Properties of Polysiloxane/Nanosilica Nanodielectrics for Wearable Electronic Devices

Polymer nanodielectrics characterized by good flexibility, processability, low dielectric loss and high dielectric permittivity are materials of interest for wearable electronic devices and intelligent textiles, and are highly in demand in robotics. In this study, an easily scalable and environmentally friendly method was applied to obtain polysiloxane/nanosilica nanocomposites with a large content of nanofiller, of up to 30% by weight. Nanosilica was dispersed both as individual particles and as agglomerates; in nanocomposites with a lower amount of filler, the former prevailed, and at over 20 wt% nanosilica the agglomerates predominated. An improvement of both the tensile strength and modulus was observed for nanocomposites with 5–15 wt% nanosilica, and a strong increase of the storage modulus was observed with the increase of nanofiller concentration. Furthermore, an increase of the storage modulus of up to seven times was observed in the nanocomposites with 30 wt% nanosilica. The tensile modulus was well fitted by models that consider the aggregation of nanoparticles and the role of the interface. The dielectric spectra showed an increase of the real part of the complex relative permittivity with 33% for 30 wt% nanosilica in nanocomposites at a frequency of 1 KHz, whereas the loss tangent values were lower than 0.02 for all tested nanodielectrics in the radio frequency range between 1 KHz and 1 MHz. The polysiloxane–nanosilica nanocomposites developed in this work showed good flexibility; however, they also showed increased stiffness along with a stronger dielectric response than the unfilled polysiloxane, which recommends them as dielectric substrates for wearable electronic devices.

Shape memory polyimide composites with high storage modulus and high glass transition temperature

Shape memory polymers (SMPs) are smart materials that can be programmed to change shape under external stimuli, whereas the low storage modulus limit the application of them. Herein, carbon fabric (CF) reinforced shape memory polyimide composites (SMPICs) with high storage modulus were manufactured via hot pressing molding process. Firstly, we synthesized one kind of thermoplastic shape memory polyimide (SMPI) with glass transition temperature of 205°C by the two-step high-temperature solution polycondensation. In addition, the triamine was added in the SMPI system as a crosslinking agent to form the thermosetting SMPI with different crosslinking degree. In order to improve the storage modulus of SMPI, the CFs with three layers were embedded in thermosetting SMPI matrix. The storage modulus of the obtained SMPICs was as high as 26 GPa. The glass transition temperature and thermal decomposition temperature of SMPICs were up to 213°C and 505°C, respectively. Moreover, the shape fixation rate and recovery rate of SMPICs were both more than 94%. These SMPICs with high storage modulus is of great significance, proving more application potential in many fields such as aerospace.

Effect of highly thermally conductive Ag@BN on the thermal and mechanical properties of phthalonitrile resins

Ag@BN/phthalonitrile resin composites were prepared using highly thermally conductive BN modified by Ag plating. The effects of different contents of Ag@BN particles on the dynamic mechanical properties, thermal stability, and thermal conductivity of composites were examined. The results of Fourier-transform infrared spectroscopy, X-ray powder diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy analyses showed that Ag was successfully deposited on the surface of BN. The prepared Ag@BN was subjected to KH550 grafting treatment. With the increase in the content of Ag@BN/KH550, the storage modulus, thermal stability, and thermal conductivity of the composite increased. The storage modulus, decomposition temperature, and thermal conductivity of the Ag@BN/phthalonitrile composite with 20 wt.% Ag@BN/KH550 were 5.0 GPa, 539°C, and 0.80 W/(mK), respectively, which are 1.35, 1.18, and 3.33 times higher than those of pure resin, respectively. The compatibility and dispersibility of BN modified by Ag plating in phthalonitrile resin were effectively enhanced, thereby providing a potential candidate to be used at high-temperature devices with high thermal conductivity.

Real-Time Life-Cycle Monitoring of Composite Structures Using Piezoelectric-Fiber Hybrid Sensor Network

In this paper, an in situ piezoelectric-fiber hybrid sensor network was developed to monitor the life-cycle of carbon fiber-reinforced plastics (CFRPs), from the manufacturing phase to the life in service. The piezoelectric lead-zirconate titanate (PZT) sensors were inserted inside the composite structures during the manufacturing process to monitor important curing parameters, including the storage modulus of resin and the progress of the reaction (POR). The strain that is related to the storage modulus and the state of resin was measured by embedded fiber Bragg grating (FBG) sensors, and the gelation moment identified by the FBG sensors was very close to those determined by dynamic mechanical analysis (DMA) and POR. After curing, experiments were conducted on the fabricated CFRP specimen to investigate the damage identification capability of the embedded piezoelectric sensor network. Furthermore, a modified probability diagnostic imaging (PDI) algorithm with a dynamically adaptive shape factor and fusion frequency was proposed to indicate the damage location in the tested sample and to greatly improve the position precision. The experimental results demonstrated that the average relative distance error (RDE) of the modified PDI method was 68.48% and 46.97% lower than those of the conventional PDI method and the PDI method, respectively, with an averaged shape factor and fusion frequency, indicating the effectiveness and applicability of the proposed damage imaging method. It is obvious that the whole life-cycle of CFRPs can be effectively monitored by the piezoelectric-fiber hybrid sensor network.

Dynamic mechanical behavior of composite materials reinforced by graphene and huntite minerals

The aim of this study is to investigate the dynamic mechanical properties of composite materials reinforced by mineral experimentally. Graphene and huntite minerals were added to epoxy resin at different weight ratios (wt.-%) as 0.5 weight percent, 1 weight percent and 3 weight percent, to examine the effect of mineral types and percentages on the resulting dynamic mechanical properties. In addition, the effect of non-layered huntite unlike graphene, with a nano-sized grain structure, was investigated. Thus, glass transition temperature (Tg), storage modulus (E’), loss modulus (E”) and damping ratio (tan δ) values were determined and compared. Moreover, a tensile test was performed in order to explain the relation between stress and strain. It was seen that adding different minerals caused different results according to types and proportions. In general, adding minerals to the pure resin increased the storage modulus and loss modulus, whereas the damping ratio (tan δ) decreased compared to the pure resin.

Effects of transglutaminase cross-linking process on printability of gelatin microgel-gelatin solution composite bioink

Three-dimensional (3D) bioprinting has emerged as a powerful engineering approach for various tissue engineering applications, particularly for the development of 3D cellular structures with unique mechanical and/or biological properties. For the jammed gelatin microgel-gelatin solution composite bioink, comprising a discrete phase of microgels (enzymatically gelled gelatin microgels) and a cross-linkable continuous gelatin precursor solution-based phase containing transglutaminase (TG), its rheology properties and printability change gradually due to the TG enzyme-induced cross-linking process. The objective of this study is to establish a direct mapping between the printability of the gelatin microgel-gelatin solution based cross-linkable composite bioink and the TG concentration and cross-linking time, respectively. Due to the inclusion of TG in the composite bioink, the bioink starts cross-linking once prepared and is usually prepared right before a printing process. Herein, the bioink printability is evaluated based on the three metrics: injectability, feature formability, and process-induced cell injury. In this study, the rheology properties such as the storage modulus and viscosity have been first systematically investigated and predicted at different TG concentrations and times during the cross-linking process using the first-order cross-linking kinetics model. The storage modulus and viscosity have been satisfactorily modeled as exponential functions of the TG concentration and time with an experimentally calibrated cross-linking kinetic rate constant. Furthermore, the injectability, feature formability, and process-induced cell injury have been successfully correlated to the TG concentration and cross-linking time via the storage modulus, viscosity, and/or process-induced shear stress. By combing the good injectability, good feature formability, and satisfactory cell viability zones, a good printability zone (1.65, 0.61, and 0.31 hours for the composite bioinks with 1.00, 2.00, and 4.00% w/v TG, respectively) has been established during the printing of mouse fibroblast-based 2% gelatin B microgel-3% gelatin B solution composite bioink. This printability zone approach can be extended to the use of other cross-linkable bioinks for bioprinting applications.

Dynamic mechanical analysis of nylon 6 fiber-reinforced acrylonitrile butadiene rubber composites

Dynamic mechanical properties of polymeric materials are of direct relevance to a range of unique polymer applications. The aim of the study is to investigate the dynamic mechanical properties of composites of short nylon 6 fiber with acrylonitrile butadiene rubber (NBR). The storage modulus (G′), loss modulus (G″), and the damping factor (tan δ) have been analyzed with reference to the effects of fiber loading, curing systems, and bonding agents over a range of temperature and at varying frequencies. The storage modulus increases with increment in fiber loading, whereas loss modulus and damping factor decrease. The glass transition temperature shifts to higher temperature upon increment in fiber loading. Dicumyl peroxide (DCP)–cured composites show higher storage modulus and lower damping than the corresponding sulfur-cured one. The addition of hexa-resorcinol and phthalic anhydride as bonding agents enhances the dynamic mechanical properties of the composites. The experimental results have been evaluated by comparing with Einstein, Guth, and Nielsen models.

Temperature-dependent dynamic property of magnetorheological gel composite: experiment and modelling

Of all the smart materials that could vary with the change of external excitations, magnetorheological gel (MRG) is one of the most preeminent composites which appear controllable and reversible responses according to the magnitude of external magnetic field. Temperature is identified as another important driver of the alteration of dynamic property of MRG, which so far has not been studied systematically. The temperature-dependent dynamic property of MRG under different magnetic field strengths are investigated by three kinds of experiments –– strain amplitude, frequency and magnetic field sweep test. The experimental results demonstrate that the storage and loss moduli of MRG display a temperature-induced stiffening effect with a magnetic field, while a temperature-induced softening effect without a magnetic field. Besides, storage modulus improves with magnetic field strength, whereas loss modulus firstly appears a rapid growth and then a gradual reduction with the increment of magnetic field strength. This temperature-dependency of dynamic property is also interpreted through different mechanisms related to the transformation of microstructures of MRG. Furthermore, a modified magnetic dipole model which could predict the relationship between storage modulus and magnetic field strength, combines with the classical Arrhenius equation expressing the effect of temperature on viscosity, to describe the temperature-dependency of storage modulus of MRG under different magnetic field strengths. This paper may provide some useful guidance for designing an MR device.

Characterization and Modeling of the Viscoelastic Behavior of Hydrocolloid-Based Films Using Classical and Fractional Rheological Models

Hydrocolloid-based films are a good alternative in the development of biodegradable films due to their properties, such as non-toxicity, functionality, and biodegradability, among others. In this work, films based on hydrocolloids (gellan gum, carrageenan, and guar gum) were formulated, evaluating their dynamic rheological behavior and creep and recovery. Maxwell's classical and fractional rheological models were implemented to describe its viscoelastic behavior, using the Vortex Search Algorithm for the estimation of the parameters. The hydrocolloid-based films showed a viscoelastic behavior, where the behavior of the storage modulus (G') and loss modulus (G'') indicated a greater elastic behavior (G' > G''). The Maxwell fractional model with two spring-pots showed an optimal fit of the experimental data of storage modulus (G') and loss modulus (G'') and a creep compliance (J) (Fmin < 0.1 and R2 > 0.98). This shows that fractional models are an excellent alternative for describing the dynamic rheological behavior and creep recovery of films. These results show the importance of estimating parameters that allow for the dynamic rheological and creep behaviors of hydrocolloid-based films for applications in the design of active films because they allow us to understand their behavior from a rheological point of view, which can contribute to the design and improvement of products such as food coatings, food packaging, or other applications containing biopolymers.

Reduced graphene oxide / epoxy nanocomposites with enhanced dielectric, mechanical, thermomechanical and thermal properties

Epoxy/conducting filler nanocomposites with high dielectric performance have emanated as a promising material in electronic and electrical industry. In this work, a facile and low-cost method, that is, thermal reduction at 400°C was adopted for the preparation of reduced graphene oxide (rGO) from graphene oxide (GO). The rGO was characterized by X-ray diffraction analysis, Fourier transform infrared spectroscopy, Raman spectroscopy, Field emission scanning electron microscopy and Transmission electron microscopy. Epoxy nanocomposite presented a dielectric permittivity of 35 at 1.8 vol.% loading of rGO (Ep/G-1.8) at 103 Hz, which was 5 times higher than neat epoxy and with a low dielectric loss. With the addition of 0.3 vol.% of rGO (Ep/G-0.3), the mechanical properties such as tensile strength, Young’s modulus and impact strength were enhanced by 34%, 56% and 54%, respectively. Dynamic mechanical analysis (DMA) revealed that in comparison to epoxy, there was a tremendous enhancement of storage modulus (55%) and the glass transition temperature (Tg) exhibited a remarkable shift of 39°C towards higher temperature for Ep/G-0.3. Cross-link density and coefficient of effectiveness (C-factor) estimated from the storage modulus improved significantlyfor Ep/G-0.3. Theoretical modelling was done on the viscoelastic properties of the composites. SEM studies indicated the uniform dispersion of rGO throughout in the epoxy matrix. Thermogravimetric analysis revealed that inclusion of rGO improved the thermal stability of epoxy nanocomposites.