A. V. Semakov

Viscoplasticity and stratified flow of colloid suspensions

Investigation of the rheology of concentrated colloid suspensions and direct observation of their flow allowed us to find several effects inherent to these media as typical soft matter. So, at low stress amplitudes, these colloids behave as mild gels with frequency independent elastic modulus and low mechanical losses. Meanwhile, suspensions demonstrate dualism of properties: at a given shear rate, they behave as viscoplastic media with clearly expressed yielding, while at a given low stress the pronounced Newtonian plateau is detected. The increase in shear rates and stresses leads to the sharp drop of the apparent viscosity, which usually is treated as the yielding effect. Transition through the yield stress is of a dynamic nature because the threshold stress depends on time and suspensions are thixotropic yielding materials. In the transient shear rate range, an unstable regime of deformation appears. It manifests itself either as deformation thickening up to jamming, or as the excitation of self-oscillations. The measuring of rheological properties in varying volume-to-surface ratio of a sample proves that flow of a suspension with high velocity at constant shear stress actually proceeds in a narrow layer inside the instrument gap. This conclusion has been confirmed by direct visual observations demonstrating that a flux is separated into three layers. A wide almost motionless layer is seen near a stationary surface. Near a moving surface, a narrow band with linear velocity profile is detected. Between them, a rather wide transient layer is observed and shear rate in this layer exceeds the average (global) shear rate by several times. Approximately, only a half of the total volume of a suspension is involved in flow. So, we observed a three-band flux of a suspension not described before. Shearing leads to an anisotropic structure of a solid phase.

The chaos-to-order transition in critical modes of shearing for polymer and nanocomposite melts

Two unusual experimental phenomena that were found for polymer melts or solutions containing the dispersed phases of Na-montmorillonite or detonation synthesis nanodiamond have been studied. These phenomena consist in the reduction of viscosity upon addition of specified amount of particles and in the formation of regular morphology by these particles in strong flows looking as a system of concentric rings. In other words, under certain conditions, there is transition to stratified shear stream and the viscosity of such a regular heterogeneous system canbe lower than that for the polymer matrix itself. Hence, both phenomena are mutually related; and the main problem here is the analysis of driving forces leading to the regular texture formation taking place in intense flows for unfilled viscoelastic polymers as well. As a preliminary explanation, the conception of the special kind of the elastic instability is discussed. This instability appears either in the regular helix-like structure formation or in the irregular elastic turbulence. The particles of the filler play a role of tracers that revealed the relief of texture.

Rheology of carbosilane dendrimers with various types of end groups

The rheological properties of high-generation carbosilane dendrimers carrying different kinds of terminal groups are studied. It is shown that the nonlinear viscoelastic behavior of dendrimers and the high-temperature relaxation transition in dendrimers are interrelated and result from the reversible breakdown of the supramolecular structure formed by the system of contacts of exterior shells of dendrimers. The strength of the supramolecular structure is dependent on the specific interaction of terminal groups of dendrimers and their mobility. The dendrimers under study demonstrate the dualism of macromolecule-particle properties: They behave as both polymer melts and colloidal systems.

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 Schrodinger 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.

Structuring during flow of polymer and colloidal systems

This review presents the description of various cases and physical causes of self-organization (development of ordered structures) provided by the shear flow of polymer and colloidal systems that are characterized by complex rheological behavior. This description is based on the common basis that, as a general rule, micellar systems in many respects behave as polymer solutions. In contrast to the development of regular dynamic structures during the flow of low-molecular-mass fluids, when the viscosity of a fluid plays a key role, in the case of complex rheological media, structuring is provided by the elasticity of a medium and/or by the existence of diverse structural modifications of the material. They arise owing to shear deformation and show different rheological characteristics, a circumstance that, in particular, lead to multivalued flow curves of the medium. Various aspects of this mode of structuring reported in the literature are revisited. Feasible mechanisms of self-organization in polymer and colloidal systems and modes of their description are discussed in detail. Special attention is focused on the similar natures of structural phenomena in polymer and colloidal systems. Various theoretical approaches to the description of instability and structural organization in polymer and colloidal systems are considered. In many cases, the possible cause of structuring can be associated with ambivalent flow curves with a region where stress decreases with an increase in the shear rate. However, in micellar and polymer systems capable of elastic strain, elasticity is the main cause of instability. In this case, such systems can coexist in two structural modifications: flow and elastic (rubbery). It is proposed that the above phenomena can be described in terms of the granular model of a fluid.

Entanglement junctions in melts and concentrated solutions of flexible-chain polymers: Macromodeling

The effect of deformation on the behavior of intermolecular entanglements in melts and in concentrated solutions of polymers is simulated with the use of knots formed by long flexible threads. Two cases are discussed: the behavior of individual junctions and the pattern of change in the entanglements in a network of statistically entangled threads. It is shown that, at low strain rates, the junctions disentangle and threads slip out of the entanglements. This situation simulates an irreversible flow of macromolecules in the classical tube model. At high rates, the junctions tighten, a phenomenon that simulates the impossibility of irreversible motion of macromolecules. The transition from slipping to tightening of entanglements simulates the change in the pattern of behavior of the melt, i.e., from flow to rubberlike deformations. A generalized dependence of elastic deformations on the Weissenberg number is constructed; it shows that the transition occurs at Weissenberg numbers on the order of 3–5, which correspond to the characteristic lifetimes of an entanglement junction. At high strain rates, redistribution and local accumulation of entanglements are observed.

Anisotropic electroconducting polymer-silicate composites based on polyaniline

Approaches for the development of anisotropic electroconducting composite materials based on polyaniline and Na-montmorillonite prepared by the me thods of boundary and intercalation polymerization of aniline and mechanical blending are proposed. Parallel plane compression of solid and plasticized dispersions is shown to lead to the development of primarily planar ordering of anisometric clay particles with sorbed or intercalated polymer; as a result, nanocomposites with anisotropic electrical conductivity are formed. In the prepared polymer-silicate films, the parameter of anisotropy in electrical conductivity achieves 6 × 103.

Combining carbon and polymeric particles in an inert fluid as a promising approach to synthesis of nanocomposites

Effective method for introduction of dispersed filler particles into a polymeric matrix was developed. The method is based on disaggregation and uniform deposition of the filler onto the surface of polymeric particles in an inert liquid medium under the action of ultrasound. Particles of detonation nanodiamond were used as the filler.

From capillary to elastic instability of jets of polymeric liquids: Role of the entanglement network of macromolecules

By example of a series of polymers with molecular masses in a wide range, it has been shown that mechanisms of the instability of jets formed by diluted and moderately concentrated polymer solutions are different. A change from the capillary mechanism of breakup of a jet to the phase-separation instability inherent in polymer solutions occurs above the critical value of the dimensionless parameter—the product of the concentration and the intrinsic viscosity. This transition is determined by the conditions of the appearance of the entanglement network, which is also responsible for the possibility of large strains at the extension of a solution. The stability of a jet is achieved only at quite high strain rates at which a transition from the liquid solution to a rubbery-like oriented jet separated from the solution occurs.

Rheological properties and phase behavior of a hydroxypropyl cellulose-poly(ethylene glycol) system

The relaxation and phase behavior of solutions of hydroxypropyl cellulose in poly(ethylene glycols) of various molecular masses has been studied by dynamic mechanical analysis. The dynamic mechanical data are compared with the results of microinterferometry and polarization-microscopy measurements. The combination of optical and mechanical characteristics makes it possible to construct generalized phase- relaxation diagrams for the binary systems under study. Solutions based on lowmolecular-mass poly(ethylene glycol) are characterized by LC equilibrium. With an increase in the molecular mass of poly(ethylene glycol) in a certain temperature-concentration region, amorphous phase separation takes place and the phase diagram is the superposition of LC and amorphous equilibria. The relaxation properties of the systems are sensitive to the phase state and its transformation.