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We studied the microstructure-rheology relationship of polypropylene (PP)/ethylene octene copolymer (EOC) blends by means of rheology in combination with scanning electron microscopy. The blends varying in blend ratio all prepared by melt mixing were considered. The results of the frequency sweep experiment performed on the blend sample with matrix-dispersed morphology showed a pronounced positive deviation of storage modulus values at a low frequency range from those predicted by Palierne's viscoelastic model. This could be explained in terms of EOC droplets interconnectivity which was generated by in situ increased chain-branched entanglements. That is, the thermo-mechanically induced PP macro-radicals produced during the melt mixing could graft onto the more stable EOC macro-radicals in the interface leading to much longer-chain-branched EOC molecules, which had the capability of creating favorable entanglements for the EOC droplets conductivity. [ABSTRACT FROM AUTHOR]

Double-network (DN) hydrogel which possesses many superior performances such as excellent toughness, viscoelasticity and self-healing ability has become promising biomaterials. However, lack of thermal conductivity and moderate adhesiveness has greatly limited their application as load-bearing cartilage substitutes. Boron nitride itself has good rigidity and thermal conductivity but poor water solubility. Hydroxylated boron nitride nanosheets (OH-BNNSs) which have been previously reported have good water solubility (~ 0.6 mg/mL) and to obtain a homogeneous and stable dispersion. In this work, we introduced OH-BNNS into DN hydrogel to obtain high thermal conductivity and toughness hydrogel. The DN hydrogel is a class of physically double-network hydrogel with hydrophobic association polyacrylamide (PAM) and partly crystalline polyvinyl alcohol (PVA). Impressively, the obtained PVA/PAM/BNNS composite hydrogel possesses super-flexibility, high toughness, good thermal conductivity, appropriate tissue adhesiveness and stimuli-free self-healing ability. Therefore, the composite hydrogel overcomes the poor mechanical strength and locally overheating issues as cartilage substitutes and may also become alternatives for antipyretic pastes. [ABSTRACT FROM AUTHOR]

To explain the hygrothermal recovery (HTR) behavior of tension wood (TW) from the physical and chemical point of view in relation to the time, species and microfibril angle, a theoretical discussion using an analytical one-dimensional viscoelastic modeling was made. The chosen model includes an elastic element, a deformation mechanism and two viscoelastic elements called also as Kelvin-Voigt model. In this analysis, a top-down approach between the model and the experimental data was introduced to find the realistic parameters for the model. It enables us to fit the model to the HTR experimental data for three wood species: konara oak ( Quercus serrata Murray), urihada maple ( Acer rufinerve Siebold et Zucc.) and keyaki wood ( Zelkova serrata Makino). The fitted experimental data show that the two compliances of the two viscoelastic elements are the most important parameters that explain the evolution of TW longitudinal strain during the thermal treatment. [ABSTRACT FROM AUTHOR]

Experimental observations have shown that most of plant fibres are characterised by an intricate structure, morphology and organisation. This complex geometrical characteristics may affect the mechanical behaviour of this kind of natural fibres and contribute to the high variability of their mechanical properties. So, this study proposes a numerical investigation on the relationship between the cross-sectional shape of primary hemp bast fibres and their tensile behaviour. A 3D viscoelastic model based on finite element method is used for this study. Real and elliptical simplified cross-sectional shapes are considered. Results of the tensile test simulations clearly show the strong influence of the degree of ellipticity on the tensile response of the fibre, and more exactly on the shape of the nonlinearity of the response. Results also show that this morphologic effect is strongly related and coupled to structural parameters and physical mechanisms, such as the cellulose microfibrils angle and the viscoelastic behaviour of the material of the fibre wall. Geometric issues could then contribute to explain the different types of tensile behaviour experimentally observed and deserve to be taken into account in plant fibre models. [ABSTRACT FROM AUTHOR]

In this work, ternary nanocomposites of poly(lactic acid)/ethylene-co-vinyl-acetate (PLA/EVA, 70:30) containing varying concentrations (0.4-9.1 wt%) of halloysite nanotubes (HNTs) were prepared through melt compounding. Enhanced tensile modulus and impact strength demonstrated the strengthening and toughening effects of halloysite in the nanocomposites, simultaneously. The impact-fractured surface morphologies and halloysite-induced morphological changes of the nanocomposites were evaluated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. FT-IR investigation revealed interactions between HNT and PLA. Glass transition behavior of the nanocomposites, as shown by dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC), presents strong evidence in favor of phase interaction and the reinforcing effect of halloysite. Enhanced tensile strength and elongation-at-break demonstrated the toughening effect of halloysite. [ABSTRACT FROM AUTHOR]

The present article reports viscoelastic properties and melt rheology of novel nanostructured thermoplastic vulcanizates (TPVs) consisting of dispersed nanodimensional fluoroelastomer (FKM) particles into the continuous polyamide (PA6) matrix. The influence of dynamic vulcanization on the above properties of the blends was reported. The storage modulus and loss modulus were plotted against reduced frequency to understand the behavior of nanostructured TPVs. PA6/FKM TPVs showed pseudoplasticity and obeyed cross viscosity model. Insight into how the crosslink density of TPVs affects the rheological response was discussed. Also, a new relationship of zero shear viscosity ( η) with the crosslink density of TPVs was proposed to gain insight into dynamic vulcanization. It was found that up to a certain amount of curative concentration, η depends upon the crosslink density to a power equal to 3.4 and is roughly proportional to the crosslink density on further increase of curative concentration. van Gurp-Palmen plot of phase angle versus reduced complex modulus indicated the droplet-matrix morphology of the PA6/FKM blends which was further confirmed from field emission scanning electron microscopy and atomic force microscopy studies, where FKM nanoparticles (60-80 nm) were the dispersed phase. Various melt elastic properties of TPVs such as die swell, first normal stress difference, recoverable shear strain, and apparent elasticity were measured and analyzed. TPV extrudates were examined using surface finish and roughness values. TPVs exhibited smoother extrudate surface than reported earlier. Extrudate surface roughness from atomic force microscopy was used to analyze the processing behavior of TPVs. [ABSTRACT FROM AUTHOR]

The literature on the time-dependent behaviour of single bast fibres such as flax and hemp is extremely poor. The aim of this extensive study is to characterise the long-term behaviour of elementary hemp fibres and to establish suitable constitutive laws. Single hemp fibres are shown to exhibit both instantaneous strain and delayed, time-dependent strain when tensile loaded under constant climate. The creep behaviour appears to be a logarithmic function of time with a high strain rate during the primary creep and a lower and constant one during the secondary creep. A large scattering both in time-dependent properties and behaviour was observed on a batch of 25 single fibres. Three main creep behaviours were observed. Type II is truly linear as a function of the logarithm of time while Type I and Type III are strongly nonlinear and can be described, respectively, by concave and convex functions. A rheological model based on an anisotropic viscoelastic law and on a truncated inverse Gaussian spectrum of viscous mechanisms was shown to successfully describe all the experimentally observed behaviours. [ABSTRACT FROM AUTHOR]

Viscoelastic properties and sintering mechanisms of tape-casted gadolinium-doped ceria (CGO), yttrium-stabilized zirconia (YSZ), and scandium-yttrium-stabilized zirconia (ScYSZ) are characterized in order to investigate the reciprocal thermo-mechanical compatibility when arranged as a self-standing bi-layered electrolyte system. The combined use of thermo-mechanical analysis, optical dilatometry, and scanning electron microscopy ensures a systematic characterization of both the individual layers and CGO/YSZ and CGO/ScYSZ bi-layered laminates. The results of the co-firing process of the bi-layers are critical due to the mismatch of thermo-mechanical and sintering properties among the materials. Despite the better sinteractivity of ScYSZ, the self-standing CGO/ScYSZ bi-layer presents more challenges in terms of densification compared with the CGO/YSZ bi-layer. In particular, above 1200 °C, ScYSZ and CGO show residual porosity, and at higher sintering temperatures, above 1300 °C, full densification is completely inhibited by constrained sintering phenomena. [ABSTRACT FROM AUTHOR]

Micron- and submicron-sized barium titanate (BaTiO) particles, untreated and surface modified with aminopropyl triethoxy silane, were incorporated in poly(vinylidene fluoride-trifluoroethylene) to fabricate composites with up to 60 vol% of ceramic phase. The morphology and structure of solvent cast and compression-molded films, and their thermal, viscoelastic, and dielectric properties were investigated. When surface-modified BaTiO was used, it was possible to decrease both the viscoelastic and the dielectric losses of highly filled solvent cast films, while their storage modulus and relative permittivity either increased or remained equal, owing to reduced porosity and improved matrix-filler compatibility. The effect of BaTiO surface modification on the morphology of compression-molded films was less marked, leading to unchanged viscoelastic properties, and lower permittivity and dielectric losses. For all composites the frequency dependency of the dielectric properties at low frequencies was suppressed with modified BaTiO. [ABSTRACT FROM AUTHOR]

Injection molding is the most widely used processing technique for polymers. It offers several advantages over other processing conditions such as good surface finish, the ability to process complex parts without the need of secondary operations, and low cost for mass production. However, because of the complex deformation, and thermal and pressure histories that the polymer melt experiences during processing, residual stresses develop. These stresses act internally at room temperature and have the same effects on the material as externally applied stresses do, resulting in shrinkage and warpage of the product. In recent years, with the development and use of engineering plastics in an increasing number of applications, and with the tougher quality control policies in industries such as the automotive, the effects of residual stresses in product quality and performance have raised great interest. This review reports up-to-date advances in the field of residual stresses developments in polymers, with special attention given to injection molded products. Flow- and thermal-induced residual stresses are reported. Emphasis is given to the processing parameters that most influence residual stresses during injection molding as well as the effect of residual stresses not only on warpage but also on other material properties. [ABSTRACT FROM AUTHOR]

Clay is a soil component, a ceramic ware component, and a reinforcement in nanocomposites. Its viscoelastic behavior is relevant to vibration damping and slip rheology. The clay studied is hectorite, which is a trioctahedral magnesium-based smectite. Clay compacts with solid contents (50-53) vol% are studied in terms of the dynamic compressive properties at small static strains (3.1-3.5 %), low deformation amplitudes (5-8 μm), and low frequencies (0.2-10.0 Hz). The organic intercalation of hectorite (with basal spacing 12 Å prior to intercalation) to form organoclay (nanoclay in the form of disteardimonium hectorite with basal spacing 29 Å) increases both the loss tangent and the loss modulus of the solid part of the compact by up to about 27 %, while affecting the storage modulus negligibly. At 10.0 Hz, the organic intercalation increases the loss tangent of the solid part from 0.306 ± 0.009 to 0.366 ± 0.021 and increases the loss modulus of the solid part from (2.69 ± 0.08) × 10 to (3.44 ± 0.04) × 10 Pa. The organic intercalation affects the viscous behavior rather than the elastic behavior. The viscous behavior is attributed to the low-amplitude dynamic slippage at the interface between adjacent clay layers in a clay nanoplatelet and that between nanoplatelets in a clay particle. In case of organoclay, it is also due to the nanoscale viscous behavior of the organic component. The viscoelastic properties are essentially independent of the frequency. The viscous character of both unmodified clay and organoclay is weaker than that of carbon black or exfoliated graphite, but the elastic character of both unmodified clay and organoclay is stronger than that of carbon black or exfoliated graphite. [ABSTRACT FROM AUTHOR]

The dependence to temperature of the rebound of a solid polymer ball on a rigid slab is investigated. An acrylate polymer ball is brought to a wide range of temperatures, covering its glass to rubbery transition, and let fall on a granite slab while the coefficient of restitution, duration of contact, and force history are measured experimentally. The ball fabrication is controlled in the lab, allowing the mechanical characterization of the material by classic dynamic mechanical analysis. Finite element simulations of the rebound at various temperatures are run, considering the material as viscoelastic and as satisfying a WLF equation for its time-temperature superposition property. A comparison between the experiments and the simulations shows the strong link between viscoelasticity and time-temperature superposition properties of the material and the bounce characteristics of the ball. [ABSTRACT FROM AUTHOR]

Changes in the chemical, viscoelastic and hygroscopic properties of wood cell walls in Chinese fir ( Cunninghamia lanceolata) during the transition from sapwood to heartwood were studied to provide insights into the formation of heartwood. In situ imaging FTIR measurements indicated that the composition of the main components of cell walls remained almost unaltered, but more extractives were deposited in the wood cell walls during the sapwood–heartwood transition. Compared to the sapwood and transition wood, the heartwood had a higher softening temperature and greater activation energy, suggesting that the mobility restrictions of cell wall biopolymers were due to extractives obstructing the accessing of the plasticizer (ethylene glycol). The moisture sorption was the same from the sapwood to heartwood at a low relative humidity (RH), while the heartwood adsorbed less water at a high RH, probably caused by the extractives deposited in the matrix and mesopores of heartwood cell walls. [ABSTRACT FROM AUTHOR]

Aiming to develop acrylonitrile–butadiene–styrene (ABS) composites with enhanced ultraviolet stability, a series of formulations were prepared by melt compounding and evaluated by different characterization techniques. The influence of rutile titanium dioxide (TiO 2) and its combination with furnace carbon black (CB) on the viscoelastic properties of neat ABS was studied by dynamic mechanical analysis. An increase in the glass transition ( Tg) dynamics ascribed to the rubbery phase as a function of exposure time was observed. A greater contribution of CB nanoparticles in combination with TiO 2 to minimize the modifications on the Tg of the butadienic component was clearly seen. Quasi-static and spectrophotometry results are in good agreement, showing the efficiency of TiO 2 submicron particles and CB/TiO 2 against photo-oxidative degradation of neat ABS. A different behaviour was observed for modified ABS/TiO 2 and ABS/CB/TiO 2 composites with light stabilizers, antioxidants and combinations of them. While the TiO 2 efficiency was enhanced by the incorporation of combinations of light stabilizers and antioxidants, poor results were observed for modified ABS/CB/TiO 2 composites as a consequence of antagonistic interactions. It was concluded that formulations of ABS/CB/TiO 2 with light stabilizers and ultraviolet absorbers are unacceptable for common applications. [ABSTRACT FROM AUTHOR]

Three-dimensional viscoelastic properties of four-step three-dimensionally (3D) braided composites are studied in this paper. Based on the three-cell division scheme, a multi-scale model for 3D braided composites is proposed. A periodic boundary condition is applied to characterize the periodic structure of 3D braided composites and yarns. Given the viscoelastic parameters of resin matrix and the elastic constants of fibers, the viscoelastic properties of yarns are obtained by the finite element method and Prony Series fitting. The three-dimensional viscoelastic constitutive relationship of interior cells is derived based upon the viscoelastic properties of yarns and resin matrix. Moreover, the viscoelasticity of 3D braided composites is studied by creep experiment. The viscoelastic deformation obtained from the multi-scale method agrees well with the experimental results. The influence of the two independent micro-structural parameters, braiding angles, and fiber volume fractions, on the viscoelastic properties of 3D braided composites is investigated in detail. [ABSTRACT FROM AUTHOR]

Cyclic material tests were done on Divinycell PVC H100 foam to obtain out-of-plane and in-plane compression and shear material properties after foam yielding. The compression and shear stress-strain behaviors were very similar to each other except that a plateau (flow) stress occurred after yielding in compression, while the foam underwent mild strain hardening after shear yielding. The ratio of out-of-plane to in-plane stiffness and yield strength for the PVC H100 was found to be approximately 3/2 in both the compression and shear modes. After viscoplastic yielding, the foam underwent permanent damage and exhibited hysteresis, mainly in the form of viscoelasticity. Damage that occurred in the foam after it yields followed the pattern of Mullins damage, i.e., the damage was essentially fixed at a given strain amplitude, and more damage occurred with increasing the strain amplitude. Hysteresis was much more pronounced as the damage grew, suggesting that viscoelastic properties of the foam could be changing with the amount of damage. [ABSTRACT FROM AUTHOR]

The viscoelastic characteristics of ultra-high molecular weight polyethylene (UHMWPE) fibres are investigated, in terms of creep-induced recovery strain and force output, to evaluate their potential for producing a novel form of prestressed composite. Composite production involves subjecting fibres to tensile creep, the applied load being removed before moulding the fibres into a resin matrix. After matrix curing, the viscoelastically strained fibres impart compressive stresses to the surrounding matrix, to produce a viscoelastically prestressed polymeric matrix composite (VPPMC). Previous research has demonstrated that nylon fibre-based VPPMCs can improve mechanical properties without needing to increase mass or section dimensions. The viability of UHMWPE fibre-based VPPMCs is demonstrated through flexural stiffness tests. Compared with control (unstressed) counterparts, these VPPMCs typically show increases of 20-40 % in flexural modulus. Studies on the viscoelastic characteristics indicate that these fibres can release mechanical energy over a long-timescale and fibre core-skin interactions may have an important role. [ABSTRACT FROM AUTHOR]

In this study, the mechanical properties of nano-silica particulate-reinforced epoxy composites with different crosslinking densities were clarified experimentally to consider the interaction effects between nano-particles and the network structure in matrix resin. The matrices were prepared by curing with an excessive mixture of diglycidyl ether of bisphenol A type epoxy resin as the curing agent for the stoichiometric condition. The volume fraction of the silica particles with a median diameter of 240 nm was constantly 0.2 for every composite. The crosslinking densities and glass transition temperatures of the neat epoxy resins were identified from thermo-viscoelastic properties measured by dynamic mechanical analysis. Elastic moduli and strengths of the composites and the neat epoxy resins were measured by three-point bending tests. The glass transition temperatures of the neat epoxy resins decreased linearly as the crosslinking densities decreased from the stoichiometric condition. The glass transition temperatures of the composites were reduced by adding the nano-silica particles. The bending moduli of the composites in the glassy state could be predicted by using a mixture law of the composites regardless of the crosslinking densities and glass transition temperatures. The bending strengths were found to be sensitive to the crosslinking densities: they were both higher (for composites with high crosslinking densities) and lower (for composites with low crosslinking densities) than those of the neat epoxy resin. These results demonstrate that the interaction between nano-particles and network structures reduces the bending strengths, especially for low crosslinking densities. [ABSTRACT FROM AUTHOR]

The pomelo ( Citrus maxima) is the largest and heaviest fruit of the genus Citrus and can acquire considerable potential energy as it ripens hanging up to 15 m height. Its thick foam-like structured peel presumably acts inter alia as a shock absorbing layer, protecting the fruit as it impacts on the ground upon being shed. Thereby the peel dissipates kinetic energy by being compacted. In order to elucidate the compaction mechanism of the highly heterogeneous pomelo peel, we conducted incremental stress relaxation tests. Two different models describing the stress relaxation curves, namely, the well-known Maxwell model and the Peleg model were compared and found to be suitable to describe the stress relaxation. As the Peleg model involves only two constants describing the relaxation curves it was the method of choice for interpreting the compaction of the peel samples. The inverse of k reflects the initial decay rate of the relaxation process and k is a measure of the samples' solidity. The behaviour of these constants with increasing strain indicates the strong influence of the peel samples' geometry and composition which is attributed to the fruit shape, a gradual changing density of the peel, which can be considered as a stacked array of foam layers differing in density, and the turgescence of the biological cells. [ABSTRACT FROM AUTHOR]

This study applies a visco-plastic self-consistent (VPSC) model to an AZ31 alloy processed by equal-channel angular pressing. The study focuses on the possibility of reproducing a grain misorientation distribution and the distribution of coincident site lattice boundaries in the model framework. Co-rotation and a magnesium misorientation scheme are employed together with the conventional VPSC model to improve its predictions. The results of the model are then compared with experimental data. [ABSTRACT FROM AUTHOR]

Viscoelastic properties of hollow particle-reinforced composites called syntactic foams are studied using a dynamic mechanical analyzer. Glass hollow particles of three different wall thicknesses are incorporated in the volume fraction range of 0.3-0.6 in vinyl ester resin matrix to fabricate twelve compositions of syntactic foams. Storage modulus, loss modulus, and glass transition temperature are measured and related to the microstructural parameters of syntactic foams. In the first step, a temperature sweep from −75 to 195 °C is applied at a fixed loading frequency of 1 Hz to obtain temperature dependent properties of syntactic foams. In the next step, selected four compositions of syntactic foams are studied for combined effect of temperature and loading frequency. A frequency sweep is applied in the range 1-100 Hz and the temperature is varied in the range 30-140 °C. Time-temperature superposition (TTS) principle is used to generate master curves for storage modulus over a wide frequency range. The room temperature loss modulus and maximum damping parameter, Tanδ, are found to have a linear relationship with the syntactic foam density. Increasing volume fraction of particles helps in improving the retention of storage modulus at high temperature in syntactic foams. Cole-Cole plot and William-Landel-Ferry equation are used to interpret the trends obtained from TTS. The correlations developed between the viscoelastic properties and material parameters help in tailoring the properties of syntactic foams as per requirements of an application. [ABSTRACT FROM AUTHOR]

Yarn made from natural fibers, synthetic fibers, or other types of fibrous materials is all formed by twisting an assembly of short or long fibers and its performance is significantly influenced by the physical behavior of these fibers in the yarn forming region-a small triangle area called spinning triangle. In spinning process, the fibers in spinning triangle where they are finally twisted into a yarn are subject to dynamic loads. However, earlier theoretical models are all based on the static assumptions and it cannot reflect the real status of the spinning process. Therefore, in this study, a new dynamic model of spinning triangle was developed by using finite-element method with consideration of the dynamic effects such as the inertia and damping forces of constituent fibers for a more complete and accurate description. With consideration of dynamic characteristics of fiber, earlier static model of spinning triangle can be considered as a special case of the proposed dynamic model. In order to verify the developed theoretical model, comparisons of the proposed model with earlier model and experimental measurements were carried out and good agreements were achieved. Furthermore, the dynamic behavior of fibers in spinning triangle, such as the natural frequency, mode shape, resonant response, harmonic analysis, and response under a time-varying tension, has been numerically studied. The results show that dynamic parameters have a great influence on the amplitude and attenuation of the response of constituent fibers in spinning triangle. [ABSTRACT FROM AUTHOR]

The present research investigates a modified coaxial process for smoothening electrospinning of cellulose acetate (CA) and obtaining its nanofibers with high quality. With 11 w/v% CA in mixed solvent of acetone and N,N-dimethylacetamide (DMAc) (v:v, 3:1) as electrospinnable core fluid, different solvents (acetone, DMAc and their mixture) are taken as sheath fluids to conduct the modified process. SEM observations demonstrate that the modified process is effective in retarding the clogging of spinneret for a smooth electrospinning and in obtaining high quality CA nanofibers in terms of structural uniformity, diameters, and their distributions. The mechanism about the influence of sheath fluid on the process and the formation of nanofibers is discussed. The key for the modified coaxial process is the reasonable selection of sheath fluids and a sheath-to-core flow rate ratio matching the drawing process of core CA fluid during the electrospinning. The present paper provides a simple method to implement the modified coaxial process for smoothening the electrospinning and obtaining polymer nanofibers with high quality. [ABSTRACT FROM AUTHOR]

The influence of the viscous and elastic components of a visco-elastic binder phase on the extrusion behaviour of a tungsten carbide-cobalt hardmetal paste was investigated by studying the paste over a range of temperatures, 30-42 °C, where the binder changed from a semi-solid gel to a viscous liquid. The extrusion behaviour of the paste was studied by ram extrusion and quantified by the Benbow-Bridgwater (BB) method. The elastic and viscous components of the binder were studied separately by means of oscillatory and steady shear techniques in a controlled stress rheometer using rough parallel plates. The paste behaviour fitted the four parameter BB model well: the initial bulk yield stress parameter, σ, increased linearly with the binder elastic modulus, G′, while both the velocity dependence parameters α and β were linearly related to the binder steady shear viscosity. Analysis of paste-wall slip parameters gave estimated slip layer thicknesses of 2-5 particle diameters. [ABSTRACT FROM AUTHOR]

The mechanical behaviour of poly(vinyl butyral) (PVB) at small (<0.1%) and large strains (>200%) is investigated experimentally over a range of strain rates to provide data for the development and validation of constitutive models. The small-strain response is investigated using dynamic mechanical analysis at frequencies from 1 to 100 Hz and temperatures from −80 to 70 °C. It is found that a generalised Maxwell model adequately describes the material behaviour in the small-strain regime. The large-strain response is investigated using a high-speed servo hydraulic test machine at strain rates from 0.2 to 400 s. It is found that the PVB response is characterised by a time-dependent steep initial rise in stress followed by a hyperelastic type response until failure. No current material model completely captures this time-dependent behaviour at large strains. [ABSTRACT FROM AUTHOR]

The focus of this study is to explore synergy between nanomaterials such as exfoliated graphite nanoplatelets (xGnP) and micro-size reinforcements such as kenaf natural fibers, in poly(lactic acid) based composites. The nano-biocomposites are made by melt-mixing followed by injection molding. Prior to melt-mixing the kenaf fibers were coated with the xGnP using sonication. The reinforcement content used in the study was up to 5 wt% and up to 40 wt% for xGnP and kenaf fibers, respectively. The flexural strength and modulus and the viscoelastic properties such as storage modulus were determined. It was found that addition of 5 wt% xGnP did not increase the viscosity of the polymer melt, enhanced the flexural modulus by 25-30% at any fiber loading used but did not increase the strength, indicating insufficient load transfer at the polymer-xGnP or xGnP-kenaf interface. Finally, addition of xGnP had a positive effect on the heat distortion temperature but only at higher fiber loadings. [ABSTRACT FROM AUTHOR]

The article describes the effect of hydrocarbon (HC) and coumarone-indene (CI) resin tackifiers on autohesion behavior of ethylene propylene diene polymethylene (EPDM) rubber. The viscoelastic behavior and nature of compatibility of EPDM/tackifier blends were studied by means of dynamic mechanical analysis. Furthermore, scanning electron microscopy and atomic force microscopy were used to understand the compatibility of the EPDM/tackifier blends. The HC tackifying resin modified the viscoelastic properties of the EPDM rubber in such a way that it behaved as a plasticizer at lower frequency by reducing the storage modulus and filler at higher frequency by increasing the storage modulus. On the contrary, the CI modified EPDM rubber did not show similar behavior; the modulus enhanced throughout the entire frequency range. The viscosity of the matrix was found to be highly governed by the compatibility as well as amount of tackifier present in the blend. In order to explain the tack behavior, several tack governing factors such as green strength, creep compliance, entanglement molecular weight, relaxation time, self-diffusion coefficient, and monomer friction coefficient (ζ) were investigated. The tack strength increased with HC tackifier loading up to 24 parts per hundred grams of rubber (phr), beyond which a plateau region was observed. A maximum of 196% improvement was observed at 24 phr HC loaded sample as compared to gum EPDM rubber devoid of tackifier. Conversely, there was a marginal improvement of tack strength (36%) up to 8 phr loading for the system containing CI, beyond which it dropped. [ABSTRACT FROM AUTHOR]

Polycaprolactone (PCL) and chitosan (Ch) are nontoxic, biocompatible, and biodegradable polymers of vast interest for wound repair. The aim of this work was to prepare Ch/PCL membranes in different proportions (90:10 and 80:20 w/w) in the presence and absence of the surfactant Pluronic F68 (PF68). The membranes were evaluated regarding morphology, thermal behavior, and viscoelastic properties. Sample swelling and degradation in phosphate-buffered saline (PBS), simulated body fluid (SBF), and fetal bovine serum (FBS) were determined by differential scanning calorimetry (DSC) and dynamical mechanical analysis (DMA), while cell toxicity to L929 and Vero fibroblasts was evaluated using the MTT reduction assay and cell proliferation, by DNA quantification and confocal laser microscopy. After 60 days in SBF, marked Ch matrix loss and advanced degradation of PCL particles were noticed by scanning electron microscopy (SEM). No significant differences in melting temperature ( T) and enthalpy (Δ H) were detected by DSC. However, the surfactant increased the Δ H. After 30 days, the membranes obtained in the presence of PF68 had absorbed more blood serum and were more degraded after exposure to simulated blood fluid for 30 days. All membranes had low cytotoxicity, and higher cell proliferation was noticed for samples obtained in the presence of the surfactant. In conclusion, the Ch/PCL membranes showed satisfactory degradability and biocompatibility, which enhances their potential for application in wound repair. [ABSTRACT FROM AUTHOR]

Short aramid fibre reinforced vinyl ester resin based isotropic composites are fabricated with varying fibre weight fractions (20-50 wt%). The composites were evaluated for their erosion performance under a dynamic set of variables such as impingement angle (30°-90°), impact velocity (43-76 m/s), erodent size (250-600 lm) and standoff distance (55-85 μm) following design of experiments (DOE) based on Taguchi analysis approach. The thermomechanical attributes such as storage modulus, loss modulus and damping properties as viscoelastic responses of the composites were investigated in the temperature range of 0-180 °C for their possible interpretations regarding reinforcement efficiency and energy dissipation aspects relevant to erosion process. An interrelation between the full-width half-maxima (FWHM) of loss modulus peak and erosion rate has emerged indicating the erosion to be mainly controlled by the fibre-matrix interfacial characteristics. The eroded surface morphology investigation by scanning electron microscopy (SEM) revealed the nature of wear-craters, material damage mode and other qualitative attributes responsible in facilitating erosion of the composites. [ABSTRACT FROM AUTHOR]

Poly(methyl methacrylate) films were prepared by dissolving the polymer in chloroform, toluene and tetrahydrofuran, with identical concentrations of 200 mg/mL, and drop casting the solutions on Teflon surface at room temperature. The thermal, thermomechanical and structural properties have been investigated by differential scanning calorimetry, dynamic mechanical analyzer, and Fourier transform infrared spectroscopy, respectively. The dynamic mechanical behavior of the films has been measured over a temperature range of 30-150 °C, using sinusoidal stress with a frequency of 2 Hz. The samples prepared from tetrahydrofuran showed the highest storage modulus, indicating a higher polymer chain entanglement in that solvent, whereas the samples prepared from chloroform showed the lowest storage modulus. The samples prepared from chloroform, which showed the weakest mechanical properties, also showed the lowest glass transition temperature, which is evidence of the plasticization and solvent retention mechanism of chloroform. The spectroscopic analysis confirmed the solvent-polymer interactions giving rise to the above mentioned effects. [ABSTRACT FROM AUTHOR]

The strain ( γ) dependences of viscoelasticity and electrical resistance ( R) of vapor grown carbon nanofiber (VGCF)/polystyrene (PS) composites have been studied using simultaneous measurement technique. The composites containing at least 4 vol.% VGCF present two regions of strain softening at γ < 10% and γ > 30%, respectively. Using strain amplification factor introduced by hydrodynamic effects as vertical and horizontal shifting factors, the curves of dynamic storage modulus ( G′) and loss modulus ( G″) as a function of γ for the composites can be superposed, respectively, on those for the pure matrix at γ ≥ 30%. Significant deflection from the master curves can be observed at γ < 30%. R tested as a function of γ provides direct evidences for breakdown of filler-filler interactions even by a small strain perturbation. It is suggested that breakdown of filler-filler interactions plays a vital role in strain softening at small strains, whereas the matrix provides the main contribution to strain softening at large strains. Dynamic moduli $$ G_{{{\text{f,}}(0.1\% ,\varphi )}}^{\prime } $$ and $$ G_{{{\text{f,}}(0.1\% ,\varphi )}}^{\prime \prime } $$ of the filler phase at volume fraction φ at 0.1% strain are used to account for the viscoelastic contribution of the initial filler structure. Ratios of dynamic moduli of the filler phase to the composite at 0.1% strain, $$ G_{{{\text{f,}}0.1\% }}^{\prime } /G_{{{\text{c,}}0.1\% }}^{\prime } $$ and $$ G_{{{\text{f,}}0.1\% }}^{\prime \prime } /G_{{{\text{c,}}0.1\% }}^{\prime \prime } $$, exhibit percolation-like transition as a function of φ, which is in consistence with the electric percolation transition. [ABSTRACT FROM AUTHOR]

The mechanical yielding behavior of the shape memory polyurethane (SMP) and its composite samples were investigated to verify the effect of addition of hindered phenol. The composite samples were prepared using SMP as matrix and hindered phenol (AO-80) as filler, and then tensile tests were carried out at a range of temperatures and with different fixed crosshead speeds. According to the tensile data at 25 °C, the yield stress was increased by adding AO-80. In terms of the Eyring theory, the activation volume of deformation units decreased with increase of the amount of AO-80 added. This is because the packing of the hard segments is enhanced by addition of AO-80. Consequently, it was suggested that the yield process is caused by the rotation of hard segment domains within the deformed plastic domains composed of glassy soft segments. [ABSTRACT FROM AUTHOR]

This article aims to present an improved technique for the mechanical characterization of single plant fibers. In particular, our efforts lie in resolving the difficulties involved in the testing of short plant fibers with only a few millimeters in length and tens of microns in width. Such fibers are potentially used as reinforcement phase in polymer composites. A special fiber gripping system, consisting of a pair of fiber clamps, a 3D positioning stage and two digital microscopes, was developed and combined to a small commercial mechanical tester. The resultant testing system possesses great convenience and reliability for the installation, orientation adjustment, and gripping of short plant fibers, as well as powerful control and data analysis functions. The equipment was then applied to the mechanical characterization of single bamboo fibers under air-dried state. Furthermore, the capability of this equipment for studying moisture content-related mechanical and viscoelastic properties of bamboo fibers was also successfully demonstrated. [ABSTRACT FROM AUTHOR]

Small amounts of organoclays in different sizes and concentrations were added into thermotropic liquid crystalline polymer (TLCP) by a combination method of ultrasonication, centrifugation, and solution casting (and shear-induced phase separation). Four kinds of organoclay-modified TLCP composites were obtained. TC3 UP was a kind of organoclay which displayed a marked shear-induced phase separation phenomenon at 190 °C and higher temperatures. TC3 UP could be separated into a TLCP-rich part, TC3 white, and an organoclay-rich part, TC3 dark. TC3 white was an organoclay-modified TLCP with fully exfoliated organoclay of a uniform size of 15–25 nm well dispersed in the TLCP. TC3 dark had a typical intercalated model structure with some TLCP molecules confined in organoclay galleries. TC3 FS was an organoclay-modified TLCP with organoclay of comparable size to that of the fully extended TLCP molecule, i.e., 85 nm. The organoclay layers dispersed into the polymeric matrix as a few randomly organized organoclay layers or stacks of layers in the quiescent condition in organoclay-modified TLCPs. The larger organoclay size or higher concentration of organoclay in the composites (such as, TC3 UP and TC3 dark) caused a greater proportion of the layers or stacks be hydro-dynamically impeded in the quiescent state, resulting in the filler–filler interaction becoming important and in fact dominating the long-term viscoelasticity of the composites. Additionally, the ease with which the organoclay structure could be altered by flow was considerably enhanced, primarily because of filler–filler interactions. Organoclay size has primary effect on the liquid crystallinity of TLCP. Small or comparable organoclay size with the fully extended TLCP molecule in TC3 white and TC3 FS has weak or negligible negative influence on the liquid crystallinity of TLCP, while larger organoclay size totally damages the liquid crystallinity of TLCP. The morphologies, liquid crystallinity as well as the linear and non-linear viscoelastic behaviors of organoclay-modified TLCPs have been characterized in detail. [ABSTRACT FROM AUTHOR]

Brittleness of materials—whether it occurs naturally or with aging—affects significantly performance and manifests itself in various properties. In the past, brittleness was defined qualitatively, but now a definition of brittleness for viscoelastic materials exists, enabling analysis of all types of polymer-based materials. The quantity brittleness, B, has been evaluated for neat thermoplastics, but here composites and metal alloys are also assessed. The physical significance of brittleness is connected to the dimensional stability of materials. The connections of brittleness to tensile elongation and to fatigue are explored while its relationship to surface properties—specifically wear by repetitive scratching—is examined more closely. The economic impact of wear results in monetary loss associated with failure and reduced service life of plastic parts—thus its connection to brittleness finds use across a broad spectrum of industrial applications which utilize plastics for manufacturing, processing, etc. We also demonstrate a correspondence between impact strength (Charpy or Izod) and brittleness of polymers. It is shown that the assumption hardness is equivalent to brittleness is inaccurate; this fact has important implications for interpreting the results of mechanical testing of viscoelastic materials. [ABSTRACT FROM AUTHOR]

The interpenetrating polymer networks (IPNs) were formed by unsaturated polyester resin (UPR) polymerized by free radical initiators: benzoyl peroxide (BPO) or cumene hydroperoxide (CHP) and epoxy polyester (EP), cured with acid anhydrides: tetrahydrophthalic anhydride (THPA) or maleic anhydride (MA). IPNs consisting 10, 30, 50, 70, 90 wt% of EP were prepared. The effect of the EP component in the IPNs and the type of curing agent on the cure behavior, thermal, and viscoelastic properties have been investigated. The results showed that both EP content and used curing system influenced on studied properties. As the EP content increased, the glass transition temperatures ( Tg) also increased. Moreover, higher values of tanδmax and lower values of cross-linking density in a rubbery state (νe) of IPNs containing higher EP content, probably due to plasticization effect of EP component were observed. Additionally, more heterogeneous network structure (higher values of the full-width at half-maximum (FWHM) as the EP content decreased was prepared. The thermal and viscoelastic properties of the blends cured with BPO/MA or CHP/MA system were considerably better than those cured with BPO/THPA or CHP/THPA. The higher stiffness, νe, Tg and lower tanδmax values were obtained. It was probably connected with the interactions of carbon–carbon double bonds of MA with vinyl monomer (styrene), UPR and radical initiators causing to obtain more cross-linked polymer network structure. This supposition was confirmed on basis of the cure reaction monitored by DSC. The chemical interactions between two components of the blends and epoxy hardener caused that the BPO/MA or CHP/MA cure systems influenced on the cure behavior of UPR and EP components in the IPNs. The exotherm peak temperature ( Tmax1) shifted to lower values compared to these in the neat UPR whilst Tmax2 shifted to higher values than in the neat EP. However, the cure behavior of the UPR was not greatly affected by the presence of EP component when BPO/THPA or CHP/THPA cure systems were used due to the lack of chemical interactions between the components and their curatives. [ABSTRACT FROM AUTHOR]

The mechanical and the dynamic rheological behavior of solution-polymerized styrene butadiene rubber (SSBR) filled with compounding fillers organic montmorillonite (OMMT)/silica (SiO2) were studied. The mechanical properties of the vulcanizates could be improved by incorporation of low OMMT contents, and the best reinforcement effect appeared when 7 phr (parts per hundred rubber) OMMT was added. Moreover, due to the formation of the exfoliated silicate platelet in the matrix, a strong cooperative reinforcement effect to the vulcanizate could be gained for the vulcanizate containing 7 phr OMMT and 20 phr SiO2. Incorporation of SiO2 up to 50 phr to the vulcanizates with or without OMMT resulted in marked reinforcement effect, and the best mechanical properties were obtained for 7 phr OMMT filled composites. On the other hand, compounding fillers OMMT/SiO2 significantly influences the dynamic mechanical behavior of the vulcanizates. It was found that the compounding filler with a certain composition facilitated preparing SSBR vulcanizates with optimized wet traction, rolling resistance, and strength properties. [ABSTRACT FROM AUTHOR]

Tests are described to measure the creep and relaxation response of aramid fibres with the specific aim of determining whether the visco-elastic response is linear or non-linear. Hitherto, creep and relaxation tests have been carried out in different circumstances and at different loads, which has led to disagreement about the type of response that aramid fibres exhibit. Tests are carried out at stresses between 10% and 80% of the short-term strength of the fibres under controlled temperature and humidity conditions, and it is shown that both creep and relaxation are non-linear at stresses below 40% of the breaking load, but both are linear at stresses above this level. This result explains the contradictions in earlier work and also indicates that there may be two different processes underway in the visco-elasticity of aramids. [ABSTRACT FROM AUTHOR]

Studies on the gelation behaviors of the reactive blends of nylon1212 and functional elastomer were carried out. The results show that the curves of the storage modulus( G′)–frequency (ω) exhibit a gel plateau in the low ω region, and the transition from liquid-like to solid-like viscoelastic behaviors emerges with the concentration of SEBS-g-MA increasing. There exist the gelation behaviors in the blending process similar to those of crosslinking polymer. Based on Winter’s method, the gel point of blends is determined to be, φg = 17.45 wt%, and the corresponding value of tanδ is 1.44. The gel index n calculated is 0.61 and the gel strength S g is 1.08 × 104 Pa s0.61. However, the non-reactive blends of nylon1212 and elastomer have no emergence of gelation behaviors. The morphology analysis shows that the gel point for the reactive blends is a threshold of cocontinuous morphology, and morphology analysis can also be a method to determine the gel point. [ABSTRACT FROM AUTHOR]

This paper gives details of a comprehensive dynamic mechanical analysis (DMA) material characterisation activity for all constituent layers of two modern-day thermoformed soccer balls. The resulting material data were used to define a series of viscoelastic finite element (FE) models of each ball design which incorporated the through-thickness composite material properties, including an internal latex bladder, woven fabric-based carcass and polymer based outer panels. The developed FE modelling methodology was found to accurately describe the viscoelastic kinetic energy loss characteristics apparent throughout a soccer ball impact at velocities which are typical of those experienced throughout play. The models have been validated by means of experimental impact testing under dynamic loading conditions. It was found that the viscoelastic material properties of the outer panels significantly affected ball impact characteristics, with outer panel materials exhibiting higher levels of viscous damping resulting in higher losses of kinetic energy. [ABSTRACT FROM AUTHOR]

A constitutive model of finite strain viscoelasticity, based on the multiplicative decomposition of the deformation gradient tensor into elastic and inelastic parts, is presented. The nonlinear response of rubbers, manifested by the rate effect, cycling loading and stress relaxation tests was captured through the introduction of two internal variables, namely the constitutive spin and the back stress tensor. These parameters, widely used in plasticity, are applied in this work to model the nonlinear viscoelastic behaviour of rubbers. The experimental results, obtained elsewhere, related with shear deformation in monotonic and cyclic loading, as well as stress-relaxation, were simulated with a good accuracy. [ABSTRACT FROM AUTHOR]

The non-linear viscoelastic behaviour of a polyurethane (PUR) network is determined by continuous uniaxial tension tests and stepwise relaxation tests. Following the concept of internal variables on the modelling side, the finite Neo-Hookean material model combined with linear evolution equations for the internal variables is applied to include the time-dependence caused by viscoelasticity. The parameters are identified by an evolution strategy combined with a non-linear finite element analysis which solves the boundary value problem given by the specimen geometry and testing conditions. In conclusion, the combination of the described experiments and modelling provides a full description of the mechanical behaviour of the given PUR. [ABSTRACT FROM AUTHOR]

Chrome-free leather such as glutaraldehyde-tanned leather behaves very differently from chrome-tanned leather. Information regarding its viscoelasticity has not been previously reported. Hysteresis and stress relaxation are two essential properties associated with viscoelasticity. We have designed a cyclic tensile test to measure these properties to gain insight into the structural difference between chrome-free and chrome-tanned leather. Observations revealed that chrome-free leather tanned with glutaraldehyde has a higher hysteresis than chrome-tanned leather. Stress relaxation experiments, on the other hand showed chrome-free leather has very similar relaxation curves as chrome-tanned leather. Both leathers demonstrate a rapid decrease in stress for the first few seconds followed by a much slower decay thereafter. The chrome-free leather, however, has a greater initial stress than chrome-tanned leather, indicating a higher stiffness than chrome-tanned leather. Moreover, observations showed the viscoelasticity of leather was affected significantly by its fatliquor content. A decrease of loading energy in a cyclic stress–strain experiment resulted from higher fatliquor content in leather. [ABSTRACT FROM AUTHOR]

Pumice particles-based electrorheological (ER) suspensions were prepared in silicone oil and its ER behavior was investigated as a function of shear rate, electric field strength, concentration and temperature. Sedimentation stabilities of suspensions were determined. ER activity of all the suspensions was observed to increase with increasing electric field strength, concentration and decreasing shear rate. Yield stress of pumice suspensions increased linearly with increasing applied electric field strength and with concentrations of the particles. The pumice suspensions show a typical shear thinning non-Newtonian viscoelastic behavior, in which viscosity of suspension decreased sharply with increasing shear rate. Effect of high temperature onto ER activity of pumice/silicone oil system was also investigated. [ABSTRACT FROM AUTHOR]

This study was designed to determine the linear viscoelastic and extrusion properties of a series of LDPE/EVAc blends. Newtonian viscosity was found to show strong positive deviation from the double reptation model, which assumes miscibility or, at least, cooperative relaxation between the mixed species. The blends also showed enhanced steady-state compliance and elastic indices compared to those of the pure components. These features, typical of emulsion-like polymer blends, seem to be attributable to additional relaxation processes at low frequencies associated with dispersed phase deformability. Good agreement with experimental dynamic results was shown by the Palierne model for emulsions of two viscoelastic liquids, for values of α/ R = 3.5 × 103 Pa across the whole frequency range examined. The double reptation model, usually applied to miscible blends, showed good agreement with experimental data for frequencies higher than 10−2 rad s−1. This could lead to erroneous conclusions on the miscibility of certain systems, for which a complete set of experimental data is lacking. [ABSTRACT FROM AUTHOR]

Package warpage during the cooling process just after the molding is one of the critical issues in the manufacture of plastic integrated circuit (IC) packaging. Such warpage depends on the epoxy molding compound characteristics and the dimensional details of the IC package design such as downset and the chip to die-pad area ratio. In this study, the analysis methodology using a viscoelastic based material model is adopted to account the time and temperature dependent behavior of epoxy molding compound. Using such model the effect of compound thickness ratio of the top to bottom side of the package is optimized to reduce the package warpage. Secondly, the effect of the cooling rate on the warpage is also examined in this study. [ABSTRACT FROM AUTHOR]

Morphology and viscoelastic behaviour during the initial stages of crystallisation of isotactic polypropylene were explored as a function of time and angular frequency by light microscopy and dynamic oscillatory rheology. Results were evaluated according to the Krieger-Dougherty and Palierne models for viscoelastic suspensions of spheres. The data obtained from light microscopy were introduced in the rheological models reproducing quite well the viscoelastic response during crystallisation. The Palierne model was able to describe the behaviour of the system, though it was not possible to observe all the model’s features due to a limited angular frequency range. Further, at high filler contents, an ‘equilibrium’ modulus needs to be introduced for the model to fit the experimental data. The exponent required to model the changes occurring in the ‘equilibrium’ modulus over time resembles that of chemical gelation more than physical gelation. [ABSTRACT FROM AUTHOR]

The article presents a study on the effect of interfacial interaction on the characteristic structures formed and on the viscoelastic properties of polyamide-6/organoclay nanocomposites. X-ray diffraction measurements were performed to confirm exfoliation and structure of the prepared nanocomposites. Exfoliation and structure of the nanocomposites were qualitatively verified through direct visualization using transmission electron microscopy.

A study of the tension behavior of polycarbonate (PC) under room temperature and various sustained loads is presented. Time-dependent axial elongations and transverse contractions of the specimen were simultaneously measured at nine different stress levels, from 15.89 to 59.4 MPa, and modeled according to a time-stress superposition principle. The test duration was only one hour. It was shown that creep compliance vs. log time curves at different stresses can be horizontally shifted to form a smooth master curve for one year at a reference stress of 30.97 MPa. Moreover, the stress shift factors for axial extension creep curves and transverse contraction creep curves are found to be identical for the stress levels considered, and this is verified using the Poisson's ratio measurements. [ABSTRACT FROM AUTHOR]

Melt blends of poly(butylene terephalate) (PBT) and poly(ethylene naphthalate) (PEN) with 30 and 60 wt% PEN were prepared using a single screw extruder and an injection moulding machine. Stress relaxation tests for the specimens of PBT/PEN blends and the homopolymers were carried out using an Instron testing machine in an Instron environmental chamber. The Taguchi method of experimental design analysed how different levels of temperature, PEN content and initial stress affected the relaxation behaviour of PBT/PEN blends and homopolymers. From the response tables and analyses of main and interaction effects, it was shown that the most significant factor was temperature, followed by PEN content and then the initial stress. Consequently, high temperature, low PEN content and high initial stress speeded up stress relaxation rate of specimens. Interaction effects between factors were insignificant. To fit the relaxation curves of the PBT/PEN blends and the homopolymers at different temperatures, PEN contents and the initial stresses, four different equations were attempted with Matlab™, which determined the coefficients of these functions using the experimental data of stress change with time. The simulated curves from the most suitable function among them were shown using the calculated coefficients to predict the relaxation behaviour of PBT/PEN blends (50% PEN) at temperatures of 30 and 60°C with an initial stress of 7 MPa. [ABSTRACT FROM AUTHOR]