Frequency-Selective Radar-Absorbing Composites Using Hybrid Core-Shell Spheres.

Radar-absorbing materials (RAMs) covering the exterior surfaces of installed parts and assembled devices are crucial in absorbing most incident electromagnetic (EM) waves. This absorption minimizes reflected energy, thereby enhancing pilot safety and the stability of operating electronic devices without interference. Particularly, active stealth aircraft require effective protection from near- and far-field EM radiation across a wide spectrum of frequencies from both highly integrated electronic components and advanced enemy radars. Studies of RAMs often prioritize absorption over crucial tunability in frequency selectivity, revealing a research gap. In this study, we propose smart RAMs with frequency-selective absorption capabilities. Our approach involves incorporating two types of core-shell spheres in a polymer matrix, which feature shells of either wave-diffuse reflecting metal or wave-absorbing graphene. The key innovation lies in the ability to tailor absorption frequencies in the X-band range (8.2-12.4 GHz) by adjusting the interstitial spaces between the metallic spheres while the scattered waves are efficiently attenuated by graphene networks in the composites. On a metal substrate, a 2 mm-thick composite with an optimized structural composition and ratio of the two types of spheres exhibits a maximum absorption efficiency of 99.3%, effectively trapping and extinguishing incident waves. Combined with the structural tunability and frequency-selective properties of spherical fillers, our approach provides a scalable and effective method for creating functional isotropic coverings on various metallic surfaces.

Avocado seed starch utilized in eco-friendly, UV-blocking, and high-barrier polylactic acid (PLA) biocomposites for active food packaging applications.

Efficient and effective use of biopolymers, such as starch, has increasingly prompted interest due to the current environmental challenges. However, starch-based composites still show poor ductility along with water and oxygen permeability, which may not meet the requirements for food packaging standards. In this study, modified starch (m-St), isolated from the avocado seed and synthesized with tert-butyl acetoacetate (t-BAA), was embedded into polylactic acid (PLA) to design new eco-friendly composites. The developed biocomposites were found to exhibit high performance with outstanding mechanical properties in conjunction with remarkable light, water vapor, and oxygen blocking features for food packaging applications. PLA/m-St(1:6) 20 wt% composites showed a dramatic increase in elongation at break (EB%) from 3.35 to 27.80 % (about 730 % enhancement) and exhibited remarkable UV-blocking performance from 16.21 to 83.86 % for UVB, relative to pure PLA. Equally importantly, these biocomposites revealed significant improvement in oxygen and water vapor barrier performance by reducing their values from 1331 to 32.9 cc m-2 day-1 (indicating a remarkable reduction of 97.53 %) and 61.9 to 28 g m-2 day-1, respectively. This study can show the great potential of extracting starch from biowaste resources and transforming it into sustainable bio-based composites as a promising solution for food packaging applications.

Flexible Antireflection Coatings with Enhanced Durability and Antifogging Properties.

Antireflection coatings (ARCs) enhance optical clarity and improve light transmission by reducing glare and reflections. The application of conventional ARCs in flexible devices, however, is impeded by their lack of durability, particularly under bending deformation. We develop ARCs that withstand delamination and fracture, remaining intact even after 1000 bending cycles with a 5 cm bending radius. We fabricate integrated ARCs (iARCs) on a poly(methyl methacrylate) (PMMA) substrate by inducing free polymers to infiltrate the interstices of a disordered assembly of hollow silica nanochains and nanospheres. The polydispersity of PMMA creates a refractive index gradient, yielding a broadband antireflection capability. The nanochain-based iARCs are superior to the nanosphere-based coatings in both antireflection properties and mechanical durability, owing to the lower packing density and mechanical interlocking of the nanochains, respectively. Additionally, these nanochain iARCs display antifogging properties stemming from their superhydrophilicity. While our demonstrations are based on PMMA as a model substrate, this methodology is potentially extendable to other polymers, enhancing the iARC's applicability across various practical applications, including flexible and wearable devices.

Dual Porosity-Enhanced Antireflection Coatings with Continuous Gradient.

The incorporation of porous structures into films and coatings can transform their properties for applications in optics, separation, electronics, and energy generation and storage. Packing nanoparticles (NPs) is a versatile approach for fabricating nanoporous films with a tunable structure and properties. The mechanical fragility of NP packing-based films and coatings, however, significantly impedes their widespread utilization. Although infiltrating a polymer into the interstices of these NP packings has been shown to enhance their mechanical durability, this method completely eliminates the porosity of the structures, compromising their properties and functionality. This study presents a new approach to fabricate highly loaded porous nanocomposite films with a gradient in the refractive index by infiltrating subsaturating amounts of poly(methyl methacrylate) (PMMA) into disordered packings of hollow silica NPs. We demonstrate that dual porosity is a critical feature that enhances their antireflection (AR) and mechanical properties. The hollow cores of NPs prevent a substantial increase in the refractive index of the resulting films. Moreover, the interparticle voids allow for mechanical reinforcement to occur when the NP packings are infiltrated with PMMA, making them even more suitable for AR coatings. The refractive index and gradient across the nanocomposites can be tailored by adjusting the amount of PMMA infiltrated into the NP packing, the shape of hollow NPs, and the annealing time. The nanocomposite coatings with a continuous gradient in refractive index exhibit excellent AR properties and enhanced mechanical durability. Combined with the unique structural tunability afforded by the dual porosity, this approach provides a scalable and effective way to create robust and graded nanoporous structures for various applications.

Design and Control of Lightweight Bionic Arm Driven by Soft Twisted and Coiled Artificial Muscles.

Twisted and coiled actuators (TCAs), which are light but capable of producing significant power, were developed in recent times. After their introduction, there have been numerous improvements in performance, including development of techniques such as actuation strain and heating methods. However, the development of robots using TCA is still in its early stages. In this study, a bionic arm driven by TCAs was developed for light and flexible operation. The aim of this study was to gain a foothold in the future of robot development using TCA, which is considered as the appropriate artificial muscle. The main developments were with regard to the design (from actuator design to system design), system configuration for control, and control method. First, a process technology for repeatedly manufacturing TCA, which can be used practically and delivers sufficient performance, was developed. Based on the developed actuator, a joint was designed to move the elbow and hand. The final bionic arm was developed by integrating the TCA, pulley joint, and control system. It moved the elbow up to 100° and allowed the hand to move in three degrees of freedom. Using the control method for each joint, we were able to show the movement by using the hand and elbow.

Quantitative Interpretation of Electromagnetic Interference Shielding Efficiency: Is It Really a Wave Absorber or a Reflector?

As electromagnetic (EM) pollution continues to increase, electromagnetic interference (EMI) shielding materials have been intensively evaluated in terms of two main shielding mechanisms of reflection and absorption. Since the shielding effectiveness (SE) is represented in the logarithmic scale and in a coupled way of transmission (SET), absorption (SEA), and reflection (SER), often there is a misinterpretation that the EM wave reflectors are regarded as EM wave-absorbing materials. Surprisingly, we found that many materials reported as an EM wave absorber in the literature provide, in fact, less than 50% of EM wave-absorbing capability, i.e., over 50% of EM wave-reflecting feature. According to the theory and definition of EMI SE, the absorption-dominant EMI shielding materials should have the ratio of absorption to incident energy (A) as A > 0.5, which corresponds to a necessary condition that SER < 3.01 dB. The SER subsequently gives SEA in relation to SET. Using this criterion, we classified the EMI shielding materials with their shielding mechanism. The proposed methodology provides significant insight into the evaluation and development of EMI shielding materials.

Magnetic Polymer Composite Particles: Design and Magnetorheology.

As a family of smart functional hybrid materials, magnetic polymer composite particles have attracted considerable attention owing to their outstanding magnetism, dispersion stability, and fine biocompatibility. This review covers their magnetorheological properties, namely, flow curve, yield stress, and viscoelastic behavior, along with their synthesis. Preparation methods and characteristics of different types of magnetic composite particles are presented. Apart from the research progress in magnetic polymer composite synthesis, we also discuss prospects of this promising research field.

Magnetorheological Elastomers: Fabrication, Characteristics, and Applications.

Magnetorheological (MR) elastomers become one of the most powerful smart and advanced materials that can be tuned reversibly, finely, and quickly in terms of their mechanical and viscoelastic properties by an input magnetic field. They are composite materials in which magnetizable particles are dispersed in solid base elastomers. Their distinctive behaviors are relying on the type and size of dispersed magnetic particles, the type of elastomer matrix, and the type of non-magnetic fillers such as plasticizer, carbon black, and crosslink agent. With these controllable characteristics, they can be applied to various applications such as vibration absorber, isolator, magnetoresistor, and electromagnetic wave absorption. This review provides a summary of the fabrication, properties, and applications of MR elastomers made of various elastomeric materials.

Heat dissipative mechanical damping properties of EPDM rubber composites including hybrid fillers of aluminium nitride and boron nitride.

As highly integrated electronic devices and automotive parts are becoming used in high-power and load-bearing systems, thermal conductivity and mechanical damping properties have become critical factors. In this study, we applied two different fillers of aluminium nitride (AlN) and boron nitride (BN), having polygonal and platelet shapes, respectively, into ethylene-propylene-diene monomer (EPDM) rubber to ensure improved thermo-mechanical properties of EPDM composites. These two different shapes are considered advantageous in providing effective pathways of phonon transfer as well as facilitating sliding movement of packed particles. When the volume ratio of AlN : BN was 1 : 1, the thermal conductivity of the hybrid-filler system (EPDM/AlN/BN) increased in comparison to that of the single-filler system (EPDM/AlN) of 3.03 to 4.76 W m-1 K-1. The coefficient of thermal expansion (CTE) and thermal distortion parameter (TDP) substantially decreased from 59.3 ppm °C-1 and 17.5 m K-1 of EPDM/AlN, to 39.7 ppm °C-1 and 8.4 m K-1 of EPDM/AlN/BN, representing reductions of 33 and 52%, respectively. Moreover, the damping coefficient of EPDM/AlN/BN was greatly increased to 0.5 of at 50 °C, compared to 0.03 of neat EPDM. These excellent performances likely stem from the effective packing of AlN/BN hybrid fillers, which could induce facile energy transfer and effective energy dissipation by the sliding movement of the adjacent hybrid fillers in the EPDM matrix.

Author Correction: Solvent-free bulk polymerization of lignin-polycaprolactone (PCL) copolymer and its thermoplastic characteristics.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Nanoparticles Functionalized by Conducting Polymers and Their Electrorheological and Magnetorheological Applications.

Conducting polymer-coated nanoparticles used in electrorheological (ER) and magnetorheological (MR) fluids are reviewed along with their fabrication methods, morphologies, thermal properties, sedimentation stabilities, dielectric properties, and ER and MR characteristics under applied electric or magnetic fields. After functionalization of the conducting polymers, the nanoparticles exhibited properties suitable for use as ER materials, and materials in which magnetic particles are used as a core could also be applied as MR materials. The conducting polymers covered in this study included polyaniline and its derivatives, poly(3,4-ethylenedioxythiophene), poly(3-octylthiophene), polypyrrole, and poly(diphenylamine). The modified nanoparticles included polystyrene, poly(methyl methacrylate), silica, titanium dioxide, maghemite, magnetite, and nanoclay. This article reviews many core-shell structured conducting polymer-coated nanoparticles used in ER and MR fluids and is expected to contribute to the understanding and development of ER and MR materials.

Microfibrillated Cellulose Suspension and Its Electrorheology.

Microfibrillated cellulose (MFC) particles were synthesized by a low-pressure alkaline delignification process, and their shape and chemical structure were investigated by SEM and Fourier transformation infrared spectroscopy, respectively. As a novel electrorheological (ER) material, the MFC particulate sample was suspended in insulating oil to fabricate an ER fluid. Its rheological properties-steady shear stress, shear viscosity, yield stress, and dynamic moduli-under electric field strength were characterized by a rotational rheometer. The MFC-based ER fluid demonstrated typical ER characteristics, in which the shear stresses followed the Cho-Choi-Jhon model well under electric field strength. In addition, the solid-like behavior of the ER fluid was investigated with the Schwarzl equation. The elevated value of both dynamic and elastic yield stresses at applied electric field strengths was well described using a power law model (~E1.5). The reversible and quick response of the ER fluid was also illustrated through the on-off test.

Quantitative Electrode Design Modeling of an Electroadhesive Lifting Device Based on the Localized Charge Distribution and Interfacial Polarization of Different Objects.

Electroadhesive devices can lift materials of different shapes and various types using the electrostatic force developed at the interface between the device and the object. More specifically, the electrical potential generated by the device induces opposite charges on the object to give electrostatic Maxwell force. Although this technology has a great deal of potential, the key design factors based on the fundamental principles of interfacial polarization have yet to be clearly identified. In this study, we identify that the lifting force is quantitatively related to the total length of the boundary edges of the electrodes, where the induced charges are selectively concentrated. We subsequently propose a model equation that can predict the electrostatic lifting forces for different object materials as a function of the applied voltage, impedance, and electrode-boundary length. The model is based on the fact that the amount of induced charges should be concentrated where the equipotential field distance is minimal. We report that the impedance magnitude is correlated with the electroadhesive lifting forces by analyzing the impedance characteristics of objects made of different materials (e.g., paper, glass, or metal), as attached in situ to the electroadhesive device.

Non-Einstein Viscosity Phenomenon of Acrylonitrile-Butadiene-Styrene Composites Containing Lignin-Polycaprolactone Particulates Highly Dispersed by High-Shear Stress.

Lignin powder was modified via ring-opening polymerization of caprolactone to form a lignin-polycaprolactone (LPCL) particulate. The LPCL particulates were mixed with an acrylonitrile-butadiene-styrene (ABS) matrix at an extremely high rotational speed of up to 3000 rpm, which was achieved by a closed-loop screw mixer and in-line melt extruder. Using this high-shear extruding mixer, the LPCL particulate size was controlled in the range of 3395 nm (conventional twin-screw extrusion) down to 638 nm (high-shear mixer of 3000 rpm) by altering the mixing speed and time. The resulting LPCL/ABS composites clearly showed non-Einstein viscosity phenomena, exhibiting reduced viscosity (2130 Pa·s) compared to the general extruded composite one (4270 Pa·s) at 1 s-1 and 210 °C. This is due to the conformational rearrangement and the increased free volume of ABS molecular chains in the vicinity of LPCL particulates. This was supported by the decreased glass transition temperature (T g, 83.7 °C) of the LPCL/ABS composite specimens, for example, giving a 21.8% decrement compared to that (107 °C) of the neat ABS by the incorporation of 10 wt % LPCL particulates in ABS. The LPCL particulate morphology, damping characteristics, and light transmittance of the developed composites were thoroughly investigated at various levels of applied shear rates and mixing conditions. The non-Einstein rheological phenomena stemming from the incorporation of LPCL particulates suggest an interesting plasticization methodology: to improve the processability of high-loading filler/polymer composites and ultra-high molecular weight polymers that are difficult to process because of their high viscosity.

Solvent-free bulk polymerization of lignin-polycaprolactone (PCL) copolymer and its thermoplastic characteristics.

The pristine lignin molecules contain multiple reactive hydroxyl [OH] groups, some of which undergo limited polymerization depending on their configuration (aromatic or aliphatic) or conformation. The key issue in lignin-polymerization is to quantify the number of hydroxyl groups in the pristine molecules for subsequent activation to specific lignin-polymer chain lengths or degree of grafting. In this study, using ε-caprolactone (CL) as a reactive solvent, we successfully polymerized CL on the [OH] sites in the kraft lignin macromonomers (LM, Mw = 1,520 g mol-1), which resulted in a thermoplastic lignin-polycaprolactone (PCL) grafted copolymer. We found that the average number of [OH] groups in the LM was 15.3 groups mol-1, and further detected 40-71% of the [OH] groups in the CL bulk polymerization. The degree of polymerization of PCL grown on each [OH] site ranged between 7 and 26 depending on the reaction conditions ([CL]/[OH] and reaction-time) corresponding to 4,780 and 32,600 g mol-1 of PCL chains per a LM. The thermoplastic characteristics of the synthesized lignin-PCL copolymers were established by the melt viscosity exhibiting a shear-thinning behavior, e.g., 921 Pa.s at 180 °C. The thermal stability was remarkable providing a Tid (2% of weight loss) of 230 °C of the copolymers, compared with 69 °C for the pristine lignin.

Experimental Investigation on 3D Graphene-CNT Hybrid Foams with Different Interactions.

Due to the exceptional properties of graphene, numerous possibilities for real applications in various fields have been provided. However, it is a challenge to fabricate bulk graphene materials with properties arising from the nature of individual graphene sheets, and which assemble into monolithic three-dimensional structures. If 3D structured graphene foam were made instead of 2D structured graphene, it is expected that it would be a facile fabrication, with relatively low cost with the possibility of scale-up, and would maintain the intrinsic properties of graphene. To solve the weaknesses of 2D structured graphene, this study aimed to fabricate a 3D graphene-carbon nanotubes (CNT) hybrid foam. In this study, CNT was used to reinforce the graphene foams. In addition, two different surfactants, known as sodium dodecylbenzene sulphonate (SDBS) and cetyltrimethylammonium bromide (CTAB), were applied to help CNT dispersion. The π⁻π interaction was induced by SDBS/CNT, while ionic interaction was derived from CTAB/CNT. To confirm the charge effect with different surfactants, SEM, Zeta-potential, FT-IR, Raman spectroscopy, and compression tests were performed. When using a cationic surfactant, CTAB, compressive modulus, and strength increased due to the formation of relatively strong ionic bonding.

Understanding the Thermal Properties of Precursor-Ionomers to Optimize Fabrication Processes for Ionic Polymer-Metal Composites (IPMCs).

Ionic polymer-metal composites (IPMCs) are one of many smart materials and have ionomer bases with a noble metal plated on the surface. The ionomer is usually Nafion, but recently Aquivion has been shown to be a promising alternative. Ionomers are available in the form of precursor pellets. This is an un-activated form that is able to melt, unlike the activated form. However, there is little study on the thermal characteristics of these precursor ionomers. This lack of knowledge causes issues when trying to fabricate ionomer shapes using methods such as extrusion, hot-pressing, and more recently, injection molding and 3D printing. To understand the two precursor-ionomers, a set of tests were conducted to measure the thermal degradation temperature, viscosity, melting temperature, and glass transition. The results have shown that the precursor Aquivion has a higher melting temperature (240 °C) than precursor Nafion (200 °C) and a larger glass transition range (32⁻65°C compared with 21⁻45 °C). The two have the same thermal degradation temperature (~400 °C). Precursor Aquivion is more viscous than precursor Nafion as temperature increases. Based on the results gathered, it seems that the precursor Aquivion is more stable as temperature increases, facilitating the manufacturing processes. This paper presents the data collected to assist researchers in thermal-based fabrication processes.

Hierarchical Porous Chitosan Sponges as Robust and Recyclable Adsorbents for Anionic Dye Adsorption.

Biomass waste treatment and detrimental dye adsorption are two of the crucial environmental issues nowadays. In this study, we investigate to simultaneously resolve the aforementioned issues by synthesizing chitosan sponges as adsorbents toward rose bengal (RB) dye adsorption. Through a temperature-controlled freeze-casting process, robust and recyclable chitosan sponges are fabricated with hierarchical porosities resulted from the control of concentrations of chitosan solutions. Tested as the adsorbents for RB, to the best of our knowledge, the as-prepared chitosan sponge in this work reports the highest adsorption capacity of RB (601.5 mg/g) ever. The adsorption mechanism, isotherm, kinetics, and thermodynamics are comprehensively studied by employing statistical analysis. Importantly and desirably, the sponge type of chitosan adsorbents exceedingly facilitates the retrieving and elution of chitosan sponges for recyclable uses. Therefore, the chitosan sponge adsorbent is demonstrated to possess dramatically squeezable capability with durability for 10,000 cycles and recyclable adsorption for at least 10 cycles, which provides an efficient and economical way for both biomass treatment and water purification.

Cellulose-Based Smart Fluids under Applied Electric Fields.

Cellulose particles, their derivatives and composites have special environmentally benign features and are abundant in nature with their various applications. This review paper introduces the essential properties of several types of cellulose and their derivatives obtained from various source materials, and their use in electro-responsive electrorheological (ER) suspensions, which are smart fluid systems that are actively responsive under applied electric fields, while, at zero electric field, ER fluids retain a liquid-like state. Given the actively controllable characteristics of cellulose-based smart ER fluids under an applied electric field regarding their rheological and dielectric properties, they can potentially be applied for various industrial devices including dampers and haptic devices.

Tensile properties of millimeter-long multi-walled carbon nanotubes.

There have been a number of theoretical and experimental studies on tensile properties of carbon nanotubes (CNT), reporting the Young's modulus of the individual CNT up to 1 TPa. Although CNT shows the promise to be used as reinforcement in a high modulus/strength composite material, it exhibits quite disappointing in terms of modulus or strength. Along with recent advance in CNT growth technique, we will be able to directly measure tensile properties of millimeter-long MWCNTs. This study firstly tackles the direct measurement of the tensile properties of millimeter-long MWCNTs that can be used as reinforcement in a composite system. A carefully designed tensile testing technique for the MWCNTs is developed, which allows us to obtain more accurate and reliable measured values. The average tensile strength and Young's modulus of the CNTs investigated in this study are measured to be 0.85 GPa and 34.65 GPa, respectively. Also, this work statistically investigates the effect of the CNT dimensions including length, diameter and volume on the tensile properties. To the best of our knowledge, this is the very first report on the tensile properties of macroscopically long and continuous CNTs.