Dispersions of nanocrystalline cellulose in aqueous polymer solutions: structure formation of colloidal rods

The steady-state shear and linear viscoelastic deformations of semidilute suspensions of rod-shaped nanocrystalline cellulose (NCC) particles in 1.0% hydroxyethyl cellulose and carboxymethyl cellulose solutions were investigated. Addition of NCC at the onset of semidilute suspension concentration significantly altered the rheological and linear viscoelastic properties of semidilute polymer solutions. The low-shear viscosity values of polymers solutions were increased 20-490 times (depending on polymer molecular weight and functional groups) by the presence of NCC. NCC suspensions in polymer solutions exhibited yield stresses up to 7.12 Pa. Viscoelasticity measurements also showed that NCC suspended polymer solutions had higher linear elastic moduli than the loss moduli. All of those results revealed the gel formation of NCC particles and presence of internal structures. The formation of a weak gel structure was due to the nonadsorbing macromolecules which caused the depletion-induced interaction among NCC particles. A simple interaction energy model was used to show successfully the flocculation of NCC particles in the presence of nonadsorbing polymers. The model is based on the incorporation of the depletion interaction term between two parallel plates into the DLVO theory for cubic prismatic rod shaped NCC particles.     

Peculiarities of rheological properties and flow of highly concentrated emulsions: The role of concentration and droplet size

Results of a complete study of the rheological properties of highly concentrated emulsions of the w/o type with the content of the dispersed phase up to 96% are reported. The aqueous phase is a supersaturated solution of nitrates, where the water content does not exceed 20%. Dispersed droplets are characterized by a polyhedral shape and a broad size distribution. Highly concentrated emulsions exhibit the properties of rheopectic media. In steady-state regimes of shearing, these emulsions behave as viscoplastic materials with a clearly expressed yield stress. Highly concentrated emulsions are characterized by elasticity due to the compressed state of droplets. Shear storage modulus is constant in a wide range of frequencies that reflect solid-like behavior of such emulsions at small deformations. The storage (dynamic) modulus coincides with the elastic modulus measured in terms of the reversible deformations after the cessation of creep. Normal stresses appear in the shearing. In the low shear rate domain, normal stresses do not depend on shear rate, so that it can be assumed that they have nothing in common with normal stresses arising owing to the Weissenberg effect. These normal stresses can be attributed to Reynolds’ dilatancy (elastic dilatancy). Normal stresses sharply decrease beyond some threshold value of the shear rate and slightly increase only in a high shear rate domain. Observed anomalous flow curves and unusual changes of normal stresses with shear rate are explained by the two-step model of emulsion flow. Direct optical observations show that emulsions move by the mechanism of the rolling of larger droplets over smaller ones without noticeable changes of their shape at low shear rates, while strong distortions of the droplet shape is evident at high shear rates. The transition from one mechanism to the other is attributed to a certain critical value of the capillary number. The concentration dependence of the elastic modulus (as well as the yield stress) can be described by the Princen-Kiss model, but this model fails to predict the droplet size dependence of the elastic modulus. Numerous experiments demonstrated that the modulus and yield stress are proportional to the squared reciprocal size, while the Princen-Kiss model predicts their linear dependence on the reciprocal size. A new model based on dimensional arguments is proposed. This model correctly describes the influence of the main structural parameters on the rheological properties of highly concentrated emulsions. The boundaries of the domain of highly concentrated emulsions are estimated on the basis of the measurement of their elasticity and yield stress.     

Negative thixotropy of polymer solutions. 1. A model explaining time-dependent viscosity

Negative thixotropy, also called antithixotropy, is the effect of a flow-induced increase in viscosity that has been observed for many polymer solutions. Here, a simple quantitative model describing the time dependence of the shear stress or viscosity is presented. The model assumes a dynamic gel or network in the polymer solution, whose cross-links are dynamically formed and broken. The cross-links exist with or without deformation or flow of the solution. A second property of the model network is that it cannot be deformed infinitely, which is also true for any real network. The dynamic network solution is characterized by four parameters: its elastic shear modulus, its maximum degree of deformation, the rate with which the dynamic cross-links form and break and the viscous contribution of the polymer solution. The first two parameters can be related to each other, so only three independent parameters enter the model. An analytical solution is obtained which describes the flow-induced increase in viscosity, the minimum shear rate required for negative thixotropy and the dependence of the induction time on the shear rate. The results are shown to be in agreement with reported experimental results.     

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.     

Out of shear plane deformations in nematic liquid crystals

The stability of the director field to deformations out of the plane of shear is examined by use of the Taylor expansion method based on catastrophe theory. For simple shear flow of nematics, the coming out of the shear plane is found for suitable surface alignment and not too high twist elastic constant. The role of these parameters is pointed out. Non-flow-aligning nematics are also considered, and results consistent with earlier reports are obtained.     

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     

Non-Newtonian behavior of polydisperse polymer melts as a consequence of the evolution of their relaxation spectra

The non-Newtonian flow of polydisperse polymer melts is shown to be described by a model according to which an increase in the shear rate leads to the suppression of the dissipative losses of the relaxation modes of each fraction. The higher the shear rate, the greater the suppression. The relaxation spectrum of each monodisperse fraction is represented by the Rouse distribution, and only this form of spectrum leads to a ??spurt?? effect at the critical shear stress. Hence, the physical content of the model that relates the non-Newtonian behavior of polymer melts to their molecular-mass distributions consists in the fact that the relaxation modes responsible for energy dissipation are gradually truncated from the side of high relaxation times. The higher the M of a given fraction, the greater the contribution of this part of the spectrum to the total viscous losses. In this case, the truncation of the spectrum from the side of high relaxation times is equivalent to the gradual ??elimination?? of high-molecular-mass fractions of the polydisperse polymer from the contribution to dissipation. The shear-rate-dependent evolution of the relaxation spectrum of the medium is the structural mechanism that causes the non-Newtonian flow of polymer melts. The efficiency of the proposed model is shown through calculation of the flow curves for polymers with known molecular-mass distributions. The calculation results are in agreement with the experimental data. The theoretical ideas developed with the use of the ?? function to describe molecular-mass distributions have made it possible to solve the inverse problem, i.e., to establish a quantitative relationship between the shape of the flow curve and the molecular-mass distribution and, thus, to calculate the molecular-mass distributions according to the shearrate dependence of the apparent viscosity.     

Overshoots in stress and free energy change during the flow-induced crystallization of polymeric melt in shear flow

<正>The effect of pre-shear flow on the subsequent crystallization process of polymeric melt was investigated and a flow-induced crystallization(FIC) model based on the conformation tensor incorporating the pre-shear effect was proposed. The model is capable of predicting the overshoot phenomena of the stress and the flow-induced free energy change of the polymeric system at high pre-shear rates.Under the condition of flow,the increase in the activated nuclei number was contributed by the flow-induced free energy change,which showed an overwhelming effect on the nuclei formation during the pre-shear process at high shear rates.The half crystallization time(f_(1/2)) of polypropylene(PP) as functions of pre-shear rate and pre-shear time at different crystallization temperatures was predicted and compared with the experiment data.Both numerical and experimental results showed that t_(1/2) of PP decreased dramatically when the flow started but leveled off at long times.It was found that two transformation stages in t_(1/2) existed within a wide range of shear rates.For the first stage where the melting polymer experienced a relatively weak shear flow,the acceleration of crystallization kinetics was mainly contributed by the steady value of free energy change while in the second stage for high shear rates,strong overshoot in flow-induced free energy change occurred and the crystallization kinetics was thus significantly enhanced.The overshoots in stress and flow-induced free energy change reflected an important role of flow on the primary nucleation especially when the flow was strong enough.     

The Effects of Extensional Flow and Hydrodynamic Interaction on the Nonequilibrium Brownian Dynamics of Dendrimers. A Bead‐Spring Model for Dendrimeric Molecules

We present the effects of incorporating hydrodynamic interactions into the nonequilibrium Brownian dynamics model describing the rheological properties of dendrimers under simple shear flow. The model response to uniaxial extensional flow is considered and compared with the extensional flow behavior of conventional linear molecules. The dendrimers are characterized by low viscosities and display little visco‐elasticity which makes them attractive materials from the stand‐point of energy‐intensive polymer forming processes.     

Interplay between shear flow and elastic deformations in liquid crystals

We study shear flow in liquid crystal cells with elastic deformations using a lattice Boltzmann scheme that solves the full, three-dimensional Beris-Edwards equations of hydrodynamics. We consider first twisted and hybrid aligned nematic cells, in which the deformation is imposed by conflicting anchoring at the boundaries. We find that backflow renders the velocity profile non Newtonian, and that the director profile divides into two regions characterized by different director orientations. We next consider a cholesteric liquid crystal, in which a twist deformation is naturally present. We confirm the presence of secondary flow for small shear rates, and are able to follow the dynamical pathway of shear-induced unwinding, for higher shear rates. Finally, we analyze how the coupling between shear and elastic deformation can affect shear banding in an initially isotropic phase. We find that for a nematic liquid crystal, elastic distortions may cause an asymmetry in the dynamics of band formation, whereas for a cholesteric, shear can induce twist in an initially isotropic sample.     

Self-assembly and elastic instability in polymer flows

This paper is devoted to the discussion of problems related to elastic instability arising in polymer flows. A new model of the rotary dynamics of macromolecules in shear fields of different geometries is proposed. The model is based on the nonlinear finite-difference Schródinger equation describing the process of self-assembly for the system of bonded macromolecules as rotators. It is shown that the self-assembly of macromolecules is accompanied by the chaos-order transition that creates prerequisites for the flow elastic instability obeying the bifurcation mechanism. The self-assembly of macromolecules in shear fields is accompanied by the growth of the space scale in the molecular correlation and can lead to formation of rheological spiral and fibril superlattices.     

Elastic Behavior of Polymer Chains

The elastic behavior of the polymer chain was investigated in a three-dimensional off-lattice model. We sample more than 109 conformations of each kind of polymer chain by using a Monte Carlo algorithm, then analyze them with the non-Gaussian theory of rubberlike elasticity, and end with a statistical study. Through observing the effect of the chain flexibility and the stretching ratio on the mean-square end-to-end distance, the average energy, the average Helmholtz free energy, the elastic force, the contribution of energy to the elastic force, and the entropy contribution to elastic force of the polymer chain, we find that a rigid polymer chain is much easier to stretch than a flexible polymer chain. Also, a rigid polymer chain will become difficult to stretch only at a quite high stretching ratio because of the effect of the entropy contribution. These results of our simulation calculation may explain some of the macroscopic phenomena of polymer and biomacromolecular elasticity.     

Molecular simulation of the frictional behavior of polymer-on-polymer sliding

Molecular simulations of the sliding processes of polymer-on-polymer systems were performed to investigate the surface and subsurface deformations and how these affect tribological characteristics of nanometer-scale polymer films. It is shown that a very severe deformation is localized to a band of material about 2.5 nm thick at the interface of the polymer surfaces. Outside of this band, the polymer films experience a uniform shear strain that reaches a finite steady-state value of close to 100%. Only after the polymer films have achieved this steady-state shear strain do the contacting surfaces of the films show significant relative slippage over each other. Because severe deformation is limited to a localized band much thinner than the polymeric films, the thickness of the deformation band is envisaged to be independent of the film thickness and hence frictional forces are expected to be independent of the thickness of the polymer films. A strong dependency of friction on interfacial adhesion, surface roughness, and the shear modulus of the sliding system was observed. Although the simulations showed that frictional forces increase linearly with contact pressure, adhesive forces contribute significantly to the overall friction and must therefore be accounted for in nanometer-scale friction. It is also shown that the coefficient of friction is lower for lower-density polymers as well as for polymers with higher molecular weights.     

Phenomenological theory of textured mesophase polymers in weak flows

A macroscopic viscoelastic model for incompressible, isothermal, homogeneous lyotropic mesophases exhibiting the nematic polydomain texture is presented. Under equilibrium static conditions the model describes a three dimensional tessellation, where each region or nematic domain has a characteristic size and orientation, and where the polydomain texture has a random orientation. Close form expressions that define the characteristic texture size and the number of randomly oriented domains are given. When subjecting the model lyotropic liquid crystalline polymer displaying the polydomain texture to a steady rectilinear shear flow, the predicted characteristic texture size decreases with increasing shear rates. The power law scaling relations of texture size with shear rate are in excellent agreement with the experimental measurements. The steady shear flow orientation predictions, characterized by a positive shear dependent alignment angle and a low orientation, are in agreement with experimental data.     

Phase behaviour of SMA/PMMA blends: the influence of processing at low and high deformation rates

The influence of processing parameters (deformations) on SMA/PMMA blend phase behaviour is studied. It is found that injection moulding does change polymer blend phase behaviour. Phase separation kinetics are important to understand the injection moulding experiments and the kinetics are probably influenced by the deformations caused by the injection moulding proces. Capillary flow causes a complex change of polymer blend phase behaviour showing both deformation induced mixing and redemixing. Short capillaries, causing almost only uniaxial elongation in combination with pressure, cause no change to polymer blend phase behaviour. This is probably due to the short time the deformation is imposed to the material: it is expected that elongation is a main parameter causing changes in polymer blend phase behaviour. Parallel plate rheometer experiments show that applying only shear causes a complex change of phase behaviour showing both shear induced mixing and redemixing.     

Micro-Rheological Modeling of Flow-Induced Crystallization in Mixed Shear/Extensional Flows

Flow Induced Crystallization (FIC) is the common term to indicate the acceleration in polymer crystallization kinetics due to the action of flow. When modeling FIC, two major challenges are encountered. On the one hand, the model must be able to produce quantitative reliable results, while correctly describing the coupling between the intrinsic (quiescent) crystallization kinetics and the rheological response of the polymer. On the other hand, the model must be able to describe the complex kinematics taking place in real industrial processes. In this paper, we present the predictions of a recently proposed model for FIC in the case of a mixed flow, where both shear and extensional components are present at the same time. In particular, the effects of the overall flow intensity and of relative weight between shear and extension on the enhancement in nucleation rate are presented and discussed. Some guidelines for future development are also proposed.     

Microscopic theory of gelation and elasticity in polymer-particle suspensions

A simplified mode-coupling theory (MCT) of ergodic-nonergodic transitions, in conjunction with an accurate two-component polymer reference interaction site model (PRISM) theory for equilibrium structural correlations, has been systematically applied to investigate gelation, localization, and elasticity of flexible polymer-hard particle suspensions. The particle volume fraction at the fluid-gel transition is predicted to depend exponentially on reduced polymer concentration and size asymmetry ratio at relatively high colloid concentrations. In contrast, at lower particle volume fractions, a power-law dependence on polymer concentration is found with effective exponents and prefactors that depend systematically on the polymer/particle size ratio. Remarkable power-law and near universal scaling behavior is found for the localization length and elastic shear modulus. Multiple experiments for gel boundaries and shear moduli are in good agreement with the no adjustable parameter theory. The one exception is the absolute magnitude of the shear modulus which is strongly overpredicted, apparently due to nonequilibrium dense cluster formation. The simplified MCT-PRISM theory also captures the qualitative aspects of the weak depletion-driven "glass melting" phenomenon at high particle volume fractions. Calculations based on an effective one-component model of structure within a low particle volume fraction framework yield qualitatively different features than the two-component approach and are apparently all in disagreement with experiments. This suggests that volume fraction and size asymmetry dependent many-body screening of polymer-mediated depletion attractions at finite particle concentrations are important.