proposed simplified method using N. oceanica is very successful in producing CNFs with great mechanical strength which could be used in various reinforcement fields.