Rammile Ettelaie

The rheology of solutions of associating polymers: Comparison of experimental behavior with transient network theory

The properties of aqueous solutions of model HEUR associative thickeners under dynamic and steady shear have been studied as a function of concentration, molecular weight, temperature, and hydrophobic end‐cap length. It is shown that solutions of AT behave as near perfect Maxwell fluids inasmuch that Cole–Cole plots of the dynamic moduli are almost exactly semi‐circular. An Arrhenius law temperature dependence of the static viscosity and relaxation time is also observed, providing confirmation of a single relaxation process. In certain other respects, AT solutions show more complex behavior, e.g., the Cox–Merz rule is not obeyed, with the steady shear viscosity showing a weaker dependence on shear rate than does the complex viscosity upon frequency. Furthermore, weak shear thickening is seen to precede shear thinning in steady shear. The above results are consistent with the predictions of a transient network theory presented recently by Tanaka and Edwards and Jenkins (generalized Green–Tobolsky theory). ...

Network formation and its consequences for the physical behaviour of associating polymers in solution

The physical properties of hydrophobically modified polyurethane thickeners have been studied in solution. Such polymers form transient networks via association of their hydrophobic end groups. The rheological response of such systems is characterized by a Maxwellian viscoelastic response. It is shown that this is consistent with a picture in which stress relaxation occurs via disengagement of a chain end from an association followed by rapid Rouse relaxation of the stressed chain. This mechanism in principle allows the preparation of fluids with designed relaxation spectra. In this way it is possible to control the viscosity/shear-rate profile of a system thickened with such macromolecules. The concentration dependence of the shear modulus of such solutions can be described by a simple model in which there is an interplay between chains taking up loop and link configurations. A theory developed on such principles shows excellent agreement with experiment. This model is also shown to shed light on the interaction of associating polymers with surfactants. The phase behaviour of associating polymer mixed with unmodified homopolymer of the same monomer composition is also discussed. The observed phase diagram is well-described by a modified form of Flory-Huggins theory to account for the associations between the hydrophobic end groups of the associating polymer (AT) chains. It is argued that phase separation is driven by entropic rather than enthalpic considerations. This paper highlights the fact that many of the physical features of AT can be accounted for by a simple picture of the network topology combined with an understanding of the mechanism of stress relaxation. It is shown that the capability of association between chains has considerable implications regarding the physical response of polymer solutions.

Electrical double layer interactions for spherical charge regulating colloidal particles

Abstract A new way of linearising the Poisson-Boltzmann equation is presented which is capable of producing accurate answers for moderately high surface charge and surface potentials. Unlike the Debye-Huckel approximation, the linearisation is based on the fact that electric potential variation, rather than the magnitude of the potential itself, remains small between two charged plates at close separation. The method can be applied quite generally to any type of surface. The cases of constant surface charge, the constant surfaceppotential and charge regulating systems are considered as examples. The results for constant charge and potentials as high as 5 μC/cm 2 and 200 mV are within 7% of the exact answers for plate separations of one Debye length or less. In contrast, the Debye-Huckel approximation produces answers that are at least an order of magnitude too large, for the same problems. The application of the method to the problem of calculating the electrostatic forces between charge regulated spherical particles is also addressed in some detail.

Kinetic study of κ-carrageenan degradation and its impact on mechanical and structural properties of chitosan/κ-carrageenan film

The purpose of the current research was to study κ-carrageenan degradation behavior under thermal treatment, and its influence on chitosan κ-carrageenan film properties. A pseudo-first-order reaction equation was applied by using reciprocal plots of κ-carrageenan molecular mass versus heating time, which showed a strong dependence on heating time. Incorporation of thermally treated κ-carrageenan into the chitosan had diminished both water resistance and water vapor permeability of the blend, in contrast to those for intact or untreated κ-carrageenan. A dramatic decrease of equilibrium moisture content and tensile strength were noticed, and these parameters were more affected by the longer times. Furthermore, the contact angle of the films was found to be a function of the heating time. Scanning electron microscopy revealed apparent agglomeration of κ-carrageenan through the thermal process. Atomic force microscopy demonstrated that the intact blend had the flattest surface, whilst the blend containing treated κ-carrageenan had high roughness.

Computer simulation and modeling of food colloids

Recent developments in theoretical modeling and computer simulations of colloidal systems of potential relevance to foods are reviewed. Attention has been focused on those areas where simulation studies have provided major new insight, complementary to experimental results. These include formation, structure and mechanical properties of colloidal gels; viscoelastic phase separation and transient networks; competitive displacement of protein layers from interfaces and the problem of disproportionation in foams and bubble dispersions. The usefulness of many conclusions emerging from the literature reviewed here points to an increased potential use of modeling and computer simulation methods in the study of food colloidal systems in the future.

Growth and aggregation of surfactant islands during the displacement of an adsorbed protein monolayer: a Brownian dynamics simulation study

We present Brownian dynamics simulations of the displacement of a protein monolayer by competitive adsorption. The protein film is modelled as a network of spherical bonded particles adsorbed at a fluid interface. Spherical displacer particles, which have a stronger affinity for the interface than the protein film particles, are introduced into the system through the sub-phase. At early stages, these particles diffuse to the interface and are adsorbed in the gaps in the network. Soon thereafter, however, further adsorption initiates displacement of the film particles, ultimately leading to the complete removal of the protein layer from the surface. We study the evolution of the number and size of the displacer islands formed at the interface. The introduction of a direct long-range repulsion between film and displacer particles is shown to lead to a phase-separation-type behaviour at intermediate time scales. Further comparisons with the displacement of a non-bonded monolayer are also presented.

Mixed protein–polysaccharide interfacial layers: a self consistent field calculation study

Mixed interfacial films of protein and polysaccharide have been investigated using self consistent field (SCF) calculations. The colloidal interactions mediated by such composite layers between two approaching surfaces have been considered. Two types of systems have been studied: (a) covalently-bonded polysaccharide and protein, and (b) the excess presence of polysaccharide at the interface, occurring through electrostatic interaction with an already existing, oppositely charged, protein layer. Our calculations show that for covalently-bonded complexes, depending on the location of the protein-polysaccharide bond, the attachment of short uncharged chains to the protein can be detrimental to provision of repulsive colloidal forces by such complexes. We have attributed this to an increased tendency of the hybrid polymer to adopt bridging configurations in the gap between two nearby surfaces. For larger grafted chains this bridging effect is eliminated, and the expected enhanced steric stabilization of the protein-polysaccharide conjugate is achieved. For adsorbed films formed through electrostatic interactions between these two biopolymers, stronger repulsive forces between the surfaces are produced, at an intermediate level of charging for the polysaccharides. This has been related to a maximum level of adsorption of polysaccharide, as the number of charged segments of the chain is varied. The peak occurs at higher levels of charging as the salt concentration in the bulk solution is increased. We have also observed the experimentally-reported phenomenon of charge overcompensation, arising from adsorption of the polysaccharide chains onto the primary protein layer. The importance of the non-uniform charge distribution of the polysaccharide molecule, in providing an explanation for this effect, has been demonstrated.

Interactions between adsorbed layers of αS1-Casein with covalently bound side chains : a self-consistent field study

The effect on the adsorbed layer properties of the modification of alpha S1-casein by covalent bonding with an uncharged polysaccharide side chain has been investigated using lattice-based self-consistent field (SCF) theory. Interactions between two hydrophobic planar surfaces coated by a layer of adsorbed modified alpha S1-casein have been studied as a function of pH and ionic strength. While the interactions of the unmodified alpha S1-casein layers become attractive at high ionic strength, it has been shown that the presence of polysaccharide attachment to the alpha S1-casein molecule can confer net repulsive interactions over a wide range of salt concentration. The disordered protein is represented as a linear flexible polyampholyte with a sequence of hydrophobic, polar, and charged units based on the known alpha S1-casein primary structure. The hydrophilic side chain is attached at various fixed positions along the casein backbone. Different lengths and locations of the attached polysaccharide side chain are examined. Interfacial structures and colloidal stability properties of the system are determined, including the surface-surface interaction potential, the extent of protein bridging, and the distribution of protein segments from the surface under different conditions of pH and ionic strength. It has been found that the covalent bonding of short hydrophilic chains may not only enhance but can also worsen the colloidal stabilizing properties of the modified protein, depending on the position of the attachment.

Characterization of physical properties of tissue factor containing microvesicles and a comparison of ultracentrifuge-based recovery procedures

Microvesicles were isolated from the conditioned media of 3 cell lines (MDA-MB-231, AsPC-1 and A375) by ultracentrifugation at a range of relative centrifugal forces, and the tissue factor (TF) protein and activity, microvesicle number, size distribution and relative density compared. Also, by expressing TF-tGFP in cells and isolating the microvesicles, the relative density of TF-containing microvesicles was established. Nanoparticle tracking analysis (NTA) indicated that the larger-diameter microvesicles (>200 nm) were primarily sedimented at 100,000g and possessed TF-dependent thrombin and factor Xa generation potential, while in the absence of factor VII, all microvesicles possessed some thrombin generation capacity. Immuno-precipitation of TF-containing microvesicles followed by NTA also indicated the range of these microvesicles to be 200–400 nm. Analysis of the microvesicles by gradient density centrifugation showed that lower-density (<1.1 g/ml) microvesicles were mainly present in the samples recovered at 100,000g and were associated with TF antigen and activity. Analysis of these fractions by NTA confirmed that these fractions were principally composed of the larger-diameter microvesicles. Similar analysis of microvesicles from healthy or patient plasma supported those obtained from conditioned media indicating that TF activity was mainly associated with lower-density microvesicles. Furthermore, centrifugation of healthy plasma, supplemented with TF-tGFP-containing microvesicles, resulted in 67% retrieval of the fluorescent microvesicles at 100,000g, but only 26% could be recovered at 20,000g. Pre-centrifugation of conditioned media or plasma at 10,000g improved the speed and yield of recovered TF-containing microvesicles by subsequent centrifugation at either 20,000g or 100,000g. In conclusion, TF appears to be associated with low-density (1.03–1.08 g/ml), larger-diameter (200–350 nm) microvesicles.

Steric interactions mediated by multiblock polymers and biopolymers: role of block size and addition of hydrophilic side chains

Abstract To a first approximation, the primary structure of many food proteins maybe thought of as a sequence of short hydrophobic and hydrophilic blocks. The influence of this type of structure on the steric-stabilising properties of such proteins has been considered here. In line with previous studies, using Self-Consistent-Field calculations, it has been shown that the presence of such protein molecules can lead to attraction and consequently bridging flocculation of colloidal particles. In the low adsorption energy limit for the hydrophobic groups (∼−1kBT), it is found that the steric potential is significantly influenced by the changes in the number of adsorbed segments, as two surfaces are brought together. This is in contrast to the well-known results in the literature for the high adsorption limiting cases, where the number of such segments remains constant. In particular, the changes in the number of adsorbed hydrophobic units are observed not to be a monotonic function of the separation distance, but increase or decrease in reasonable accord with the oscillatory nature of the steric interactions, observed for various block sizes. Effects of the addition of a moderately sized hydrophilic side chain to the above molecules have also been studied. It is found that, in principle, such a modification can lead to a purely repulsive steric potential in solutions of these hybrid biopolymers. At the hydrophilic side chain sizes considered here, the surface affinity of the molecules is observed not to be drastically different compared to those of unmodified proteins.