Experimental Observations and Quantitative Predictions of Rheological Properties of Organoclay/Poly(propylene) Nanocomposites at the Startup of Shear Flows

We study the rheological characteristics of nanocomposites containing nano-sized plate like particles in a viscoelastic fluid at the startup of steady state in the simple shear flow mode. The nanocomposites of organoclay-polypropylene with different nanoclay contents were prepared by melt mixing. A rheological equation of state, originally formulated to predict the orientation state and viscoelastic behavior of suspensions of ellipsoidal particles in polymer melts, has been modified to describe the observed phenomena for the nanoclay/poly(propylene) composites. The rotational particle motion and alignment for a group of symmetric ellipsoids with the applied flow field are investigated. Additionally, model calculations of the macroscopic rheological properties for a simple flow case suggest the presence of nano-particles significantly modify the suspended fluid at volume concentrations as low as 0.5%. The model calculations for the startup viscosity are reasonably in agreement with the experimental results at the experimental range covered in this study. At the shear rate of , we observe pronounced stress overshoots at the three nanoclay loadings level tested which are found to be related to the fast alignment of the silicate layers with the shear direction in the polymer melt.     

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.     

Model filled rubber. II. Particle composition dependence of suspension rheology

Monodisperse size colloidal particles varying in chemical composition were synthesized by emulsifier‐free emulsion polymerization. Using a stress‐controlled rheometer, the rheological behavior of colloidal suspensions in a low molecular weight liquid polysulfide was investigated. All suspensions exhibited shear thinning behavior. The shear viscosity, dynamic moduli, and yield stress increased as interactions between particles and matrix increased. The rheological properties associated with network buildup in the suspensions were sensitively monitored by a kinetic recovery experiment. We propose that interfacial interactions by polar and hydrogen bonding between particles and matrix strongly promote affinity of matrix polymer to the filler particles, resulting in adsorption or entanglement of polymer chains on the filler surface. A network structure was formed consisting of particles with an immobilized polymer layer on the particle surface with each particle floc acting as a temporary physical crosslinking site. As the interfacial interaction increases, the adsorbed layer thickness on the filler particles, hence, the effective particle volume fraction, increases. As a result, the rheological properties were enhanced in the order PS < PMMA < PSVP. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 815–824, 1999     

Electrorheological properties of suspensions of monodispersed micron‐sized polyaniline composite particles

Monodispersed micron‐sized polyaniline (PANi) composite particles were synthesized by chemically oxidative polymerization of aniline in the presence of functional porous polymer particles. The formation of the PANi‐coated composite particles was confirmed by scanning electron microscopy. Electrorheological (ER) properties of the monosized composite particle suspensions were then investigated under different DC electric fields by altering the particle characteristics. The ER effect of the PANi composite suspensions was largely dependent on the composition ratio (PANi loading), the particle conductivity, and the particle concentration. Dynamic oscillation measurements revealed that the applied electric field induced the viscoelastic property of the ER suspensions by generating the chain structures of the suspended particles. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1163–1170, 2002     

Rheological properties of filled polymers

The rheological properties of two model suspensions using a Newtonian polymeric matrix are presented and discussed in light of results presented in the literature. It is shown that particle-particle interactions in concentrated suspensions are responsible for a gel-type behavior at very small strain and strain hardening at a critical strain. Suspensions of concentrated colloidal particles in a Newtonian matrix behave like typical viscoelastic molten polymers, but the properties are strongly dependent on the solids dispersion, and strain at small strain. A simple rheological model is proposed to describe the shear viscosity of these suspensions.     

Rheological Properties of Silica Particle Suspensions in Mineral Oil

The rheological properties of particles suspended in a non‐polar mineral oil have been investigated as a function of volume fraction of particles, particle size, surface properties and shear rate. Three different types of particles were investigated; glass microspheres, monodisperse silica particles and fumed silica. The suspensions showed shear thinning behavior at higher volume fractions, and the viscosity increased with decreasing particle size. The hydrophobic particles display lass shear thinning effects. The relative viscosity of all the suspensions was well fitted to the Krieger and Dougherty model.     

Assessing crystalline lamellae orientation impact on the properties of semi‐crystalline polymer‐clay nanocomposites

Semi‐crystalline polymer‐clay nanocomposite properties are often considered only by their clay dispersion state. The purpose of this work is to highlight texture effects on semi crystalline polymer‐clay properties. Maleic anhydride‐grafted polyethylene nanocomposites with two different processing techniques (Blown Extrusion and Compression) were studied. The processing was shown to induce different crystalline lamellae orientation in the films but with no significant changes in the crystalline lamellae long period, degree of crystallinity, clay particle orientation morphology and dispersion. The impact of these specific textures on the nanocomposites barrier and tensile properties were reported. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1966–1975, 2008     

Fiber motion and rheology of suspensions with uniform fiber orientation

One of the main goals in the studies of fiber suspensions is the prediction of fiber orientation in a short fiber composite part, using the processing variables, mold geometry, and material characteristics. The rheological properties of the fiber suspensions are strongly associated with the fiber orientation distribution. The understanding of the relations between the fiber structure in the suspension and its rheological properties is a key step in the design and implementation of processing operations. The fiber motion in shear flow is analyzed in this article. The study is focused on the relation between fiber orientation and rheological properties for a suspension with uniform (delta function) fiber orientation distribution in a Newtonian fluid. The study shows that the rheological properties of the suspension, measured during the start up of steady shear flow, can be used to determine the fiber orientation in the sample. The first normal stress coefficient is the property to measure in order to determine whether or not the suspension has a random fiber orientation. Any of the shear flow transient rheological properties can be used to determine the fiber initial orientation. It was found that the normal stress coefficients can show negative or positive values depending on the fiber orientation. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1788–1799, 2000     

Rheological Modeling of Dispersion System of Nano/Microsized Polymer Particles Considering Swelling Behavior

Rheological behavior of dispersion system containing nano/microsized cross-linked polymer particle was studied considering particle hydration and swelling. Viscosity of the dispersion system depends on swelling kinetics of polymer particles. Under shear flow, dispersion of swollen polymer particles is shear thinning. According to experimental results, kinetics of particle swelling and hydration was described well by second-order kinetic equation. Relational expression between equilibrium particle size and influencing factors of swelling such as salt concentration and temperature was presented. Assume that swollen polymer particles are uniform and have a simple core-shell structure, interacting through a repulsive steric potential. The rheological modeling of such dispersion system at low shear rate was presented using the concept of effective volume fraction, which depends on swelling kinetics and interparticle potential. Cross model was introduced to describe shear-thinning behavior. The viscosity equation allows correlation of experimental data of relative viscosity versus shear rate or hydration time; accounting for effect of temperature and salt concentration on viscosity. Predictions of the model have a good agreement with experimental results.     

Multiscale modeling of viscoelastic properties of polymer nanocomposites

A methodology for simple multiscale modeling of mechanical properties of polymer nanocomposites has been developed. This methodology consists of three steps: (1) obtaining from molecular dynamics simulations the viscoelastic properties of the bulklike polymer and approximating the position-dependent shear modulus of the interfacial polymer on the basis of the polymer-bead mean-square displacements as a function of the distance from the nanoparticle surface, (2) using bulk- and interfacial-polymer properties obtained from molecular dynamics simulations and performing stress–relaxation simulations of the nanocomposites with material-point-method simulations to extract the nanocomposite viscoelastic properties, and (3) performing direct validation of the average composite viscoelastic properties obtained from material-point-method simulations with those obtained from the molecular dynamics simulations of the nanocomposites. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1005-1013, 2005     

Morphology and viscoelasticity of PP/TiO2 nanocomposites prepared by in situ sol–gel method

PP/TiO₂ nanocomposites were prepared from an original method based on the hydrolysis‐condensation (sol–gel method) reactions of titanium alkoxide inorganic precursor premixed with polypropylene (PP) under molten conditions. Nanocomposites with a mean diameter of primary particles lower than 5 nm were then prepared. The TiO₂ particle dispersion in the PP matrix was characterized over a wide length scale from the combination of small angle X‐ray scattering, transmission electron microscopy, and linear viscoelasticty of molten nanocomposites. As a result, a fractal structure of these particles was highlighted at the highest concentration (φ_r ≥ 0.014) with a characteristic aggregation size d_aggr ≈ 130 nm. The relationships between fractal structure and linear viscoelastic have been discussed from the main works of the literature on the reinforcement of nanocomposites. The drastic alteration of the terminal relaxation zone (solid‐like behavior) is correlated to the formation of an aggregate‐particle network. The study of the nonlinear viscoelastic behavior (Payne effect) agrees qualitatively with this reinforcement mechanism. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1213–1222, 2010     

Flow of non‐newtonian fluids in porous media

The study of flow of non‐Newtonian fluids in porous media is very important and serves a wide variety of practical applications in processes such as enhanced oil recovery from underground reservoirs, filtration of polymer solutions and soil remediation through the removal of liquid pollutants. These fluids occur in diverse natural and synthetic forms and can be regarded as the rule rather than the exception. They show very complex strain and time dependent behavior and may have initial yield‐stress. Their common feature is that they do not obey the simple Newtonian relation of proportionality between stress and rate of deformation. Non‐Newtonian fluids are generally classified into three main categories: time‐independent whose strain rate solely depends on the instantaneous stress, time‐dependent whose strain rate is a function of both magnitude and duration of the applied stress and viscoelastic which shows partial elastic recovery on removal of the deforming stress and usually demonstrates both time and strain dependency. In this article, the key aspects of these fluids are reviewed with particular emphasis on single‐phase flow through porous media. The four main approaches for describing the flow in porous media are examined and assessed. These are: continuum models, bundle of tubes models, numerical methods and pore‐scale network modeling. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Electroosmotic flow of non‐Newtonian fluids in a constriction microchannel

Insulator‐based dielectrophoresis has to date been almost entirely restricted to Newtonian fluids despite the fact that many of the chemical and biological fluids exhibit non‐Newtonian characteristics. We present herein an experimental study of the fluid rheological effects on the electroosmotic flow of four types of polymer solutions, i.e., 2000 ppm xanthan gum (XG), 5% polyvinylpyrrolidone (PVP), 3000 ppm polyethylene oxide (PEO), and 200 ppm polyacrylamide (PAA) solutions, through a constriction microchannel under DC electric fields of up to 400 V/cm. We find using particle streakline imaging that the fluid elasticity does not change significantly the electroosmotic flow pattern of weakly shear‐thinning PVP and PEO solutions from that of a Newtonian solution. In contrast, the fluid shear‐thinning causes multiple pairs of flow circulations in the weakly elastic XG solution, leading to a central jet with a significantly enhanced speed from before to after the channel constriction. These flow vortices are, however, suppressed in the strongly viscoelastic and shear‐thinning PAA solution.     

Chain confinement in polymer nanocomposites and its effect on polymer bulk properties

The issue of chain confinement in nanocomposites remains largely unanswered because experimental systems are plagued by additional complicating variables such as particle–polymer interactions and free volume increases brought upon by the addition of the particles. Using computer simulation of high length chain polymers, we show that simple excluded volume interactions between polymer and nanoparticles lead to a wealth of changes in the diffusion coefficient and entanglement density of the chains. This opens up the possibility of using nanoparticles for tuning polymer properties, such as toughness, melt viscosity, and transient rubberlike behavior. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 687–692, 2010     

Mechanistic model for deformation of polymer nanocomposite melts under large amplitude shear

We report the mechanical response of a model nanocomposite system of poly(styrene) (PS)-silica to large-amplitude oscillatory shear deformations. Nonlinear behavior of PS nanocomposites is discussed with the changes in particle dispersion upon deformation to provide a complete physical picture of their mechanical properties. The elastic stresses for the particle and polymer are resolved by decomposing the total stress into its purely elastic and viscous components for composites at different strain levels within a cycle of deformation. We propose a mechanistic model which captures the deformation of particles and polymer networks at small and large strains, respectively. We show, for the first time, that chain stretching in a polymer nanocomposite obtained in large amplitude oscillatory deformation is in good agreement with the nonlinear chain deformation theory of polymeric networks. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Flow-Induced Anisotropy in Mixtures of Associative Polymers and Latex Particles

The effect of associative polymers on the structure and rheological behavior of colloidal suspensions is discussed. Adding associative polymer is known to increase the viscosity of the suspensions. At high shear rates the increase is close to what could be expected on the basis of the hydrodynamic effects of the added polymer. At low shear rates the viscosity increases much more. Small-angle light scattering (SALS) during flow is used here to investigate the underlying structural mechanisms. The SALS patterns indicate that the associative polymer changes the particulate structure: characteristic butterfly patterns appear even at relatively low particle volume fractions. They are not present in the suspensions without associative polymer. The patterns indicate that fluctuations in particle concentration are more pronounced in the flow direction than in the vorticity direction and that anisotropic particulate structures with an orientation along the vorticity direction develop. The evolution of their characteristic length scale during flow has been followed over time. Changing the hydrophilic part of the polymer from polyacrylamide to polyacrylic acid induces stronger associative interactions. In the suspensions this results in a reduction of the relative viscosity rather than an increase. The difference in degree of associativity between the polymers also has an effect on the SALS patterns in the suspensions both at rest and during flow. The rheology as well as the SALS suggest the presence of a strong polymer network in the second system. The competition between adsorption of the associative polymer on the particles with the intermolecular associations between the polymer chains seems to be responsible for the observed differences. Copyright 2000 Academic Press.     

Viscoelastic properties of silica‐grafted poly(styrene–acrylonitrile) nanocomposites

The melt state viscoelastic properties of styrene–acrylonitrile random copolymers grafted to silica nanoparticles and prepared by in situ atom transfer radical polymerization are examined. Linear dynamic oscillatory viscoelastic measurements indicate that nanocomposites with silica contents as low as 1.2 vol % exhibit solid‐like response. Steady shear results in considerable alteration to the mesoscale structure and results in the failure of the Cox‐Merz rule. Flow reversal studies confirm the elastic nature of the mesoscale structure and demonstrate that the relaxation of the flow altered structure is slow, non‐Brownian, and possibly cooperative. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2014–2023, 2006     

Modeling multiphase latex particle equilibrium morphology

Multiphase waterborne polymer particles provide advantages in more demanding applications and their performance depends on particle morphology. Currently, no general approach to predict the morphology of multiphase latex particles is available. In this work, a model based on Monte Carlo methods was developed for the prediction of equilibrium morphologies of multiphase waterborne systems, such as polymer‐polymer and polymer‐polymer‐inorganic hybrids. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2579–2583, 2010     

Viscoelastic properties of ionic polymer composites reinforced by soy protein isolate

Both linear and nonlinear viscoelastic properties of ionic polymer composites reinforced by soy protein isolate (SPI) were studied. Viscoelastic properties were related to the aggregate structure of fillers. The aggregate structure of SPI is consisted of submicron size of globule protein particles that form an open aggregate structure. SPI and carbon black (CB) aggregates characterized by scanning electron microscope and particle size analyzer indicate that CB aggregates have a smaller primary particle and aggregate size than SPI aggregates, but the SPI composites have a slightly greater elastic modulus in the linear viscoelastic region than the CB composites. The composite containing 3–40 wt % of SPI has a transition in the shear elastic modulus between 6 and 8 vol % filler, indicating a percolation threshold. CB composites also showed a modulus transition at <6 vol %. The change of fractional free volume with filler concentration as estimated from WLF fit of frequency shift factor also supports the existence of a percolation threshold. Nonlinear viscoelastic properties of filler, matrix, and composites suggested that the filler‐immobilized rubber network generated a G′ maximum in the modulus‐strain curves and the SPI formed a stronger filler network than the CB in these composites. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3503–3518, 2005