Viscous Loss Modulus Research Articles

A Traumatic Impact Immediately Changes the Mechanical Properties of Articular Cartilage.

To investigate whether and how a single traumatic impact changes the mechanical properties of talar articular cartilage. A marble was placed on the joint surface and a weight was dropped on both medial and lateral caprine talus to create a well-defined single focal impact. The mechanical properties of intact and impacted talar cartilage were measured with a micro-indenter. Elastic (storage) and viscous (loss) moduli were determined by oscillatory ramp and dynamic mechanical analysis protocols. We found significant differences between ankles and within the same ankle joint, with the medial talus having significantly higher storage- and loss moduli than the lateral talus. The storage- and loss moduli of intact articular cartilage increased with greater indentation depths. However, postimpact the storage- and loss moduli were significantly and consistently lower in all specimens indicating immediate posttraumatic damage. The deeper regions of talar cartilage were less affected by the impact than the more superficial regions. A single traumatic impact results in an immediate and significant decrease of storage- and loss moduli. Further research must focus on the development of non- or minimally invasive diagnostic tools to address the exact microdamage caused by the impact. We speculate that the traumatic impact damaged the collagen fibers that confine the water-binding proteoglycans and thereby decreasing the hydrostatic pressure of cartilage. As part of the treatment directly after a trauma, one could imagine a reduction or restriction of peak loads to prevent the progression of the cascade towards PTOA of the ankle joint.

Dynamic Ocular Response to Mechanical Loading: The Role of Viscoelasticity in Energy Dissipation by the Cornea.

We have used vibrational optical coherence tomography (VOCT) to measure the resonant frequency, elastic modulus, and loss modulus of components of the anterior segment of pig eyes in vitro. Such basic biomechanical properties of the cornea have been shown to be abnormal not only in diseases of the anterior segment but also in posterior segment diseases as well. This information is needed to better understand corneal biomechanics in health and disease and to be able to diagnose the early stages of corneal pathologies. Results of dynamic viscoelastic studies on whole pig eyes and isolated corneas indicate that at low strain rates (30 Hz or less), the viscous loss modulus is as high as 0.6 times the elastic modulus for both whole eyes and corneas. This large viscous loss is similar to that of skin, which has been hypothesized to be dependent upon the physical association of proteoglycans with collagenous fibers. The energy dissipation properties of the cornea provide a mechanism to dissipate energy associated with blunt trauma, thereby preventing delamination and failure. The cornea possesses the ability to store impact energy and transmit excess energy to the posterior segment of the eye through its serial connection to the limbus and sclera. In this manner, the viscoelastic properties of the cornea, in concert with that of the posterior segment of the pig eye, function to prevent mechanical failure of the primary focusing element of the eye. Results of resonant frequency studies suggest that the 100-120 Hz and 150-160 Hz resonant frequency peaks reside in the anterior segment of the cornea since the removal of the anterior segment of the cornea decreases the peak heights at these resonant frequencies. These results suggest that there is more than one collagen fibril network found in the anterior portion of the cornea that provides structural integrity to prevent corneal delamination and that VOCT may be useful clinically to diagnose corneal diseases.

Viscoelastic properties of suspended cells measured with shear flow deformation cytometry.

Numerous cell functions are accompanied by phenotypic changes in viscoelastic properties, and measuring them can help elucidate higher level cellular functions in health and disease. We present a high-throughput, simple and low-cost microfluidic method for quantitatively measuring the elastic (storage) and viscous (loss) modulus of individual cells. Cells are suspended in a high-viscosity fluid and are pumped with high pressure through a 5.8 cm long and 200 µm wide microfluidic channel. The fluid shear stress induces large, ear ellipsoidal cell deformations. In addition, the flow profile in the channel causes the cells to rotate in a tank-treading manner. From the cell deformation and tank treading frequency, we extract the frequency-dependent viscoelastic cell properties based on a theoretical framework developed by R. Roscoe [1] that describes the deformation of a viscoelastic sphere in a viscous fluid under steady laminar flow. We confirm the accuracy of the method using atomic force microscopy-calibrated polyacrylamide beads and cells. Our measurements demonstrate that suspended cells exhibit power-law, soft glassy rheological behavior that is cell-cycle-dependent and mediated by the physical interplay between the actin filament and intermediate filament networks.

Nixtamalization of durum wheat and the effect on protein secondary structure, gliadins, dough, and breadmaking quality

The objective of this study was to evaluate new nixtamalization approaches in durum wheat to overcome its rheological characteristics and poor breadmaking quality. To test this possibility, a durum-wheat variety was nixtamalized using either Calcium hydroxide [Ca(OH)2], calcium chloride (CaCl2), and calcium carbonate (CaCO3) at 0.3% and 0.7%. Nixtamalized durum wheat (NDW) dough using either CaCl2 or CaCO3, showed high elastic (storage) modulus (G′) and viscous (loss) modulus (G″), with dough elasticity values ranging from 10,000 to 35,000 Pa, which are appropriate for breadmaking. The FT-IR spectra showed that nixtamalized samples had in general higher antiparallel β-sheet values with samples treated with CaCl2 at 0.3% (37.29%), Ca(OH)2 at 0.3% (40.02%), and CaCO3 at 0.7% (39.55%) exhibiting the highest values. Bread from NDW exhibited significantly improved bread-loaf volume compared with non-nixtamalized the durum control, but not better crumb quality than the bread wheat control. Protein electrophoretic analysis demonstrated that after nixtamalization, new gliadin bands appeared in the region of the α-gliadins and ω-gliadins, which cannot be observed in the durum wheat without nixtamalization. To the best of our knowledge, this is the first study to investigate the effect of nixtamalization in durum wheat-dough composition, rheological quality, breadmaking potential, and protein structure.

A DMA-Based Approach to Quality Evaluation of Digitally Manufactured Continuous Fiber-Reinforced Composites from Thermoplastic Commingled Tow

Direct digital manufacturing of continuous fiber-reinforced thermoplastics exhibits the potential to relieve many of the constraints placed on the current design and manufacture of composite structures. At present, the additive manufacturing of continuous fiber-reinforced thermoplastics is demonstrated to varying extents; however, a comprehensive investigation of manufacturing defects and the quality of additively manufactured high fiber volume fraction continuous fiber-reinforced thermoplastic composites is limited. Considering the preliminary nature of the additive manufacturing of continuous fiber-reinforced thermoplastics, composites processed in this manner are typically subject to various manufacturing defects, including excessive void content in the thermoplastic matrix. Generally, quality evaluation of processed composites in the literature is limited to test methods that are largely influenced by the properties of the continuous fiber reinforcement, and as such, defects in the thermoplastic matrix are usually less impactful on the results and are often overlooked. Hardware to facilitate the direct digital manufacturing of continuous fiber-reinforced thermoplastic matrix composites was developed, and specimens were successfully processed with intentionally varied void content. The quality of the additively manufactured specimens was then evaluated in terms of the measured maximum storage modulus, maximum loss modulus, damping factor and the glass transition temperature by means of dynamic mechanical analysis (DMA). DMA allows for thermomechanical (i.e., highly matrix sensitive) evaluation of the composite specimens, specifically in terms of the measured elastic storage modulus, viscous loss modulus, damping factor and the glass transition temperature. Within the tested range of void contents from roughly 4–10%, evaluation by DMA resulted in a distinct reduction in the maximum measured storage modulus, maximum loss modulus and glass transition temperature with increasing void content, while the damping factor increased. Thus, the results of this work, which focused on the effect of void content on DMA measured properties, have demonstrated that DMA exhibits multi-faceted sensitivity to the presence of voids in the additively manufactured continuous fiber-reinforced thermoplastic specimens.

Dynamic, 3D Schiff base networks for medical applications

AbstractIn this study, we utilized the reversible property of aqueous Schiff bases to design a dynamic three‐dimensional (3D) network. Two derivatized polysaccharides—acrylated chitosan (aCHN) and oxidized dextran (oDEX)—form a 3D network rapidly (<10 s) in situ without the use of external crosslinking agents. Reversible Schiff base (NC) pairs are formed between the amines of aCHN and aldehydes of oDEX. These bonds are mobile and reversible, allowing the network to form additional bonds, leading to lower energy state and densification. In the presence of a primary organic amine, the network dissolves completely, demonstrating network reversibility. The network has strong cohesion (85 ± 7 mmHg) and adhesion (0.017 ± 0.003 MPa) strengths. Rheological examination of the network demonstrates that both elastic storage (G') and viscous loss (G") moduli do not plateau but keep increasing over time. The network shrinks continuously over time, expelling fluid even when completely immersed in physiological solution, in contrast with conventional crosslinked networks that imbibe water and swell. Both rheology and shrinkage demonstrate the dynamic nature of the aCHN–oDEX networks. The dynamic nature of these networks is tunable and the properties of gelation, rheology, cohesion, adhesion, and densification can be modulated depending on the application of interest.

Viscoelastic properties of a novel hydrogel/foam composites for nucleus pulposus replacement

Neck and back pathologies cause considerable pain and suffering, and treatment costs tens of billions per year. A common source of problems is degeneration and herniation of the gel-like nucleus pulposus (NP) component of the intervertebral disc, and NP repair/replacement has been a long-term health care goal. We have investigated a novel class of hydrogel/foam composites to identify biocompatible materials with similar viscoelastic characteristics to the native tissue. Low acyl gellan gum (G) and agarose (A) in varying weight percentages were infused into Sugi® cellulose sponge material (F) to produce three candidate materials in addition to those samples without foam. Dynamic oscillatory shear tests and dynamic oscillatory axial compression test tests were conducted on the materials at frequencies between (f = 0.1–10 Hz) to measure elastic (storage) and viscous (loss) moduli in shear and compression conditions. The results show that hydrogel/foam composites show greater viscoelastic properties than hydrogel alone. Various materials of hydrogel and hydrogel/foam composite closely match the viscoelastic properties of native NP tissue.

Microstructural Origins of Nonlinear Response in Associating Polymers under Oscillatory Shear.

The response of associating polymers with oscillatory shear is studied through large-scale simulations. A hybrid molecular dynamics (MD), Monte Carlo (MC) algorithm is employed. Polymer chains are modeled as a coarse-grained bead-spring system. Functionalized end groups, at both ends of the polymer chains, can form reversible bonds according to MC rules. Stress-strain curves show nonlinearities indicated by a non-ellipsoidal shape. We consider two types of nonlinearities. Type I occurs at a strain amplitude much larger than one, type II at a frequency at which the elastic storage modulus dominates the viscous loss modulus. In this last case, the network topology resembles that of the system at rest. The reversible bonds are broken and chains stretch when the system moves away from the zero-strain position. For type I, the chains relax and the number of reversible bonds peaks when the system is near an extreme of the motion. During the movement to the other extreme of the cycle, first a stress overshoot occurs, then a yield accompanied by shear-banding. Finally, the network restructures. Interestingly, the system periodically restores bonds between the same associating groups. Even though major restructuring occurs, the system remembers previous network topologies.

Impact of Water on Rheological Behavior of Polyurethane Glues

Abstract Gluing of wood with great moisture content has challenge of water interaction with glue to give unwanted changes in glue properties and formed bond-line. Rheological measurements of polyurethane glues are sufficiently effective method to understand the polymer curing process and viscoelastic properties. Gel formation point is important parameter for a glue from the point of its application. When glue reaches the gel formation point, its viscosity drastically increases so that it could not properly wet the wood and it can result in poor bonds. One-component moisture curing polyurethane glues require water molecules for the polyadditional reaction. When water is involved in the curing reaction, it reacts with the isocyanate groups of glue and can lead to the formation of excessive carbon dioxide and thus can lead to glue penetration out of bond-line, additional backpressure. The dynamic viscoelastic parameters such as complex dynamic viscosity (ή), phase angle (δ), elastic storage modulus (G”), the viscous loss modulus (G”) of two kinds of polyurethane glues depending on added water amount (from 0.9% to 10.0%) at a constant frequency and deformation (oscillation mode) were determined using rheometer. The aim of the present study was to investigate impact of added water on rheological behavior of polyurethane glues. At the end of this study it was concluded that tested glues indicated various behavior depending on chemical composition and added water content. Gel formation point is higher in case of GlueI. However, gel formation time was higher in case of GlueII and was not dependent on added water content. Gel formation time was faster for GlueII. Nevertheless, faster formed gel is stronger than longer formed gel of GlueI.

Functionalized Nanolipobubbles Embedded Within a Nanocomposite Hydrogel: a Molecular Bio-imaging and Biomechanical Analysis of the System

The purpose of this study was to explore the use of molecular bio-imaging systems and biomechanical dynamics to elucidate the fate of a nanocomposite hydrogel system prepared by merging FITC-labeled nanolipobubbles within a cross-linked hydrogel network. The nanocomposite hydrogel system was characterized by size distribution analysis and zeta potential as well as shears thinning behavior, elastic modulus (G'), viscous loss moduli (G"), TEM, and FTIR. In addition, molecular bio-imaging via Vevo ultrasound and Cell-viZio techniques evaluated the stability and distribution of the nanolipobubbles within the cross-linked hydrogel. FITC-labeled and functionalized nanolipobubbles had particle sizes between 135 and 158nm (PdI = 0.129 and 0.190) and a zeta potential of -34mV. TEM and ultrasound imaging revealed the uniformity and dimensional stability of the functionalized nanolipobubbles pre- and post-embedment into the cross-linked hydrogel. Biomechanical characterization of the hydrogel by shear thinning behavior was governed by the polymer concentration and the cross-linker, glutaraldehyde. Ultrasound analysis and Cell-viZio bio-imaging were highly suitable to visualize the fluorescent image-guided nanolipobubbles and their morphology post-embedment into the hydrogel to form the NanoComposite system. Since the nanocomposite is intended for targeted treatment of neurodegenerative disorders, the distribution of the functionalized nanolipobubbles into PC12 neuronal cells was also ascertained via confocal microscopy. Results demonstrated effective release and localization of the nanolipobubbles within PC12 neuronal cells. The molecular structure of the synthetic surface peptide remained intact for an extended period to ensure potency for targeted delivery from the hydrogel ex vivo. These findings provide further insight into the properties of nanocomposite hydrogels for specialized drug delivery.

Molecular characterization of acid‐thinned wheat, potato and pea starches and correlation to gel properties

Molecular parameters and gel properties of systematically prepared acid‐thinned starch products (wheat, potato, and pea) were investigated. The impacts of acid type (HCl, H2SO4), acid concentration (0.36 and 0.72 N), and hydrolysis time (4 and 24 h) were found to be statistically significant on weight‐average molar mass (Mw) of the starch as revealed by analysis of variance (ANOVA). Especially for potato starch samples, the blue value (BV) was found to increase after short‐time hydrolysis and decrease after longer modification, indicating a reduced amount of polysaccharide chains able to complex with iodine, which was found for all products. Simultaneously, the wavelength of the absorption maximum (λmax) decreased systematically with an increasing degree of modification, giving evidence for decreasing length of linear polymer chains. Enzymatic debranching of amylopectin and dextrins and molecular characterization by means of size exclusion chromatography‐multi angle laser light scattering (SEC‐MALS) enabled the detection of Mw and the amount of amylose fraction. Generally, declining Mw of amylose was accompanied by a mass‐specific loss. Starch pastes with very good solution states were prepared in general. Pastes of the acid‐thinned starches showed dominant viscous behavior, whereat storage and loss modulus decreased with increasing degradation. Additionally, a significant reduction of the sol‐to‐gel‐transition temperature was found applying oscillation measurements. Increasing gel strength of the starch samples was detected due to acid‐thinning up to a certain degree of modification and optimal molecular parameters of the starch as well as the amylose fraction were determined resulting best strength properties. The gel clarity was reduced due to acid modification in general.

Dyskretni vlastyvosti kvaziridkykh plivok vody v oblasti peredplavlennya l'odu. 1. Temperaturni zalezhnosti tovshchyny nanoplivok vody ta v"yazkopruzhnykh vlastyvostey polikrystalichnoho l'odu

Peculiarities in the temperature dependences of the properties of quasiliquid water films on the surface of ice crystallites have been studied experimentally under ice premelting conditions. Viscoelastic properties of polycrystalline ice in the temperature interval from 60 to 20 oC have been analyzed. Peculiarities in the temperature dependences of the water nanolayer thickness, L(T), and the imaginary part of the shear modulus (modulus of viscous losses), G2(T), are found. Quasiequidistant temperature variations of the viscous loss modulus are revealed for the first time. A comparison of the results obtained with literature data on the temperature dependences L(T), the density of water in nanolayers, and the ice surface roughness allowed us to associate the observed features with a discrete cluster structure of quasiliquid water nanofilms. Temperature intervals of the enhanced stability for a cluster structure of water nanofilms are revealed, which manifest themselves in the form of extrema in viscoelastic ice parameters in the premelting interval. The interrelation between the phenomena of ice premelting and temperature discretization at the melting in ice nanolayers is considered for the first time.

Changes in the rheological behavior of the vagina in women with pelvic organ prolapse.

Pelvic organ prolapse is a common condition impacting the quality of life of millions of women worldwide. Although vaginal estrogen is widely used in women with prolapse, little is known regarding estrogen's benefits on the biomechanical properties of vaginal tissue. Here, we aimed to examine the effect of prolapse on the biomechanical properties of the vagina and determine alterations in vaginal mechanics in the presence and absence of hormone therapy (HT). We characterized the viscoelastic properties of vaginal biopsies from age-matched premenopausal women without (n = 12) and with prolapse (n = 8) and postmenopausal women with prolapse on (n = 18) and off HT (n = 9). Utilizing a single-lap shear testing protocol, full-thickness anterior vaginal biopsies were subjected to ±10% shear strain over a range of frequencies (1-90 Hz). This applied energy is either dissipated (viscous) or stored (elastic) as a function of frequency due to compositional or structural differences in the tissue. Prolapsed tissue was more stiff (higher complex modulus) under shear deformation resulting from increases in both elastic (elastic modulus) and viscous (loss modulus) contributions, with non-prolapsed premenopausal women being the least stiff. Postmenopausal women with prolapse currently on HT were the most stiff of all the groups. These data suggest that prolapsed tissue has an increased elastic contribution likely resulting from changes in biochemical constituents, and hormones increase the viscous contribution of prolapsed tissue. Overall, this study design characterized the viscoelastic properties of vaginal biopsies and may be utilized to conduct longitudinal studies to better understand the mechanisms of prolapse development and progression.

Rheological Behaviour of Core-Shell Emulsion/Amine Crosslinking Process

Isothermal kinetics studies of thermal-induced gelation for epoxy functionalized poly(vinyl acetate) emulsion and diamine were investigated rheologically. The change in the viscoelastic material functions such as elastic storage modulus, G’, viscous loss modulus, G” and complex dynamic viscosity, η* during the gelation process was evaluated. The isothermal gelation kinetics was also analyzed using an isoconversional method that was based on replicated experimental data and model-free kinetics calculations. This isoconversional method evaluated an effective activation energy.

Microrheology and Characteristic Lengths in Wormlike Micelles made of a Zwitterionic Surfactant and SDS in Brine

We study the Brownian motion of probe particles embedded in a wormlike micellar fluid made of a zwitterionic surfactant N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (TDPS), sodium dodecyl sulfate (SDS), and salty water to get structural and dynamical information of the micellar network. The motion of the probe particles was tracked with diffusing wave spectroscopy, and the mean square displacement as a function of time for the particles was obtained. This allowed us to obtain the long-time diffusion coefficient for microspheres moving in the micellar network and the cage size where each particle is harmonically bound at short times in that network. The bulk mechanical susceptibility of the fluid determines the response of the probe particles excited by the thermal stochastic forces. As a consequence, the mean square displacement curves allowed us to calculate the elastic (storage) and the viscous (loss) moduli as a function of the frequency. From these curves, spanning a wide frequency range, we estimated the characteristic lengths as the mesh size, the entanglement length, the persistence length, and the contour length for micellar solutions of different zwitterionic surfactant concentration, surfactant ratio ([SDS]/[TDPS]), salt concentration, and temperature. Mesh size, entanglement length, and persistence length are almost insensitive to the change of these variables. In contrast, the contour length changes in an important way. The contour length becomes shorter as the temperature increases, and it presents a peak at a surfactant ratio of ∼0.50-0.55. When salt is added to the solution, the contour length presents a peak at a salt concentration of ∼0.225 M, and in some solutions, this length can reach values of ∼12 μm. Scission energies help us to understand why the contour length first increases and then decreases when salt is added.

Control of the Flow Properties of DNA by Topoisomerase II and Its Targeting Inhibitor

The flow properties of DNA are important for understanding cell division and, indirectly, cancer therapy. DNA topology controlling enzymes such as topoisomerase II are thought to play an essential role. We report experiments showing how double-strand passage facilitated by topoisomerase II controls DNA rheology. For this purpose, we have measured the elastic storage and viscous loss moduli of a model system comprising bacteriophage λ-DNA and human topoisomerase IIα using video tracking of the Brownian motion of colloidal probe particles. We found that the rheology is critically dependent on the formation of temporal entanglements among the DNA molecules with a relaxation time of ∼1 s. We observed that topoisomerase II effectively removes these entanglements and transforms the solution from an elastic physical gel to a viscous fluid depending on the consumption of ATP. A second aspect of this study is the effect of the generic topoisomerase II inhibitor adenylyl-imidodiphosphate (AMP-PNP). In mixtures of AMP-PNP and ATP, the double-strand passage reaction gets blocked and progressively fewer entanglements are relaxed. A total replacement of ATP by AMP-PNP results in a temporal increase in elasticity at higher frequencies, but no transition to an elastic gel with fixed cross-links.

The Wormlike Micellar Solution made of a Zwitterionic Surfactant (TDPS), an Anionic Surfactant (SDS), and Brine in the Semidilute Regime

Structural and dynamical properties of a micellar solution are studied mainly through examining its rheological behavior in the semidilute regime. The micellar solution is made of a zwitterionic surfactant N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, sodium dodecyl sulfate, and salty water. In particular, we are interested in how the system is affected when the ionic strength of the media is modified by adding salt. Until recently, it was known that this solution forms wormlike micelles. In a range of chemical composition, the solution behaves as a viscoelastic Maxwellian fluid at low frequencies. We present measurements of the elastic (storage) modulus and the viscous (loss) modulus varying the surfactant ratio (R = [SDS]/[TDPS]), and how the Maxwellian relaxation time abruptly increases when the NaCl concentration is also varied. Reptation and breaking/recombination times were estimated. The effect of temperature in the viscoelastic solution is also studied. Shear stress versus shear rate flow curves were measured under shear and stress control, for different micellar solutions with different composition, brine concentration, and temperature, showing a nonlinear behavior. Flow curves present two branches, one corresponding to high viscous fluid and another to low viscous fluid, separated by a stress plateau. We were able to develop a master dynamic phase diagram, which summarizes the nonlinear behavior by appropriately reducing the rheological variables. In the stress plateau, the micellar solution presents gradient shear banding, which was observed with the scattered light of a sheet of light perpendicular to the fluid flow velocity in the gap of a transparent Couette rheometer.

Control of the Viscoelasticity of the Genome By Topoisomerase Type II and Anti-Cancer Drugs

The mechanical, viscoelastic properties of the genome are important for our understanding of cell division and, indirectly cancer therapy. Topology controlling enzymes (topoisomerase type II) are thought to play an essential role, but so far quantitative measurements of the effect on the viscoelasticity of DNA are lacking. We report experiments showing how double strand passage facilitated by human topo II controls the disentanglement of DNA. For this purpose, we have measured the elastic storage and viscous loss modulus of a model system consisting of bacteriophage DNA in buffer solution using video tracking of the Brownian motion of colloidal probe particles. We found that the viscoelastic response is critically dependent on the formation of entanglements among the DNA molecules with relaxation times on the order of seconds. For the first time we observed that topo II effectively removes these entanglements and converts the system from an elastic gel to a viscous fluid depending on the dissipation of energy through the hydrolysis of ATP. A second aspect of this study is the effect of a generic topo II inhibitor on the viscoelasticity. Topo II inhibitors constitute an important class of anti-cancer drugs, because they impede the division of cancerous cells by forming clamps between DNA segments. We continue interpreting our results in terms of a molecular model, which includes the reaction kinetics and functional mechanism. Our work contributes to the understanding of energy dependent, non-equilibrium dynamics of biomolecules, which is a key feature of life.

Electric-Field-Induced Patterns in Soft Viscoelastic Films: From Long Waves of Viscous Liquids to Short Waves of Elastic Solids

We show that the electric field driven surface instability of viscoelastic films has two distinct regimes: (1) The viscoelastic films behaving like a liquid display long wavelengths governed by applied voltage and surface tension, independent of its elastic storage and viscous loss moduli, and (2) the films behaving like a solid require a threshold voltage for the instability whose wavelength always scales as approximately 4xfilm thickness, independent of its surface tension, applied voltage, loss and storage moduli. Wavelength in a narrow transition zone between these regimes depends on the storage modulus.

Rheological behavior of waterborne polyurethane/starch aqueous dispersions during cure

Isothermal rheological behaviors of waterborne polyurethane (WPU)/starch aqueous dispersions during cure were investigated with a small-amplitude oscillatory shear flow experiment to evaluate their crosslinked structure and to predict their mechanical properties, for the first time. An abrupt increase in the elastic storage modulus (G′), the viscous loss modulus (G″), the complex dynamic viscosity (η∗) and the loss tangent (tanδ) was observed during the curing process of the dispersions, as a result of the formation of a fractal polymer gel. The gel point (tgel) was determined from the intersection in tanδ vs curing time for different constant shear frequencies, where tanδ was frequency independent and all curves crossed over, indicating the validity of the Winter–Chambon criterion for the complex system. The values of the power law exponent (n) and the gel strength (Sg) at the gel point indicated that with an increase of starch content the crosslinked WPU/starch gels underwent a transition from weak fractal to strong elastic ones. Moreover, the WPU/starch composite sheets, obtained from the aqueous dispersions with relatively high Sg values, also exhibited the increased tensile strength (σb) and Young's modulus (E). Their structure–mechanical properties relationship and the phase transitions of dispersed starch–dual-phase continuity–starch matrix were revealed. This work confirmed that the rheological characters could be used to predict the mechanical properties of the WPU materials blended with natural polymer.