Soft Composites Research Articles

Microwave Absorption in MWNTs‐Based Soft Composites Containing Nanocrystalline Particles as Magnetic Core and Intrinsically Conducting Polymer as a Conductive Layer

AbstractMicrowave absorbers derived using poly(vinylidene fluoride) (PVDF), super paramagnetic nanocrystalline calcium ferrite (CaFe2O4) and multiwall nanotubes (MWNTs) were developed in this study. Effect of in‐situ modification of CaFe2O4 and MWNTs with a conducting layer, (polyaniline, PANI) on different properties of the composite has been investigated systematically. Two approaches were investigated here to gain insight into the mechanism of microwave absorption. Firstly, coating PANI onto CaFe2O4 and blending along with MWNTs; secondly, coating PANI onto MWNTs and blending along with CaFe2O4. The electrical and magnetic properties of various composites containing hybrid particles were evaluated. Interestingly, the PVDF composites containing PANI coated CaFe2O4 blended together with MWNTs showed excellent shielding effectiveness (‐57 dB at 18 GHz) as compared to the second approach where PANI was coated onto MWNTs and blended along with CaFe2O4. This was discussed here with respect to the relative permittivity and permeability in a wide range of frequency.

Droplet spreading characteristics observed during 3D printing of aligned fiber-reinforced soft composites

The 3D printing of fiber-reinforced soft composites (FrSCs) is a hybrid process that combines conventional inkjet-based 3D printing with the directed deposition of electrospun polymer fiber mats. This paper investigates the spreading characteristics of droplets when deposited on fibrous substrates, under conditions relevant to 3D printing of aligned FrSCs. Both single and multi-droplet impingement studies are conducted on substrates with varying fiber number densities. High-speed imaging is used to study the characteristic time-scales and the spreading behavior of the droplets. The single droplet impingement studies on stationary substrates reveal that the presence of fibers promotes droplet spreading along the length of the fibers. Occasional surface energy variations in the fiber mats in the form of voids and fiber bundles are also seen to affect the droplet shape and the characteristic spreading times. In the case of a moving substrate, the droplets are seen to spread the most during in-line printing, i.e., when the direction of the printing velocity coincides with the direction of fiber alignment. They spread the least during orthogonal printing, i.e., when the direction of the printing velocity is perpendicular to the direction of fiber alignment. The printing of straight lines shows an interesting edge retraction phenomenon that gets accentuated the most in the case of in-line printing. The findings of the high-speed imaging studies have been confirmed by 3D printing comparable artifacts using UV curable inks. These studies indicate that for a given fiber mat and UV curable ink combination, the choice of the in-line or orthogonal printing strategy has implications for the overall printing time, fiber content, edge resolution and surface quality of the 3D printed FrSC part.

Lithium intercalation into disordered carbon/SiCN composite. Part 2: Raman spectroscopy and 7Li MAS NMR investigation of lithium storage sites

Within this work, we analyze the lithium storage sites within carbon/silicon carbonitride (SiCN) composites. Commercial carbons, HD3 (hard carbon) and LD1N and LD2N (soft carbons), of varying porosity are impregnated with polysilazane (HTT 1800) and pyrolysed at 1100 °C. It is found in the first part of this study (Graczyk-Zajac et al. J Solid State Electrochem 19:2763–2769, 2015) that the initial porosity of the carbon phase plays an important role in determining the lithium insertion capacity and rate capability of the composite material. By applying Raman spectroscopy and solid-state 7Li MAS NMR on pristine, lithiated, and delithiated samples, we investigate the lithium storage sites within the composite materials. By means of Raman spectroscopy, it has been found that lithium storage in hard carbon-derived composites occurs in a significant extent via adsorption-like process within unorganized carbon, whereas for the soft carbon composites, storage in turbostratic carbon is identified. 7Li solid-state NMR confirms these findings revealing that more than 33 % of lithium stored in HD3/SiCN is adsorbed in ionic form at the surface and in pores of the composite, while around 38 % is stored between carbon layers. LD1N and LD2N composites store more than 50 % of lithium in the intercalation-type sites.

Stationary variational estimates for the effective response and field fluctuations in nonlinear composites

This paper presents a variational method for estimating the effective constitutive response of composite materials with nonlinear constitutive behavior. The method is based on a stationary variational principle for the macroscopic potential in terms of the corresponding potential of a linear comparison composite (LCC) whose properties are the trial fields in the variational principle. When used in combination with estimates for the LCC that are exact to second order in the heterogeneity contrast, the resulting estimates for the nonlinear composite are also guaranteed to be exact to second-order in the contrast. In addition, the new method allows full optimization with respect to the properties of the LCC, leading to estimates that are fully stationary and exhibit no duality gaps. As a result, the effective response and field statistics of the nonlinear composite can be estimated directly from the appropriately optimized linear comparison composite. By way of illustration, the method is applied to a porous, isotropic, power-law material, and the results are found to compare favorably with earlier bounds and estimates. However, the basic ideas of the method are expected to work for broad classes of composites materials, whose effective response can be given appropriate variational representations, including more general elasto-plastic and soft hyperelastic composites and polycrystals.

Macromolecular topology and rheology: beyond the tube model

The motion of entangled polymers is marked by their ability to make conformational adjustments, which is mediated by their free ends. A credible account of the basic features of entanglement release in linear and nonlinear rheology is offered by the tube model which, despite its limitations and shortcomings, is considered as the “standard” model in the field, accounting for the dynamics of linear and branched polymers with homogeneous monomer density. Here, we challenge the two central elements of the molecular picture of entanglements by exploiting the consequences of absence of free ends and monomer density distribution. Non-concatenated ring polymers of high molar mass do not form an entanglement network with plateau modulus, but instead relax stress self-similarly, while they deform much less than their linear counterparts in nonlinear shear flow. Their rheology is extremely sensitive to the presence of unlinked linear chains. On the other hand, star polymers with many arms have a dual nature: polymeric, which governs arm relaxation, and colloidal, which controls their subsequent center-of-mass motion and completes the stress relaxation process. Appropriate choice of number and size of arms allows to tune their dynamic and structural properties, and therefore bridge the gap between polymers and colloids. These examples demonstrate a different manifestation of topological interactions, with distinct linear and nonlinear rheology, which cannot be described in full by the tube model. They also provide an avenue for taking advantage of macromolecular architecture in order to engineer the rheology of polymeric structures and soft composites. Still, a number of outstanding issues remain and we outline some perspectives in this exciting field of molecular rheology.

Tunable defect mode in a soft wrinkled bilayer system

Here we study the defect mode in a wrinkled soft bilayer induced by a local geometrical defect introduced into the system. We show that the band gap of the studied composite system depends on the wrinkling-induced stress pattern and is not sensitive to the compression strain ε in the given loading range. However, the frequency of the defect mode apparently varies with ε. This interesting phenomenon enables us to widely tune the spatial extension of the defect mode from highly localized to completely extended by simply controlling the imposed external load. Our strategy to create a tunable defect mode in a soft layered composite is simple and straightforward and may find such broad applications as the development of advanced soft metamaterials and corresponding devices.

Delamination behavior of UHMWPE soft layered composites

Abstract Ultra-high-molecular-weight polyethylene (UHMWPE) fiber based soft polymeric composite laminates are widely used in ballistic applications due to their high tensile strength and low density. The delamination between the layers is an important energy dissipation mode for ballistic resistance. In this study, two experimental setups are proposed and examined for the delamination behavior of UHMWPE composites. The first mode-I setup consists of a modified double cantilever beam (DCB) test. The second is a new interlaminar shear (ILS) test proposed to characterize delamination failure in mode-II, which is known to be a major drawback of these composites due to poor matrix-fiber interphase bond. Digital Image Correlation (DIC) method is employed to extract the full field shear strain in mode-II and to examine its uniformity. Crack length in mode-I was also quantified using optical means. Crack surfaces are characterized using Scanning Electron Microscopy (SEM) to study the propagation mechanism. The mode-I test results of commercial Dyneema HB26 (DH) cross-ply multi-layered composite provide insights on the material and crack behavior. The proposed ILS tests provide a unique way to impose close to uniform and dominant interlaminar shear strain. The ILS strength of DH is found to be significantly lower than its tensile strength.

3D printing of fiber-reinforced soft composites: Process study and material characterization

Fiber-reinforced soft composites (FrSCs) are composites that are made up of polymeric fibers with specific material properties and hierarchical length-scales, embedded within another soft-polymer matrix. This paper is aimed at systematically studying the effect of key processing parameters viz., fiber mat alignment, area coverage, and surface energy of the fiber carrier substrate, on the tensile properties and failure mechanisms seen in 3D printed FrSCs. A novel electrospinning-based “direct-write” system is used for creating the aligned and random nylon fiber mats. The fiber mats are then characterized for their diameter distributions, effective area coverage, number density, and tensile properties. The surface energy of the fiber carrier substrate is found to be critical to the fiber transfer efficiency of the stamping operation used in the 3D printing process, with polytetrafluoroethylene-coated aluminum films being more effective due to their low surface energy. Tensile testing results show that depending on the extent of alignment and the fiber content present in the 3D printed composite, it can have a 40–260% improvement in the elastic modulus over that of the base UV-curable polymer. The composites also show evidence of characteristic failure mechanisms seen in the domain of nanocomposite materials, viz., fiber-induced local plastic deformation (crazing), crack arrest and deflection, fiber strengthening, and fiber pull-out. The evidence of fiber pull-out also points to the formation of an interfacial polymer sheath around the fibers. The elastic modulus of this sheath is estimated to be an order of magnitude higher than the base polymer.

3D printing of versatile reactionware for chemical synthesis.

In recent decades, 3D printing (also known as additive manufacturing) techniques have moved beyond their traditional applications in the fields of industrial manufacturing and prototyping to increasingly find roles in scientific research contexts, such as synthetic chemistry. We present a general approach for the production of bespoke chemical reactors, termed reactionware, using two different approaches to extrusion-based 3D printing. This protocol describes the printing of an inert polypropylene (PP) architecture with the concurrent printing of soft material catalyst composites, using two different 3D printer setups. The steps of the PROCEDURE describe the design and preparation of a 3D digital model of the desired reactionware device and the preparation of this model for use with fused deposition modeling (FDM) type 3D printers. The protocol then further describes the preparation of composite catalyst-silicone materials for incorporation into the 3D-printed device and the steps required to fabricate a reactionware device. This combined approach allows versatility in the design and use of reactionware based on the specific needs of the experimental user. To illustrate this, we present a detailed procedure for the production of one such reactionware device that will result in the production of a sealed reactor capable of effecting a multistep organic synthesis. Depending on the design time of the 3D model, and including time for curing and drying of materials, this procedure can be completed in ∼3 d.

Ultra-soft magnetic properties and correlated phase analysis by 57Fe Mössbauer spectroscopy of Fe74Cu0.8Nb2.7Si15.5B7 alloy

A detailed study of magnetic softness has been performed on FINEMENT type of ribbons by investigating the BH loop with maximum applied field of 960 A/m. The ribbon with the composition of Fe74Cu0.8Nb2.7Si15.5B7 was synthesized by rapid solidification technique and the compositions volume fraction was controlled by changing the annealing condition. Detail phase analysis was performed through X-ray diffraction (XRD), Differential scanning calorimetry (DSC), Vibrating sample magnetometer (VSM) and Mössbauer spectroscopy in order to correlate the ultrasoft magnetic properties with the volume fraction of amorphous and α-Fe(Si) soft nano composites. Bright (BF) and dark field (DF) image with selective area diffraction (SAD) patterns by the transmission electron microscopy (TEM) of the sample annealed for the optimized annealed condition at 853 K for 3 min reveals nanocrystals with an average size between 10-15 nm possessing the bcc structure which matches with the grain size revealed by the X-ray diffraction. Kinetics of crystallization of α-Fe(Si) phases has been determined by DSC curves. Extremely small coercivity of 30.9 A/m and core loss of 2.5 W/Kg for the sample annealed at 853 K for 3 min was found. Similar values for other crystalline conditions were determined by using BH loop tracer with a maximum applied field of around 960 A/m. Mössbauer spectroscopy was used to determine chemical shift, hyperfine field distribution (HFD), and peak width of different phases. The volume fractions of the relative amount of amorphous and crystalline phases are also determined by Mössbauer spectroscopy. High saturation magnetization along with ultrasoft magnetic properties exhibits very high potentials technological applications.

Soft Color Composites with Tunable Optical Transmittance

A class of soft color composites whose light transmittance can be actively tuned and controlled through mechanical actuation is studied. The design comprises thin sheets of polydimethylsiloxane, an optically clear silicone‐based rubber, that is mixed with a colloidal suspension of black micrometer‐sized dye particles to provide tunable opacity to the specimens. The thickness of the samples can be reduced by mechanical loading (e.g., pneumatically), which modulates the thickness and, in turn, the transmittance by as much as 40%. The mechanism is independent of the specific method of actuation chosen for loading. Scaling analysis and finite element modeling are combined to predictively describe and rationalize the evolution of the transmittance of our samples as a function of the applied mechanical loading and validate the predictions against biaxial tensile experiments. Compared to existing solutions, the main advantages of this mechanism are that it is remarkably simple and robust, as well as fast and fully reversible. Making use of this framework, pneumatic bulging is then chosen as a representative loading strategy, for which a series of design guidelines is presented, which may be implemented in practical applications, such as smart windows and other visually active materials.

Harnessing viscoelasticity and instabilities for tuning wavy patterns in soft layered composites.

In this study, we combine the elastic instability and non-linear rate-dependent phenomena to achieve microstructure tunability in soft layered materials. In these soft composites, elastic instabilities give rise to formation of wrinkles or wavy patterns. In elastic materials, the critical wavelength as well as amplitude at a particular strain level are exclusively defined by the composite microstructure and contrast in the elastic moduli of the phases. Here, we propose to use rate-dependent soft constituents to increase the admissible range of tunable microstructures. Through the experiments on 3D printed soft laminates, and through the numerical simulation of the visco-hyperelastic composites, we demonstrate the existence of various instability-induced wavy patterns corresponding to the identical deformed state of the identical soft composites.

Micromechanics methods for evaluating the effective moduli of soft neo-Hookean composites

Most biological soft tissues are multiphase composite materials, and the determination of their effective constitutive relations is a major concern in medical engineering. In this paper, we consider a class of soft two-phase composites, in which both phases are isotropic and hyperelastic neo-Hookean materials. For such an isotropic composite consisting of inclusions uniformly distributed but randomly oriented in a matrix, two constitutive parameters are required to characterize its hyperelastic constitutive relation. Micromechanics methods, including dilute concentration method, Mori–Tanaka method, self-consistent method, and differential method are extended to predict the effective properties of this kind of composites. Analytical solutions are given for the hyperelastic neo-Hookean composites reinforced by spherical particles, long fibers, and penny-shaped platelets, respectively. Finite element simulations are performed to evaluate the accuracy of these theoretical methods.

Buckling of an elastic fiber with finite length in a soft matrix.

Elastic fibers embedded in a soft matrix are frequently encountered in nature and engineering across different length scales, ranging from microtubules in cytosol and filament networks to dissociative slender fish bones in muscles and fiber-reinforced soft composites. Fibers may buckle when the composite is subjected to compression; this study investigates this issue through a combination of experiments, finite-element simulations and theoretical analysis. Analysis reveals the important role of the interfacial shear forces and leads to an explicit solution to predict the occurrence of buckling for a slender fiber with finite length. The results reported in this paper will help understand the formation of shapes in some natural systems and provide guidelines for the design of soft biocomposites.

Viscoelasticity of Shear Thickening Fluid Based on Silica Nanoparticles Dispersing in 1-butyl-3-methylimidizolium Tetrafluoroborate

The viscoelasticity of shear thickening fluid (STF), a crucial property in the protective composite applications, with different silica nanoparticle concentrations in ionic liquid, 1-butyl-3-methylimidizolium tetrafluoroborate ([C4min]BF4), was studied at different temperatures and with shear frequencies through oscillatory shear, respectively. All STFs present strain thickening behavior. With increasing silica nanoparticle concentration, the critical shear strain for the onset of strain thickening decreased, while the complex viscosity, storage modulus, and loss modulus increased significantly. The critical shear strain increased with an increase of temperature, while the complex viscosity, storage modulus, and loss modulus decreased notably. The critical shear strain was constant with increasing the frequency of strain, while the complex viscosity decreases slightly. The storage modulus and loss modulus were independent with frequency in the strain thickening region. Nanoparticle clusters leading to strain thickening were demonstrated. The viscoelastic response of STFs to varying silica nanoparticle content, temperature, and frequency investigated here will help to design the specific application of STFs in soft protective composites and damping devices.

Homogenization and multiscale stability analysis in finite magneto-electro-elasticity. Application to soft matter EE, ME and MEE composites

Soft matter electro-elastic (EE), magneto-elastic (ME) and magneto-electro-elastic (MEE) composites exhibit coupled material behavior at large strains. Examples are electro-active polymers and magneto-rheological elastomers, which respond by a deformation to applied electric or magnetic fields, and are used in advanced industrial environments as sensors and actuators. Polymer-based magneto-electric-elastic composites are a new class of tailor-made materials with promising future applications. Here, a magneto-electric coupling effect is achieved as a homogenized macro-response of the composite with electro-active and magneto-active constituents. These soft composite materials show different types of instability phenomena, which even might be exploited for future enhancement of their performance. This covers micro-structural instabilities, such as buckling of micro-fibers or particles, as well as material instabilities in the form of limit-points in the local constitutive response. Here, the homogenization-based scale bridging links long wavelength micro-structural instabilities to material instabilities at the macro-scale. This work outlines a comprehensive framework of an energy-based computational homogenization in electro-magneto-mechanics, which allows a tracking of postcritical solution paths such as those related to pull-in instabilities. It provides variational-based definitions of multiscale structural and material stability phenomena. The starting point is a minimization principle of averaged electro-magneto-elastic energy, that is discretized by using electric and magnetic vector potentials. Next, computationally more effective representations based on scalar potentials are considered by a reformulation of the energy in terms of an averaged enthalpy functional. Structural stability is analyzed based on perturbations of the averaged energy, while local material stability is defined by a generalized quasi-convexity condition. It is shown that the incremental arrays which govern the stability criteria appear within the convenient enthalpy-based representation in a distinct diagonal form, containing mechanical and electro-magnetic partitions. Representative simulations demonstrate a tracking of inhomogeneous EE, ME and MEE composites, showing the development of postcritical zones in the microstructure and a possible unstable homogenized material response.

On the generation of soft magneto‐electric effects through Maxwell interactions

AbstractThis contribution presents a finite‐strain computational homogenization framework for soft magneto‐electro‐elastic solids. We use it in order to determine effective magneto‐electric coupling coefficients of soft composites, which are driven by the interconnection of electrical and magnetical Maxwell stresses. (© 2015 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Tension-induced tunable corrugation in two-phase soft composites: Mechanisms and implications

We numerically investigate the elastic deformation response of a two-phase soft composite under externally applied concentric tension. We show that by carefully designing the inclusion pattern, it is possible to induce corrugations normal to the direction of stretch. By stacking 1D composite fibers to form 2D membranes, these corrugations collectively lead to the formation of membrane channels with shapes and sizes tunable by the level of stretch. Furthermore, we show that by using specific inclusion patterns in laminated plates, it is possible to create pop-ups and troughs enabling the development of complex 3D geometries from planar construction. We have found that the corrugation amplitude increases with the stiffness of inclusion and its eccentricity from the tension axis. We discuss the mechanisms leading to the development of corrugations as well as their different implications. We hypothesize that the techniques discussed in this paper provide greater flexibility and controllability in pattern design and have potential applications in a variety of fields including tunable band gap formation and water treatment.

Phenylboronic Acid Appended Pyrene-Based Low-Molecular-Weight Injectable Hydrogel: Glucose-Stimulated Insulin Release.

A pyrene-containing phenylboronic acid (PBA) functionalized low-molecular-weight hydrogelator was synthesized with the aim to develop glucose-sensitive insulin release. The gelator showed the solvent imbibing ability in aqueous buffer solutions of pH values, ranging from 8-12, whereas the sodium salt of the gelator formed a hydrogel at physiological pH 7.4 with a minimum gelation concentration (MGC) of 5 mg mL(-1) . The aggregation behavior of this thermoreversible hydrogel was studied by using microscopic and spectroscopic techniques, including transmission electron microscopy, FTIR, UV/Vis, luminescence, and CD spectroscopy. These investigations revealed that hydrogen bonding, π-π stacking, and van der Waals interactions are the key factors for the self-assembled gelation. The diol-sensitive PBA part and the pyrene unit in the gelator were judiciously used in fluorimetric sensing of minute amounts of glucose at physiological pH. The morphological change of the gel due to addition of glucose was investigated by scanning electron microscopy, which denoted the glucose-responsive swelling of the hydrogel. A rheological study indicated the loss of the rigidity of the native gel in the presence of glucose. Hence, the glucose-induced swelling of the hydrogel was exploited in the controlled release of insulin from the hydrogel. The insulin-loaded hydrogel showed thixotropic self-recovery property, which hoisted it as an injectable soft composite. Encouragingly, the gelator was found to be compatible with HeLa cells.

A new computational method for homogenized tangent moduli of a soft magnetoelastic composite

A finite element methods based homogenization approach is presented to simulate the nonlinear behavior of magnetoactive composites under a macroscopic deformation and an external magnetic field. The coupled magnetoelastic constitutive law and governing equations are developed in micro-scale for large deformations. Micro-scale formulation is employed on a characteristic volume element, taking into account periodic boundary conditions. A periodic homogenization method is utilized to compute macroscopic properties of the magnetoelastic composite at different mechanical and magnetic loading paths. A new and cost effective numerical scheme is used to develop the magnetoelastic tangent moduli tensors. A sensitivity analysis is proposed to compute the overall tangent moduli tensors of the composite through the finite difference method. The presented approach is useful in the characterization of magnetoactive and electroactive composites and FE2 methods. Results are presented for typical equilibrium states.