Showing papers by "Liqun Zhang published in 2020"

A wearable, self-adhesive, long-lastingly moist and healable epidermal sensor assembled from conductive MXene nanocomposites

Abstract: Flexible wearable conductive hydrogel-based epidermal sensors have attracted tremendous attention due to their versatile potential applications in soft robotics, personal healthcare monitoring and electronic skins. However, it remains a critical challenge for hydrogel-based epidermal sensors to simultaneously achieve self-healing capability, self-adhesiveness and long-lasting moisture retention for full-scale human motion biomonitoring. Herein, a conductive, self-healing, adhesive and long-lastingly moist MXene nanocomposite organohydrogel is prepared from the conformal coating of the MXene nanosheet network by the polymer networks of dopamine grafted sodium alginate (Alg-DA), phenylboronic acid grafted sodium alginate (Alg-PBA) and polyacrylamide (PAAm) with a glycerol/water binary solvent as the dispersion medium. The obtained MXene nanocomposite organohydrogel exhibits excellent self-healing capability, superior self-adhesive performance and long-lasting moisture retention (10 days). Furthermore, the MXene nanocomposite organohydrogel can be assembled as a wearable epidermal sensor to detect human motion including large deformation (finger bending and wrist bending) and tiny deformation (swallowing, breathing, and pulse) with durable stability. Meanwhile, the assembled epidermal sensor could be employed to monitor human activities wirelessly via connecting a wireless transmitter. This work sheds new light on the development of flexible, self-healing, adhesive and long-lastingly moist epidermal sensors and electronic skins for personalized healthcare monitoring, human–machine interfaces, and artificial intelligence.

Improvement of Compatibility and Mechanical Performances of PLA/PBAT Composites with Epoxidized Soybean Oil as Compatibilizer

Abstract: With excellent comprehensive properties, polylactic acid/poly(butylene adipate-co-butylene terephthalate) (PLA/PBAT) blends have found wide applications as degradable materials. During the processi...

Jelly-Inspired Construction of the Three-Dimensional Interconnected BN Network for Lightweight, Thermally Conductive, and Electrically Insulating Rubber Composites

Abstract: The improvement of modern electronics, which is constantly getting faster, has raised higher requirements for thermally conductive and electrically insulating thermal interface materials. Developin...

Design of next-generation cross-linking structure for elastomers toward green process and a real recycling loop

Abstract: Currently adopted cross-linking methods in rubber industry are suffering from variable persistent issues, including the utilization of toxic curing packages, release of volatile organic compounds (VOCs) and difficulties in the recycling of end-of-life materials. It is of great importance to explore a green cross-linking strategy in the area. Herein, we report a new “green” strategy based on hydrolyzable ester cross-links for cross-linking diene-typed elastomers. As a proof of concept, a commercial carboxylated nitrile rubber (XNBR) is efficiently cross-linked by a bio-based agent, epoxidized soybean oil (ESO), without any toxic additives. ESO exhibits an excellent plasticization effect and excellent scorch safety for XNBR. The cross-linking density and mechanical properties of the ESO-cured XNBR can be manipulated in a wide range by changing simply varying the content of ESO. In addition, zinc oxide (ZnO) performs as a catalyst to accelerate the epoxide opening reaction and improve the cross-linking efficiency, serving as reinforcement points to enhance the overall mechanical properties of the ESO-cured XNBR. Furthermore, the end-of-life elastomer materials demonstrate a closed-loop recovery by selectively cleaving the ester bonds, resulting in very high recovery of the mechanical performance of the recycled composites. This strategy provides an unprecedented green avenue to cross-link diene elastomers and a cost-effective approach to further recycle the obtained cross-linked elastomers at high efficiency.

Advances in Electrospinning of Natural Biomaterials for Wound Dressing

Abstract: Electrospinning has been recognized as an efficient technique for the fabrication of polymer nanofibers. Various polymers have been successfully electrospun into ultrafine fibers in recent years. These electrospun biopolymer nanofibers have potential applications for wound dressing based upon their unique properties. In this paper, a comprehensive review is presented on the researches and developments related to electrospun biopolymer nanofibers including processing, structure and property, characterization, and applications. Information of those polymers together with their processing condition for electrospinning of ultrafine fibers has been summarized in the paper. The application of electrospun natural biopolymer fibers in wound dressings was specifically discussed. Other issues regarding the technology limitations, research challenges, and future trends are also discussed.

Performance enhancement of rubber composites using VOC-Free interfacial silica coupling agent

Abstract: Nano-silica is an important component for producing elastomer composites used for fabricating “green tires.” However, the poor dispersion of silica particles in the rubber matrix and the emission of volatile organic compounds (VOCs) during the silica modification limit the applications of the modifiers. Here, bis-epoxypropyl polysulfide (BEP), a novel epoxy-type coupling agent, was designed and synthesized for nano-silica modification to cause an interfacial interaction between nano-silica and the rubber matrix and avoid VOC emission. The thermogravimetric analysis result and the bound rubber content show that BEP effectively built a bridge between the nano-silica and the rubber, which led to a strong interfacial effect and promising mechanical performance characteristics. The silica dispersion in solution-polymerized styrene-butadiene rubber (SSBR) was studied using a transmission electron microscope and a rubber process analyzer, and the results demonstrate that BEP could significantly improve silica dispersion. The static and dynamic mechanical performance results indicate that BEP is a valid coupling agent that can achieve silica/SSBR composites with high moduli and reinforcement indices. Moreover, the combination of BEP and bis-(γ-triethoxysilylpropyl)-tetrasulfide (TESPT) was also found to demonstrate a synergistic effect, which resulted in excellent static and dynamic performances of silica/SSBR composites for preparing higher-energy-efficient “green tires.”

A flexible and wide pressure range triboelectric sensor array for real-time pressure detection and distribution mapping

Abstract: Force distribution measurements are useful for wearable electronic devices designed for monitoring human health and disease diagnosis. However, the measuring pressure of traditional pressure sensors based on conductive materials is only in the kPa range, and therefore they are not conducive to sustain pressures in the MPa range. In this study, a self-powered and flexible pressure sensor array (PSA) was developed for the detection of both magnitude and distribution of pressures around 1.1 MPa generated during mechanical movement. The cuboid-array-structured single-electrode mode triboelectric nanogenerator (C-TENG) showed output voltage signals generated by changes in deformation-induced electrostatic potential. The output voltage of a single C-TENG device varied linearly with the applied pressure. Furthermore, the sensitivity and measuring range of the PSA can be regulated by designing the cuboid-array-structure parameter and adjusting the Young's modulus of silicone rubber (SR) by incorporating dopamine modified BaTiO3 (BT). Dopamine was employed as a modifier of BT nanoparticles to improve the interfacial compatibility between the inorganic filler and SR matrix. In summary, the proposed self-powered PSA with a wide working pressure range and high flexibility looks very promising for the construction of future advanced wearable medical electronic devices.

A supramolecular silicone dielectric elastomer with a high dielectric constant and fast and highly efficient self-healing under mild conditions

Abstract: Dielectric elastomer (DE) materials suffer from high driving voltages and cracks or breaks during repetitive actuation cycles. New DE materials with a simultaneous high dielectric constant (e′) and fast and efficient self-healing ability are in urgent need. Herein, we report a self-healable silicone DE by constructing a supramolecular network assembled by coordination bonds between carboxylated polymethylvinylsiloxane (PMVS-COOH) and FeCl3 as well as hydrogen bonds between carboxyl groups through introducing FeCl3 into PMVS-COOH. Both experimental results and density functional theory indicate that the Fe3+/COO− complex is formed and can further aggregate into clusters, which can simultaneously realize the crosslinking of PMVS-COOH and introduce interfacial polarization. Interestingly, the interfacial polarization largely increases with increasing FeCl3 content and FeCl3 can promote dipole polarization by disrupting some of the hydrogen bonds and releasing carboxyl groups when the FeCl3 content is higher than 5%, leading to a significant enhancement in e′. The as-prepared PMVS-COOH/FeCl3 (SiR-Fe) DE with 8% FeCl3 shows comparatively high e′ (12.3 at 104 Hz), much higher than that of the commercial silicone DE (∼2.7) or PMVS-COOH (6.1). Meanwhile, a self-healing efficiency of 99% in tensile strength and 100% in tensile toughness are achieved for SiR-Fe with 8% FeCl3 after being treated for 1 h at room temperature. This is ascribed to the high chain mobility of SiR-Fe and the robust supramolecular dynamic network resulting from the combination of coordination bonds and hydrogen bonds. It is demonstrated that SiR-Fe can be effectively used as a dielectric elastomer sensor. And it is promising that this highly efficient self-healable SiR-Fe DE with high e′ finds applications especially in biological and medical fields.

Enhancing the Performance of Rubber with Nano ZnO as Activators.

Abstract: The vulcanization of rubber is a chemical process to improve the mechanical properties by cross-linking unsaturated polymer chains. Zinc oxide (ZnO) acts as an activator, boosting the rubbers' sulfur vulcanization. Maintaining the level of ZnO content in the rubber compounds as low as possible is desirable, not only for economic reasons but also to reduce the environmental footprint of the process. In this contribution, octylamine (OA) capped ZnO nanoparticles (5 nm diameter), prepared through a thermal decomposition method, were demonstrated to be efficient activators for the sulfur vulcanization of natural rubber, enabling the reduction of the required amount of ZnO as compared to commercial systems. The effect of different ZnO activators (OA capped ZnO/commercial indirect process ZnO) on the curing characteristics, cross-linking densities, and mechanical performance, as well as the thermal behavior of rubber compounds, were investigated. Compared to the commercial indirect process ZnO, OA capped ZnO nanoparticles not only effectively enhanced the curing efficiency of natural rubber but also improved the mechanical performance of the composites after vulcanization. This was interpreted as, by applying the OA capped ZnO nanoparticles, the ZnO levels in rubber compounding were significantly reduced under the industrial vulcanization condition (151 °C, 30 min).

Interfacial nanomechanical properties and chain segment dynamics of fibrillar silicate/elastomer nanocomposites

Abstract: For fiber reinforced rubber composites, the interface is the most critical factor affecting the stress transfer efficiency and the ultimate reinforcing effect. In this study, we investigated the interfacial interaction of three kinds of fibrillar silicate (FS) filled natural rubber (NR) composites via a combination of quantitative nanomechanical technique of atomic force microscopy (AFM-QNM) and broadband dielectric spectroscopy (BDS) measurements. The force-deformation curves at nanoscale of these composites was revealed to quantitatively obtain the interfacial thickness and Young's modulus, and the Johnson−Kendall−Robert (JKR) contact model was used to fit the force-deformation curves to verify the reliability of the AFM-QNM results. The interfacial thickness of these composites with double-layer structure was further verified by High Resolution Transmission Electron Microscope (HRTEM) directly. The BDS results indicate that apart from the bulklike α-relaxation, a slower αinf -relaxation by 1−2 orders of magnitude than bulklike α-relaxation was observed in these nanocomposites attributed to restricted segmental relaxation of interfacial NR chains by FS, and the relaxation times of these composites agree well with the results of interfacial thickness and Young's modulus. An in-depth analysis of the interfacial interaction mechanism of these FS/NR composites was given. This study helps us to deep understand the interface of nanofiber reinforced composites, and it provides guidance for the interfacial design of high performance rubber composites.

Plasticization Effect of Bio-Based Plasticizers from Soybean Oil for Tire Tread Rubber.

Abstract: Modified soybean oil (MSO) is synthesized from soybean oil (SO) and sulfur, aiming to reduce the double bond quantity of SO and avoid harmful effects on the crosslink density and mechanical properties of rubber. MSO modified with different weight percentages of sulfur is then used to plasticize tire tread rubber (TR). It is found that the crosslink density and modulus of MSO- plasticized rubber are significantly improved compared with that of SO-plasticized TR. MSO modified with 6 wt % sulfur (MSO-6%) exhibits the best plasticization effect on TR, thus, the plasticization effect of MSO-6% on TR was further studied by adjusting its additive content. Thereafter, the Mooney viscosity, Payne effect, mechanical property of different amount of MSO-6% plasticized TR are studied to investigate their plasticization effect. At the same additive content of plasticizer, the plasticization effect of MSO-6% and a commonly used aromatic hydrocarbon plasticizer (AO) is compared to determine the potential application of MSO on tire tread rubber. It is found MSO shows similar plasticization effect on TR compared with AO. More important, the aging resistance property and wear resistance property of MSO-6% plasticized rubber are better than those of AO-plasticized rubber. Therefore, MSO-6% is a promising bio-based plasticizer for tire tread rubber.

High-power hybrid GaN-based green laser diodes with ITO cladding layer

Abstract: Green laser diodes (LDs) still perform worst among the visible and near-infrared spectrum range, which is called the “green gap.” Poor performance of green LDs is mainly related to the p-type AlGaN cladding layer, which on one hand imposes large thermal budget on InGaN quantum wells (QWs) during epitaxial growth, and on the other hand has poor electrical property especially when low growth temperature has to be used. We demonstrate in this work that a hybrid LD structure with an indium tin oxide (ITO) p-cladding layer can achieve threshold current density as low as 1.6 kA/cm2, which is only one third of that of the conventional LD structure. The improvement is attributed to two benefits that are enabled by the ITO cladding layer. One is the reduced thermal budget imposed on QWs by reducing p-AlGaN layer thickness, and the other is the increasing hole concentration since a low Al content p-AlGaN cladding layer can be used in hybrid LD structures. Moreover, the slope efficiency is increased by 25% and the operation voltage is reduced by 0.6 V for hybrid green LDs. As a result, a 400 mW high-power green LD has been obtained. These results indicate that a hybrid LD structure can pave the way toward high-performance green LDs.

Design and growth of GaN-based blue and green laser diodes

Abstract: GaN-based laser diodes (LDs) extend the wavelength of semiconductor LDs into the visible and ultraviolet spectrum ranges, and are therefore expected to be widely used in quantum technology, bio & medical instruments, laser displays, lighting and materials processing. The development of blue and green LDs is still challenging, even though they are based on the same III-nitride materials as GaN-based light-emitting diodes. The challenges and progress of GaN-based blue and green LDs are reviewed from the aspects of epitaxial growth and layer structure design. Due to large differences in lattice constants and growth conditions for InN, GaN, and AlN, considerable effort is required to improve the quality of InGaN multiple quantum well (MQW) gain medium for blue and especially green LDs. p-type doping profiles, conditions and layer structures are critical to reduce the internal losses and to mitigate the degradation of InGaN MQWs. Hole injection is also a key issue for GaN-based LDs.

Elastomer nanocomposites containing MXene for mechanical robustness and electrical and thermal conductivity

Abstract: A novel 2D nanomaterial, Ti3C2Tx MXene, added conductivity and reinforcement to a common elastomer, nitrile butadiene rubber (NBR). X-ray diffraction revealed the intercalation of lithium ions and elastomer chains into the MXene interlayer spacing, which enabled exfoliation in the elastomer. The reaction between MXene and NBR was proved by a stepwise Fourier transform infrared spectroscopy. With increase in MXene fractions, electrical and thermal conductivity of the composite increased to 9 × 10-5 S cm-1 and 0.69 W m-1 K-1, respectively. At only 2.8 vol% MXene, a swelling ratio of 1.61 was achieved, representing a 75% reduction compared to NBR containing either graphene or carbon nanotubes at the same filler fraction. Tensile tests showed that with the increase in MXene content, Young's modulus increased while both tensile strength and elongation at break first increased and then decreased. Overall, latex compounding proved to be an efficient technique for forming NBR/MXene nanocomposites. The revealed reaction between MXene and NBR to create functional polymer nanocomposites could provide a platform for utilising MXene for other polymers.

Multidirectional Triple-Shape-Memory Polymer by Tunable Cross-linking and Crystallization

Abstract: Medical fixing is one of the very important applications of the shape-memory polymer material, and the two important properties of the medical fixing material are that it perfectly fits the body during the fixing and easily detaches after being used. As the fixing and detachment are triggered by two independent stimuli in two opposite directions, it is necessary to develop multidirectional triple-shape-memory polymers. In this research, a series of polymer materials composed of trans-polyisoprene (TPI) and paraffin were prepared by melt blending and compression molding, and then the TPI was cross-linked by vulcanization. As a result of the large difference in the melting temperature and crystallization temperature between TPI and paraffin, the obtained polymer materials exhibit a triple-shape-memory behavior. According to the analysis of crystal behavior, microscopic morphology, and mechanical properties of the materials with different paraffin contents and TPI cross-linking density by differential scanning calorimetry, X-ray diffraction, scanning electron microscopy, and dynamic mechanical thermal analysis, the shape-memory behavior of the obtained materials was tunable by the cross-linking density of TPI and the crystallization degree of TPI or paraffin. Compared with the traditional triple-shape-memory material, our samples are prepared in a more facile way and can recover at human body temperature (37 °C). Moreover, our TPI/paraffin material can realize more flexible multidirectional recovery, as well as can be reprogramed and used multiple times. To the best of our knowledge, there are few polymer materials reported, which can realize multidirectional recovery. These unique multidirectional and reprogramable properties will enable the application of this polymer material, especially in the medical fixing materials.