Dynamic Viscoelastic Behavior Research Articles

Dynamic viscoelastic behavior of wood under drying conditions

In this study heartwood from a Chinese fir [Cunninghamia lanceolata (Lamb.) Hook] plantation was treated using a high-temperature drying (HTD) method at 115°C, a low-temperature drying (LTD) method at 65°C, and freeze vacuum drying (FVD), respectively. The dynamic viscoelastic properties of dried wood specimens were investigated. The measurements were carried out at a temperature range of −120 to 250°C at four different frequencies (1, 2, 5, and 10 Hz) using dynamic mechanical analysis (DMA). We have drawn the following conclusions: 1) the storage modulus E′ and loss modulus E″ are the highest for HTD wood and the lowest for FVD wood; 2) three relaxation processes were detected in HTD and LTD wood, attributed to the micro-Brownian motion of cell wall polymers in the non-crystalline region, the oscillations of the torso of cell wall polymers, and the motions of the methyl groups of cell wall polymers in the non-crystalline region in a decreasing order of temperatures at which they occurred; and 3) in FVD wood, four relaxation processes were observed. A newly added relaxation is attributed to the micro-Brownian motions of lignin molecules. This study suggests that both the HTD and the LTD methods restrict the micro-Brownian motion of lignin molecules somewhat by the cross-linking of chains due to their heating history.

Dynamic Viscoelastic Properties and Glass Transition Behavior of Corn Kernels

Information on viscoelastic properties is useful in industrial processing of foods as it provides insight into the structure of the material as a function of processing conditions. Corn kernels were hand sanded to obtain smooth and closely uniform samples with minimum damage to the kernel structure. The sample was tested using a dynamic mechanical analyzer. A miniature three point bending clamp was used to test the samples over a temperature range of 5–110°C at frequencies of 1 and 10 Hz in the moisture content ranging from 11.9–%24.7% (db). The average value of storage modulus (G′) showed a decreasing trend with increase in the moisture content. The corresponding tan δ curve showed a peak representing the glass transition temperature. The kernels exhibited a transition in the range of 20–60°C, which is expected to be the glass transition range. The transition range is consistent with the results of similar studies on cornstarch. It embodies an average of several transition temperatures that arise due to the compositional diversity in the kernel. The individual corn kernels exhibit a wide sample-to-sample variation and structural heterogeneity. A very large number of test replications are required for consistent results. The accuracy and applicability of the dynamic testing in corn is limited due to the rigid sample specifications and testing conditions. Sample preparation and selection of appropriate loading conditions are also addressed.

Electrorheological behavior of titanate nanotubes suspensions under oscillatory shear

In this study, titanate nanotubes (TNTs) were synthesized by hydrothermal treatment of TiO2 powder (P25) in a NaOH solution. The as-synthesized TNTs exhibit high surface area and large aspect ratio. Rheological properties of TNTs suspensions were then investigated under oscillatory shear. The TNTs fluid shows viscoelastic behavior and the dynamic moduli (G',G") increase significantly by about 4 orders of magnitude as the electric field strength is up to 2.0 kV/mm. The complex modulus of TNTs fluids is sensitive to temperature while that of P25 fluid become insensitive at higher temperature. Dynamic viscoelastic behavior suggests that structure of P25 to TNTs transition merits the enhancement of ER activity of TNTs fluid.

Dynamic Viscoelasticity Of Individual Bacterial Cells

Understanding how the shape of cells is regulated in time and space during cell growth and division is a fundamental problem of contemporary biology and requires quantitative estimates of cell mechanical properties. In the present paper, we have used an AFM-based approach to probe the mechanical properties of single bacterial cells (gram-negative Escherichia coli K12) by applying a constant compressive force to the cell under fluid conditions while measuring the time-dependent displacement (creep) of a colloidal AFM tip due to the viscoelastic properties of the cell. It was seen that the cells exhibited a viscoelastic solid-like behavior with retarded elasticity, i.e. both an instantaneous and a delayed elastic deformation. We found that this behavior is well described by a three-parameter mechanical model. Using the best fit parameter values, we have calculated the dynamic viscoelastic behavior of the cells over a wide range of frequencies based on a numerical time-frequency transform technique and we have compared the calculated behavior with that measured experimentally. Comparison of the results obtained for E. coli with previously reported data on the mechanical properties of others gram-negative bacterial cells and their isolated surface layers suggests that the elastic component of the cell viscoelastic response is dominated by the properties of the peptidoglycan layer, whereas the viscous component likely arises from the liquid-like character of the cell membranes. This work represents a new attempt to understand how molecular structure leads to the cell mechanical properties, which could be of practical value for elucidating the biomechanical effects of drugs on pathogenic bacteria.

Dynamic viscoelastic behavior of individual Gram-negative bacterial cells

We have used atomic force microscopy (AFM) to measure the viscoelastic properties of individual Escherichia coli K12 cells under fully hydrated conditions by collecting AFM force–indentation and force–time curves. Spherical colloidal tips were used to reduce the local strain ensuring that the measurements were performed in the linear viscoelastic regime. We find that the cells exhibit a time-dependent viscoelastic response to the constant compressive force applied by the AFM tip. The results are interpreted using the standard solid model which describes both an instantaneous and a delayed elastic deformation. The values obtained for the three viscoelastic parameters were used to generate a full description of the dynamic viscoelastic behavior of the cells over a wide range of frequencies derived from the differential equation governing the standard solid model. The results obtained using this methodology are compared to experimental data obtained for the E. coli K12 cells and these results are compared to previously reported viscoelastic data on Pseudomonas aeruginosa PAO1 cells. The differences between the viscoelastic behaviors of these two types of Gram-negative bacterial cells are discussed in terms of their structural differences.

Structure and properties of poly(ethylene oxide)‐organo clay nanocomposite prepared via melt mixing

AbstractPoly(ethylene oxide) (PEO)/organo clay nanocomposites were prepared by melt‐mixing using a laboratory kneader followed by compression molding. The nanocomposites were characterized by X‐ray diffraction, atomic force microscopy, and scanning electron microscopy. Their crystallization behavior in hot stage was investigated by polarized optical microscopy (POM). A decrease in size and regularity were observed as a result of incorporation of clay into the PEO matrix. The dynamic viscoelastic behavior of PEO/organo clay nanocomposites was assessed using a strain‐controlled parallel plate rheometer. The effects of clay concentration and the processing temperature on the rheological properties of the nanocomposites were extensively studied. A significant increase in the viscosity and storage modulus of the nanocomposites was found with the increasing clay content. The flow activation energy decreased with the incorporation of clay. The reinforcing effect of the organoclay was determined in dynamic mechanical analysis and tensile testing. POLYM. ENG. SCI., 2009. © 2008 Society of Plastics Engineers

Seismic isolation effect of lined circular tunnels with damping treatments

The Havriliak-Negami model for dynamic viscoelastic material behavior and Biot’s theory of poroelasticity are employed to develop an exact solution for three-dimensional scattering effect of harmonic plane P-SV waves from a circular cavity lined with a multilayered fluid-filled shell of infinite length containing viscoelastic damping materials and embedded within a fluid-saturated permeable surrounding soil medium. The analytical results are illustrated with numerical examples where the effects of liner/coating structural arrangement, viscoelastic material properties, liner-soil interface bonding condition, seismic excitation frequency, and angle of incidence on the induced dynamic stress concentrations are evaluated and discussed to obtain representative values of the parameters that characterize the system. It is demonstrated that incorporating viscoelastic damping materials with a low shear modulus in the constrained layer configuration is an efficient means of enhancing the overall seismic isolation performance, especially for near-normally incident seismic shear waves where the amplitudes of induced dynamic stresses may be reduced by up to one-third of those without isolation in a relatively wide frequency range. Some additional cases are considered and good agreements with solutions available in the literature are obtained.

Rheological and thermal properties of poly(ethylene oxide)/multiwall carbon nanotube composites

AbstractPoly(ethylene oxide) (PEO) based nanocomposites were prepared by the dispersion of multiwall carbon nanotubes (MWCNTs) in aqueous solution. MWCNTs were added up to 4 wt % of the PEO matrix. The dynamic viscoelastic behavior of the PEO/MWCNT nanocomposites was assessed with a strain‐controlled parallel‐plate rheometer. Prominent increases in the shear viscosity and storage modulus of the nanocomposites were found with increasing MWCNT content. Dynamic and isothermal differential scanning calorimetry studies indicated a significant decrease in the crystallization temperature as a result of the incorporation of MWCNTs; these composites can find applications as crystallizable switching components for shape‐memory polymer systems with adjustable switching temperatures. The solid‐state, direct‐current conductivity was also enhanced by the incorporation of MWCNTs. The dispersion level of the MWCNTs was investigated with scanning electron microscopy. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Modeling and Simulation of Viscoelastic Behavior of Three-Phase Polymer Blends with Multiple Droplet Morphology

Three-phase polymer blends with ``multiple droplet" morphology are important industrially. They are referred to as ``double emulsions" as the droplets of these blends themselves contain a number of smaller droplets inside. In this work, the dynamic viscoelastic behavior of double emulsions composed of three immiscible polymeric liquids is modeled. Simulations show that such double emulsions exhibit three relaxation domains due to relaxation of two different interfaces (internal and external) and the matrix polymer. The factors affecting the dynamic viscoelastic behavior of double emulsions composed of three immiscible polymeric liquids are discussed.

Dynamic viscoelastic effects on sound wave scattering by an eccentric compound circular cylinder

The classical method of separation of variables in conjunction with the translational addition theorem for cylindrical wave functions are employed to obtain an exact solution for two-dimensional interaction of a harmonic plane acoustic wave with an infinitely long (visco)elastic circular cylinder which is eccentrically coated by another (visco)elastic material and is submerged in an ideal unbounded acoustic medium. The novel features of Havriliak–Negami model for dynamic viscoelastic material behaviour are used to take the rheological properties of the coating (and/or core) material into consideration. The analytical results are illustrated with numerical examples in which a steel rod eccentrically coated with (an eccentric steel shell filled with) dissipative materials of distinct viscoelastic properties is insonified by plane sound waves at selected angles of incidence. The effects of incident wave frequency, angle of incidence, core eccentricity and dynamic viscoelastic material properties on the backscattered form function spectra are examined. Limiting cases are considered and fair agreements with available solutions are obtained.

Shear-thickening flow of nanoparticle suspensions flocculated by polymer bridging

The steady-shear viscosity, dynamic viscoelasticity, and stress relaxation behavior were measured for suspensions of silica nanoparticles dispersed in aqueous solutions of poly(ethylene oxide) (PEO). The suspensions of silica with diameters of 8–25 nm show striking shear-thickening profiles in steady shear and highly elastic responses under large strains in oscillatory shear. Since the silica particles are much smaller than the polymer coils, one molecule can extend through several particles by intrachain bridging. Each polymer coil may remain isolated as a floc unit and the silica particles hardly connect two flocs. Therefore, the flow of suspensions is Newtonian with low viscosity at low shear rates. When the polymer coils containing several nanoparticles are subjected to high shear fields, three-dimensional network is developed over the system. The shear-thickening flow may arise from the elastic forces of extended bridges. But, the polymer chain is easily detached from particle surface by thermal energy because of large curvature of particles. As a result, the network structures are reversibly broken down in a quiescent state and the suspensions behaves as viscoelastic fluids with the zero-shear viscosity.

Dynamic Viscoelasticity of Poly(butylene terephthalate) during Isothermal Crystallization

The dynamic viscoelastic behavior during isothermal crystallization of poly(butylene terephthalate) (PBT) has been investigated as a function of crystallization temperature (Tc). The enthalpy of crystal formation during the isothermal crystallization is also examined by differential scanning calorimetry (DSC). The equilibrium degree of crystallization in the long time limit is about 30%, almost independently of Tc. Correspondingly, the viscoelastic parameters at the equilibrium of crystallization are also independent of Tc. The crystallization rate decreases with increasing Tc, and the Tc dependence is well approximated by the Arrhenius type equation. The effect of crystallization on the viscoelasticity is considerably small in the initial stage of crystallization whereas it becomes pronounced in the later stage.

Viscoelastic properties of polyarylene ether nitriles/thermotropic liquid crystalline polymer blend

Polyarylene ether nitriles (PEN)/thermotropic liquid crystalline polymer (TLCP) blend was prepared via melt mixing. The immiscible phase morphologies, linear and nonlinear, as well as transient viscoelastic properties of the blend were studied using SEM, rheometer, and DMA. The linear dynamic viscoelastic behavior of the blend shows temperature dependence due to further evolution of the immiscible morphology and, as a result, the principle of time-temperature superposition (TTS) is invalid. In the steady shear flow, the discrete TLCP phase is difficult to be broken up because of the high viscosity ratio of the blend systems, while is easy to be coarsened and followed by elongation, and finally, to form fibrous morphology at high TLCP content and high shear level. During this morphological evolution process, the transient stress response presents step increase and nonzero residual relaxation behavior, leading to increase of the dynamic viscoelastic responses after steady preshear. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Dynamic viscoelastic behavior of triple helical Lentinan in water: Effect of temperature

This study deals with the dynamic viscoelastic property for Lentinan fractions with different molecular weight in water under different constant temperatures and polysaccharide concentrations. The results revealed that a transition exists between a sol-like and a weak gel behavior for the Lentinan/water system. The gel point, T gel, was determined from the point of intersection in tan δ vs. temperature ( T) for different constant frequencies, indicating the validity of Winter–Chambon criteria. The T gel decreased with decreasing polymer concentration and molecular weight. Moreover, the value of exponent n at the gel point decreased with increasing polymer concentration but exhibited an independence of molecular weight, indicating that the gels of Lentinan had similar fractal structure. The dynamic strain sweep measurements proved that the gelation of Lentinan in water is induced by the extremely entangled and stiff triple helices forming continuous network, and the Lentinan gel is structurally more like a solution that is unable to flow within a timescale of usual observation. The heating–cooling process proved that the sol–gel transition of Lentinan in water was thermally reversible.

Rheological Characterization of Jute/HDPE Composites

The present paper summarizes an experimental study on the molten viscoelastic behavior of HDPE / jute composites under steady and dynamic mode. Variations of the melt viscosity and die swell of the composites with an increase in shear rate, fibre loading and coupling agent concentration have been investigated using capillary rheometer. It was observed that the melt viscosity of the composites increased with addition of fibres and MAPE. Further, the dynamic viscoelastic behavior of the composites was measured employing parallel plate rheometer. Time temperature superposition was applied to generate various viscoelastic master curves. The fibrematrix morphology of the extrudates were also examined using scanning electron microscopy.

Dynamic viscoelastic behavior and molecular mobility of acrylonitrile–butadiene copolymer nanocomposites with various organoclay loadings

Layered silicate nanostructures of NBR nanocomposites prepared using the various organoclays (OCs) and silicate loadings were investigated by analyzing the viscoelastic behavior, activation energy ( E a), and average relaxation times ( τ HN) obtained from dynamic mechanical measurements. Viscoelastic behavior revealed that NBR nanocomposite with OC having polar organic modifier (NBROC30B) has the highest elasticity in NBR nanocomposites, indicating the formation of the largest gallery height. In addition, E a and τ HN showed that the mobility of NBROC30B was the closest to that of NBR, providing further evidence that NBROC30B has the largest gallery height. Interestingly, viscoelastic behavior, E a, and τ HN of the nanocomposites with silicate loadings in excess of 7 wt% were remarkably higher than those of the nanocomposites with silicate loadings below 7 wt%. These results imply the sudden change of silicate structure and suggest the formation of three-dimensional network-like percolated silicate structure at NBR nanocomposites with silicate loadings over 8 wt%, which was consistent with HR-TEM results.

Rheological characterization of HDPE/sisal fiber composites

AbstractThe present paper summarizes an experimental study on the molten viscoelastic behavior of HDPE/sisal composites under steady and dynamic mode. Variations of the melt viscosity and die swell of the composites with an increase in shear rate, fiber loading, and coupling agent concentration have been investigated using capillary rheometer. The shear rate γ at the wall was calculated using Rabinowitsch correction applied to the apparent shear rate values. It was observed that the melt viscosity of the composites increased with the addition of fibers and maleic anhydride‐grafted PE (MAPE). Die swell of HDPE also decreased with the addition of sisal fibers and MAPE. Further, the dynamic viscoelastic behavior of the composites was measured employing parallel plate rheometer. Time–temperature superposition was applied to generate various viscoelastic master curves. Temperature sweeps were also carried out to study the flow activation energy determined from Arrhenius equation. The fiber–matrix morphology of the extrudates was also examined using scanning electron microscopy. POLYM. ENG. SCI., 47:1634–1642, 2007. © 2007 Society of Plastics Engineers

Dynamic viscoelastic behavior of triple helical Lentinan in water: Effects of concentration and molecular weight

The dynamic viscoelasitc behavior of Lentinan, one triple helical β-(1→3)-d-glucan from the fruiting body of Lentinus edodes, in water was investigated as a function of concentration and molecular weight at 25°C by using dynamic rheology. It was revealed that the shear storage moduli (G′), viscous loss moduli (G″), and the dynamic complex viscosity (η∗) exhibited strong dependence on concentration and molecular weight. At low concentrations, the Lentinan/water systems displayed liquid-like behavior with G′ lower than G″ at low frequencies and crossing-over at high frequencies. With increasing concentration, the elastic response of the Lentinan/water system was stronger than the viscous response, leading to the conclusion that the Lentinan/water systems displayed a predominantly solid-like behavior. The gel point (cgel) was determined from Winter–Chambon method (frequency-independence of tanδ). The most important point is that the cgel was much lower than some synthesized polymers and other flexible polysaccharides, which may be attributable to the high stiffness of triple helical Lentinan and strong intra- and intermolecular interactions among polysaccharide chains. Furthermore, a decrease in molecular weight leads to a sharp increase of cgel. The dynamic strain sweep measurements proved that the gelation of Lentinan in water is induced by the extremely entangled and stiff triple helices forming continuous network, and the Lentinan gel is structurally more like a solution that is unable to flow within a timescale of usual observation.

On the viscoelastic behavior of multiple emulsions

The dynamic viscoelastic behavior of multiple emulsions is investigated. A modified Palierne model is used to predict the storage and loss moduli of multiple emulsions. The multiple emulsions exhibit two relaxation domains due to relaxation of two different interfaces—internal and external. The internal interface is the interface between internal droplets and their suspending medium (primary emulsion continuous-phase). The external interface is the interface between multiple-emulsion droplets and their suspending medium (external continuous-phase). The factors affecting the dynamic viscoelastic behavior of multiple emulsions are discussed.

Dynamic Viscoelastic Behavior of High Pressure Treated Soybean Protein Isolate Dispersions

Commercial soy protein isolate (SPI) dispersions (10, 15, and 20% concentrations) were subjected to high pressure treatment at selected pressure levels (350, 450, 550, and 650 MPa) for 15 minutes at 23 ± 1.5°C. Calorimetric studies confirmed denaturation of SPI dispersions at 350 MPa irrespective of concentration. Frequency sweep data (0.1 to 10 Hz) of SPI dispersions during oscillation rheological measurement demonstrated that elastic modulus (G′) predominate over viscous component (G″) for all concentrations. Gel rigidity of pressurized samples, estimated by mechanical spectra analysis, showed no systematic pattern with applied pressure however concentration significantly increased mechanical strength. Contrary to thermal effect, high pressure treated samples exhibited predominant viscous property and overall there was no significant change on viscoelastic properties of treated samples to control. Electrophoresis results (both Native and SDS) confirmed rheological data with insignificant conformational change in protein subunits of post-process samples. As expected, thermal induced gel was firmer than that of pressure treated samples at similar concentration.