A. Ya. Malkin

Some conditions for rupture of polymer liquids in extension

Rupture is one of the least investigated and least understood features of the rheological behaviour of polymeric liquids. Some key experimental results on the rupture of polymer melts in uniaxial extension are discussed. Three features of experimental results for narrow molecular weight distribution (MWD) polymers may be highlighted. Firstly, steady elongational flow becomes impossible and rupture of the liquid filament occurs when and if the stored elastic (Hencky) strain reaches 0.5 units. This can be interpreted in terms of a critical Weissenberg number. Secondly, at higher rates of strain (when the elastic strain becomes more than 0.5) the relationship between limiting stress and elastic strain (at the breaking point) is linear. In this case the limiting elastic strain can become at least as high as 2. Thirdly, the strength of a fluid polymer is not a constant but may be characterised by the lifetime, or durability, t* of the filament. This leads to an empirical criterion for rupture: M=(σ3t*/η)=0.30±...

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.

Fracture of ultrafine fibers in the flow of mixtures of non-newtonian polymer melts

Abstract Experimental and theoretical investigations into the fracture of liquid streams resulting from the coextrusion of mixtures of polymer melts are generalized. It is shown that the fracture of a liquid cylinder (fiber) for viscoelastic polymer systems differs from the known theoretical predictions concerning Newtonian fluids. This is expressed by the following facts: 1. (a) the wavelength of the destructive disturbance and the size of the resulting drops for polymer systems are greater than for Newtonian fluids; 2. (b) the growth of amplitude of the destructive wave slows down at the final stages of fracture; 3. (c) theoretical and experimental values of the lifetime of a polymer stream differ by 2–3 orders. These discrepancies between experiment and theory are due to non-Newtonian effects caused by the elasticity of polymer melts. Ultrafine fibers produced during the flow of mixtures of polymer melts through dies are maximally unstable if the viscosity ratio of the initial polymer melts is close to unity. The relation between the lifetime of a stream and the ratio of viscosities of both phases is determined by the matrix viscosity. The relative radius of the drops which results from microfiber fracture is independent of the chemical nature of mixed polymers and equals 2.2.

The rheology of binary mixtures of highly concentrated emulsions: Effect of droplet size ratio

Binary mixtures of highly concentrated emulsions (HCE) with three droplet size ratios and different compositions were prepared. It was found that by the proper selection of droplet size ratio and composition of binary mixtures, the shear modulus, viscosity, yield stress, and yield strain can be dropped lower than mixing rules and even primary HCE. This effect is similar to what is known for dispersions with volume fraction less than 0.7 but has not been described for HCE. For such formulations, the caged mechanism of droplets dynamics is not dominant due to the provided free volume that can be occupied by smaller droplets during flow. This is originated from the increase in maximum closest packing and thus more efficient spatial packing. By studying the scaling behavior of shear modulus and yield stress, the significance of interdroplet interaction was distinguished.

A new mechanism of aging of highly concentrated emulsions: Correlation between crystallization and plasticity

The aging of highly concentrated w/o emulsions is studied upon variations in their compositions and the concentration of the dispersed phase. The dispersed phase consists of a supercooled aqueous solution of nitrate salts. The aging leads to an increase in the rigidity of a composition: the elastic modulus and yield stress increase, and the flow curve shifts toward higher viscosities. The evolution of emulsion structure during aging is studied by X-ray diffraction. It is shown that the instability of emulsions is due to the slow crystallization of the droplets of the dispersed phase that results in an increase in the yield stress. It is found that there is a direct correlation between the degree of crystallinity and the relative increase of the yield stress as a measure of the structure strength. The main result of the investigation is experimental evidence that there is a quantitative relation between the degree of crystallinity and an increase in the yield stress reflecting the transition from emulsion to suspension. An empirical equation describing this relation is proposed.

The role of interdroplet interaction in the physics of highly concentrated emulsions

The osmotic pressure and shear modulus of highly concentrated emulsions were modelled by considering both interfacial energy and interdroplet interaction. This was performed for two- and three-dimensional cases and by optimization and approximation methods of predicting film thickness. The results show that even a small source of interaction can result in non-superimposition of scaled osmotic pressure and shear modulus by Laplace pressure for different droplet sizes, and also significant deviation from the models which consider interfacial interaction as the sole source of energy. The model was used to explain the reciprocal squared diameter dependency of elastic modulus: an interaction similar to the van der Waals type can be responsible for this observation. The model can also be used to analyze the interdroplet interactions in highly concentrated emulsions.

Master curves for elastic and plastic properties of highly concentrated emulsions

Critical comparison of dependences of elastic and plastic properties of highly concentrated emulsions (so-called “compressed” emulsions) on the concentration and droplet sizes is performed. The studied emulsions of water-in-oil type are so-called “liquid explosives.” They are characterized by different mean sizes and different droplet size distributions of the dispersed phase. Different average values (Dav, D32, and D43) are used as characteristics of droplet sizes. Experiments are carried out with emulsions of two concentrations. Aqueous phase (dispersed droplets) is presented by supercooled solutions of inorganic salt in water in a metastable state. The concentration limit of the existence of highly concentrated emulsions is determined by the condition of the closest packing of liquid droplets, which lies in the φ* = 0.77–0.80 range. In addition, there is a limiting value of the maximal size of droplets. This limiting value depends on the concentration and meets the requirement that droplets should be small enough for the solution to exist in a supercooled state. The elastic modulus and the yield stress of emulsions studied are proportional to the square of the reciprocal linear size of droplets, which contradicts some theoretical models, according to which these parameter should be proportional to the reciprocal size of droplets. Using the obtained experimental data, we constructed generalized dependences of the elastic modulus and the yield stress on the concentration and size of droplets. These characteristics are in good agreement with the experimental data.

Flow with impregnation of a rheokinetic liquid

Abstract Flow with impregnation of a porous layer for a special class of non-Newtonian liquids is discussed. The particular feature of the rheological properties is the assumption that the viscosity of a liquid can change due to a chemical reaction. Change in the degree of conversion is described by a standard kinetic equation and the dependence of viscosity on the degree of conversion is written by means of an exponential equation. Moreover, it is assumed that, when approaching some critical degree of conversion, the viscosity grows without limit, i.e. chemical “curing” of the liquid takes place. Flow of such a “rheokinetic” liquid along a plane feeding channel with simultaneous impregnation of a porous layer in contact with this channel is simulated by a system of balance equations (taking into account non-Newtonian effects provided by time-dependent viscosity), supplemented by a kinetic equation. This system of equations is rewritten and solved in a dimensionless form. The principle possible solutions are obtained, including the situation where—due to premature loss of fluidity—a liquid cannot completely impregnate a porous layer. An approximate relationship determining the condition of complete impregnation is formulated.

IR Studies of Interfacial Interaction of the Succinic Surfactants with Different Head Groups in Highly Concentrated W/O Emulsions

The interfacial properties of three succinic surfactants (PIBSA) with different hydrophilic end groups and sorbitan monooleate (SMO) in water-in-oil emulsions of the liquid explosive type were studied. The aqueous phase contained 40% of ammonium nitrate (AN). The trend in equilibrium interfacial tension was found to be PIBSA-MEA > PIBSA-UREA > PIBSA-MEA/SMO mixture > PIBSA-IMIDE > SMO, where MEA, UREA, IMIDE are amid/amide, urethane and imide end groups, respectively. The same trend was observed for the interfacial elastic modulus. The FTIR study revealed interactions between surfactant head groups and an AN solution. The interactions depend on the polarity of head groups determined by their chemical structure. The packing efficiency of the surfactants under study is also influenced by the chemical structure of head groups. Attempts to model the conformation of the surfactants at the interface were made. The investigation of mixed interfacial cover (PIBSA-MEA/SMO) showed that SMO remained at the interface reducing the interfacial tension at the W/O interface. It was also demonstrated that variation of the nature of head groups influences the rheological properties of emulsions. Bulk elasticity and yielding correlate with interfacial interaction characteristics.

A method for monitoring polymer reactions in very dilute solutions

Abstract Physical and/or chemical transformations in polymer systems can be studied by different analytical methods. However, most of them are applicable to systems of rather high concentration of polymer, or that takes long periods to prepare samples. In this paper, the Toms effect (drag reduction (DR) in adding very small amounts of a polymer to a solvent if flow takes part in a turbulent regime) is proposed as a method suitable for monitoring chemical reactions of polymers in very dilute solutions (down to tens of ppm). Then, by measuring the Toms effect, there is the possibility of observing slight changes occurring in the course of a reaction taking part in a dilute solution. This publication deals with two typical polymer–polymer transformations — the formation of stereocomplexes between iso- and syndio-tactic poly(methyl methacrylate)s (PMMAs), as an example of physical transition, and denaturating of DNA, as an important chemical reaction.