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The significant drop of the storage modulus under uniaxial deformation (Payne effect) restrains the performance of the elastomer-based composites and the development of possible new applications. In this paper molecular-dynamics (MD) computer simulations using LAMMPS MD package have been performed to study the mechanical properties of a coarse-grained model of this family of nanocomposite materials. Our goal is to provide simulational insights into the viscoelastic properties of filled elastomers, and try to connect the macroscopic mechanics with composite microstructure, the strength of the polymer-filler interactions and the polymer mobility at different scales. To this end we simulate random copolymer films capped between two infinite solid (filler aggregate) walls. We systematically vary the strength of the polymer-substrate adhesion interactions, degree of polymer confinement (film thickness), polymer crosslinking density, and study their influence on the equilibrium and non-equilibrium structure, segmental dynamics, and the mechanical properties of the simulated systems. The glass-transition temperature increases once the mesh size became smaller than the chain radius of gyration; otherwise it remained invariant to mesh-size variations. This increase in the glass-transition temperature was accompanied by a monotonic slowing-down of segmental dynamics on all studied length scales. This observation is attributed to the correspondingly decreased width of the bulk density layer that was obtained in films whose thickness was larger than the end-to-end distance of the bulk polymer chains. To test this hypothesis additional simulations were performed in which the crystalline walls were replaced with amorphous or rough walls.

The Journal of Chemical Physics, 148 (18), ISSN:0021-9606, ISSN:1089-7690

There are two contributions to the friction force when a rubber block is sliding on a hard and rough substrate surface, namely, a contribution Fad = τf A from the area of real contact A and a viscoelastic contribution Fvisc from the pulsating forces exerted by the substrate asperities on the rubber block. Here we present experimental results obtained at different sliding speeds and temperatures, and we show that the temperature dependency of the shear stress τf, for temperatures above the rubber glass transition temperature Tg, is weaker than that of the bulk viscoelastic modulus. The physical origin of τf for T > Tg is discussed, and we propose that its temperature dependency is determined by the rubber molecule segment mobility at the sliding interface, which is higher than in the bulk because of increased free-volume effect due to the short-wavelength surface roughness. This is consistent with the often observed reduction in the glass transition temperature in nanometer-thick surface layers of glassy polymers. For temperatures T < Tg, the shear stress τf is nearly velocity independent and of similar magnitude as observed for glassy polymers such as PMMA or polyethylene. In this case, the rubber undergoes plastic deformations in the asperity contact regions and the contact area is determined by the rubber penetration hardness. For this case, we propose that the frictional shear stress is due to slip at the interface between the rubber and a transfer film adsorbed on the concrete surface. Locked until 12.6.2019 due to copyright restrictions.Published by AIP Publishing. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Journal of Chemical Physics and may be found at https://doi.org/10.1063/1.5037136

This paper describes the theoretical prediction, finite element simulation, and experimental studies of extrudate swell in monodisperse and bidisperse polystyrenes. We present a molecular approach to understanding extrudate swell using the tube-model-based Rolie-Poly constitutive equation within a Lagrangian finite element solver. This yields theoretical predictions of swelling which show a close universality: The molecular weight dependence of the swelling can be removed when the flow speed is scaled by the Rouse Weissenberg number. The roles that both chain orientation and stretch play in determining extrudate swell are clearly identifiable from plots of swelling ratio against each Weissenberg number. We also present isothermal extrusion experiments on the same polymers and can obtain good predictions well into the strong chain stretching regime. The predictions for swelling ratios match those from experiments up to Rouse Weissenberg numbers of ∼7, above which swelling is overpredicted by the Rolie-Poly equation.

Gelling colloidal suspensions represent an important class of soft materials. Their mechanical response is characterized by a solid-to-liquid transition at a given shear stress level. Moreover, they often exhibit a complex time-dependent rheological be- havior known as thixotropy. The viscosity changes find their origin in the microstruc- ture, which depends on flow history. Yet, the structural response to flow of colloidal gels differs fundamentally from most complex fluids, where flow induces orientation. Upon yielding, low to intermediate volume fraction gels break down in a spatially anisotropic way. Bonds in the velocity-velocity gradient plane are broken, whereas mi- crostructural features in other planes are less affected. The subsequent flow-induced microstructural anisotropy is characterized by typical butterfly scattering patterns. However, as yet there was no evidence for the pertinence of this anisotropy for the rheological properties of these systems. In the present work, orthogonal superposition rheometry was used to first evaluate how the flow-induced microstructure affects the viscoelastic properties. It was shown to retain significant elasticity in the velocity- vorticity plane, even when the structure liquefied. Further, the shear-induced me- chanical anisotropy was measured using two-dimensional small amplitude oscillatory shear, exploiting the fact that for suitable thixotropic samples the recovery after ar- resting the flow is relatively slow. It was hence possible to measure the anisotropy of the moduli upon cessation of flow. The mechanical anisotropy was shown to be spectacular, with the storage moduli in perpendicular directions differing by as much as two orders of magnitude. ispartof: Journal of Rheology vol:61 issue:5 pages:1035-1048 status: published

The non-Newtonian properties of blood are of great importance since they are closely related with incident cardiovascular diseases. A good understanding of the hemodynamics through the main vessels of the human circulatory system is thus fundamental in the detection and especially in the treatment of these diseases. Very often such studies take place in vitro for convenience and better flow control and these generally require blood analogue solutions that not only adequately mimic the viscoelastic properties of blood but also minimize undesirable optical distortions arising from vessel curvature that could interfere in flow visualizations or particle image velocimetry measurements. In this work, we present the viscoelastic moduli of whole human blood obtained by means of passive microrheology experiments. These results and existing shear and extensional rheological data for whole human blood in the literature enabled us to develop solutions with rheological behavior analogous to real whole blood and with a refractive index suited for PDMS (polydymethylsiloxane) micro-and milli-channels. In addition, these blood analogues can be modified in order to obtain a larger range of refractive indices from 1.38 to 1.43 to match the refractive index of several materials other than PDMS. (C) 2013 AIP Publishing LLC.

We show that the viscoelastic properties of polycrystalline ferroelectric ceramics can be significantly altered over a wide range of mechanical frequencies when domain switching is controlled by cyclic electric fields. The dynamic stiffness of lead zirconate titanate is shown to vary by more than 30%, while damping increases by an order of magnitude. Experimental results are interpreted by the aid of a continuum-mechanics model that captures the nonlinear electro-mechanically coupled material response for the full electric hysteresis.

Tian J., Ogi H., Hirao M. "Vibration analysis of an elastic-sphere oscillator contacting semi-infinite viscoelastic solids in resonant ultrasound microscopy", Journal of Applied Physics, 95(12), 8366-8375 (2004) https://doi.org/10.1063/1.1737472., Resonant-ultrasound microscopy evaluates local Young’s modulus of materials by the resonant-frequency shift of a vibrating oscillator. This study presents a dynamic-contact model to analyze free vibrations of an isotropic elastic-sphere oscillator contacting two semi-infinite viscoelastic solids, which sandwich the sphere. Assuming frictionless contacts and smaller vibrational amplitude, dynamic-contact pressure distributions are obtained with the linearized maximum contact pressure and contact radius. Combining the sphere oscillation and the solid motions through contact-displacement boundary conditions, resonant frequencies of the elastic sphere are obtained. Unlike the quasistatic model, this dynamic model agrees well with the measurements.

The design and development of a novel category of bi-component polymeric fibers, made of a polymeric sheath and a liquid core is described. To understand and to analyze the flow of molten polymer and liquid within a spin pack, we built a macroscale analogon where co-flow of polymer and liquid is investigated systematically. The macroscale analogon is composed of a glass cylinder with a finite length and a coaxial capillary. The inner fluid flows in the capillary and at its outlet meets the outer fluid flowing inside the glass cylinder. The fluids are pumped into the setup using pressurized bottles with controlled flow rates. A high-speed camera is placed in front of the glass cylinder to record the transient and the steady flow of the two phases and the morphology of their interface during co-flow. We systematically analyzed the combination of various fluids and different injection channel geometries, and used these findings to design a customized spin pack. Extrusion trials with molten polymers and specific liquids resulted in the production of liquid filled polymeric fibers (LFF) with different amounts of liquid from 3% to 33%vol. and diameters from 150μm to 1mm. Fiber morphology has been investigated with the help of optical microscopy and computer tomography, in order to analyze both surface and core morphology. Mechanical characterization of fibers has been carried out with special care to the damping capacity of such fibers using two different methods: two point bending tests to evaluate the energy dissipation of fibers upon bending at low frequency values (0.005-0.5Hz), and analysis of vibration reduction to determine the quality factor at the resonance frequency of the 1st and 3rd vibration mode.

Liquid crystal colloid materials are described based on the liquid crystal (LC) E7 and submicron sized poly(methyl methacrylate-co-divinylbenzene) particles. Application of a thermal treatment to the composite material produces a finely dispersed network of the internally crosslinked polymeric inclusions in the LC-E7. Dynamic rheological measurements on the LC colloids show that the presence of this network imposes pronounced viscoelastic behavior on the material, which may be exploited in the manufacturing of large-area twisted nematic (TN) electro-optical cells via continuous methods as an alternative to the currently available batchwise routes. The electro-optical characteristics of TN cells based on the composite material are approximately comparable to the electro-optical characteristics of a reference cell filled with pure LC E7, which ensures that the largely increased viscoelasticity of the composite does not lead to a degradation of electro-optical properties.

Viscoelasticity of entangled semiflexible polymers in the plateau frequency regime is investigated. We combine computer simulations and scaling arguments to develop a unified view of polymer entanglement based on primitive path analysis. The plateau modulus is obtained as a function of the dimensionless segment density. Our theoretical and numerical results agree well with the experimental data over a wide range of stiffness/density.

We present a molecular dynamic study of the interaction between an amorphous silica tip (SiO_2) and an amorphous poly-(methyl-methacrylate) substrate under conditions relevant for tapping-mode atomic force microscopy. To capture the actual dynamics of the tip, we use the dynamic contact simulation method [Kim et al., J. Appl. Phys. 112, 094325 (2012)]. We obtain force-displacement relationships both for neat polymer substrates and a sample with a sub-surface nanotube and extract the local stiffness and energy dissipation per cycle. The simulations capture non-trivial aspects of the interaction that originate from the viscoelastic nature of the polymer including an increase in repulsive interaction force during approach with tip velocity and an increase in adhesion during retraction with decreasing tip velocity. Scans of local stiffness and dissipation over the samples reveal intrinsic variability in the amorphous polymer but also the effect of local surface topography on the extracted properties as well as the ability of the method to detect a sub-surface nanotube. This insight and quantitative data should be valuable to interpret the results of atomic force microscopy studies.

Using amplitude-modulated atomic force microscopy with the cantilever having 1-nm radius sharp tip, nanoscale mechanical heterogeneity in glassy polymer was investigated. The correlation length of heterogeneous viscoelasticity was measured to be 2.0 nm. The value is consistent with the length scale of cooperatively rearranging regions at glass transition.

Instability and dewetting engendered by the van der Waals force in soft thin (

In a previous paper [T. S. K. Ng and G. H. McKinley, J. Rheol.52(2), 417–449 (2008)], we demonstrated that gluten gels can best be understood as a polymericnetwork with a power-law frequency response that reflects the fractal structure of the gluten network. Large deformation tests in both transient shear and extension show that in the absence of rigid starch fillers these networks are also time-strain factorizable up to very large strain amplitudes (γ∗>5). In the present work, we further explore the nonlinear rheological behavior of these critical gels by considering the material response obtained in large amplitude oscillatory shear over a wide range of strains and frequencies. We use a Lissajous representation to compare the measured material response with the predictions of a network theory that is consistent with the proposed molecular structure of gluten gels. In the linear viscoelastic regime, the Lissajous figures are elliptical as expected and can be quantitatively described by the same power-law relaxation parameters determined independently from earlier experiments. In the nonlinear regime, the Lissajous curves show two prominent additional features. First is a gradual softening of the network indicated by the rotation of the major axis of the stress ellipse. This feature is accounted for in the model by the inclusion of a simple nonlinear network destruction term that reflects the reduction in network connectivity as the polymer chains are increasingly stretched. Second, a distinct upturn in the viscoelastic stress is discernable at large strains. We show that this phenomenon can be modeled by considering the effects of finitely extensible segments in the elasticnetwork. We use this model to quantitatively predict the material response in other large amplitude transient flows such as the start-up of steady shear and transient uniaxial extension up until the onset of strongly nonlinear unsteady phenomena such as edge fracture in shear and sample rupture during extension., Kraft Foods Company

The evolution of specific heat near the glass transition temperature, Tg, of a Zr55Al10Ni5Cu30 glassy alloy subjected to different degrees of non-Newtonian flow conditions was investigated. At conditions in which non-Newtonian flow begins, a gradual exothermic relaxation near Tg was observed. This is attributed to the change in configuration of the long-range liquid structure of the alloy. At conditions in which a high degree of non-Newtonian flow is favored, the alloy exhibits both high-temperature and low-temperature relaxation due to a change in local short-range order. The enthalpy of relaxation is found to scale as the logarithm of the normalized viscosity and is interpreted in terms of the free volume model of viscous flow. The decrease in viscosity in the non-Newtonian flow region is associated with a highly disordered structure and induced free volume.

Solutions and gels of semiflexible polymers provide a template for the molecular description of a large class of soft materials including dispersed polysaccharides, carbon nanotubes, cytoskeletal proteins, and even living cells. These chemically very diverse systems all seem to share characteristic macroscopic mechanical properties, such as power‐law rheology, transitions from shear stiffening to softening, and plastic flow. We attempt to trace back the observed universality to the common meso‐scale structure, which is argued to conceal much of the molecular details and held responsible for the pronounced rheological redundancy.

We study the time-dependent contact area as a viscoelastic solid is squeezed against a randomly rough substrate. Using a recently developed contact mechanics theory we study how the contact area depends on time and on the magnification zeta. Numerical results are presented for self-affine fractal surfaces, and applications to tack, rubber friction, and sealing are given.

It is demonstrated that an AT-cut quartz crystal driven in the thickness-shear-wave mode and typically used as a sensor to monitor the viscoelastic shear-wave properties of a fluid also produce longitudinal pressure waves. Unlike the shear wave, these waves are capable of long-range propagation through the fluid and of reflection at its boundaries, notably at an outer fluid–air interface. They introduce a component into the measured electrical impedance and resonance frequency shift of the crystal, which reflects the setting up of cyclic pressure-wave resonances in the fluid. This has important implications for the practical employment of these crystal as sensors. Under appropriate conditions, as demonstrated for water and n-octane, it is possible to determine the propagating properties of sound waves in a fluid simultaneously with the viscoelastic shear-wave properties. These experiments are supported by an analysis of the appropriate hydrodynamic equations for waves in the crystal–fluid system, which predicts electrical characteristics in close agreement with those found experimentally.

A theoretical model for the adhesive friction between elastomers and rough solid surfaces is proposed on the basis of opening crack propagation processes at the boundary of the contact interfaces and the rate processes of formation of molecular bonds on the solid surface. This model, which is expressed as a product of the terms related to the two abovementioned processes, requires some measurable and fitted parameters such as the frictional shear strength expressed as a function of viscoelastic dissipation, rate-dependent elasticity, density of bonded molecular chains at a contact junction, critical velocity related to viscoelastic relaxation, and critical velocity related to the rate process of formation of molecular bonds on the solid surface. The friction-velocity relationship exhibits a remarkable fit to previously obtained experimental results for polymers such as engineering rubber, gels, and plastics (glassy polymers), and all fitting parameters are physically reasonable. The viscoelastic index "n" is also related to the "glass-to-rubber transition" of a nanometer-thick polymer layer for frictional behavior. Thus, from a practical viewpoint, this model can be used effectively for fitting the adhesive friction behavior of polymers.

Using ultrasonics, the acoustic characteristics of porous Pd42.5Cu30Ni7.5P20 glassy alloys were examined in terms of complex elasticity. Bulk modulus (K) and Lame parameter (λ) decrease in a parabolic manner as porosity increases, while Young (E) and shear (G) moduli decrease linearly. Consequently, the order of K>λ>E>G, which is monopolistically characterized by nonporous glassy alloys, changes to the order of E>K>λ⩾G at 58% porosity. The decrease in Poisson’s ratio and the increase in G∕K ratio up to 58% porosity indicate crystallinelike characteristics that resist three-dimensional volume-nonpreserving deformation. An increase in porosity induces frequency convergence for longitudinal waves and an increment of the imaginary parts in the complex waves. The viscoelasticity of the porous glassy alloys is predominated by shear motion. In contrast to the continuous increment in dilational damping, the porosity-dependent increment in shear damping could be elucidated based on the accumulation of strains indu...

Creep deformation under a constant applied load in a Zr55Al10Ni5Cu30 glassy alloy at the glass transition region is investigated. At an initial stress, σ0, less than a critical stress, σc=80 MPa, the glass shows a Newtonian flow. When σ0⩾σc, the flow viscosity, η, initially decreases and attains a minimum. It then increases as the true stress, σ, decreases with further deformation. The initial decrease in η and the attendance of viscosity minimum are due to the stress-induced structural disorder and the structural equilibration with the applied stress, respectively. For stress, σ, less than the viscosity minimum stress, the stress dependence of viscosity, η(σ), curves all tend to merge together, and is fitted well with a master curve, η(σ), established previously for the steady-state flow under constant strain-rate experiments. These results render further support to the hypothesis of stress-induced structural disorder and the concept of fictive stress.

A simple model for porous rocks is proposed based on classification of pores as either crack‐like or sphere‐ or tube‐like. Analytic results are used for the strain of each class to obtain expressions for effective moduli. Attenuation results from fluid flow among cracks and pores, and depends on a characteristic frequency. The parameters in the model can be estimated or measured independently. The model gives a very reasonable interpratation laboratory data on sandstone, where attenuation is due to cracks with a narrow distribution of aspect ratios.