Formation Of Polymer Bridges Research Articles

Colloidal Deposition by Polymer-Surfactant Complexes with Dilution and Shear.

Deposition of silica microparticles on glass substrates was measured as a function of cationic polymer-anionic surfactant composition and shear rate. Particles were initially deposited in quiescent conditions in different polymer-surfactant compositions, which were chosen based on prior measurements of composition-dependent polymer-surfactant interactions and deposition behavior (up to 0.5 wt % polymer and 12 wt % surfactant). Programmed shear and dilution profiles in a flow cell together with optical microscopy observation were used to continuously track particle deposition, detachment, and redeposition. Knowledge of the shear-dependent torque on each particle provides information on adhesive torque mediated by polymer-surfactant complexes. Detachment of colloids initially deposited by depletion interactions occurs at low shear rates (∼100 s-1) due to lack of tangential forces or an adhesive torque. Further dilution produced redeposition of particles that resisted detachment (up to ∼2000 s-1) as the result of strong cationic polymer bridge formation, presumably due to preferential surfactant removal. Dilution from different initial compositions indicates a pathway dependence of polymer-surfactant de-complexation into shear-resistant cationic bridges. These findings demonstrate the ability to program deposition behavior via the informed design of initial polymer-surfactant compositions and shear profiles. The particle trajectory analysis developed in this work provides an assay to screen composition-dependent colloidal deposition in diverse materials and applications.

Designing Stress-Adaptive Dense Suspensions Using Dynamic Covalent Chemistry

The non-Newtonian behaviors of dense suspensions are central to their use in technological and industrial applications and arise from a network of particle–particle contacts that dynamically adapt to imposed shear. Reported herein are studies aimed at exploring how dynamic covalent chemistry between particles and the polymeric solvent can be used to tailor such stress-adaptive contact networks, leading to their unusual rheological behaviors. Specifically, a room temperature dynamic thia-Michael bond is employed to rationally tune the equilibrium constant (Keq) of the polymeric solvent to the particle interface. It is demonstrated that low Keq leads to shear thinning, while high Keq produces antithixotropy, a rare phenomenon where the viscosity increases with shearing time. It is proposed that an increase in Keq increases the polymer graft density at the particle surface and that antithixotropy primarily arises from partial debonding of the polymeric graft/solvent from the particle surface and the formation of polymer bridges between particles. Thus, the implementation of dynamic covalent chemistry provides a new molecular handle with which to tailor the macroscopic rheology of suspensions by introducing programmable time dependence. These studies open the door to energy-absorbing materials that not only sense mechanical inputs and adjust their dissipation as a function of time or shear rate but also can switch between these two modalities on demand.

Three-Step Thermal Drawing for Rapid Prototyping of Highly Customizable Microneedles for Vascular Tissue Insertion.

Microneedles (MNs) have been extensively developed over the last two decades, and highly efficient drug delivery was demonstrated with their minimal invasiveness via a transdermal route. Recently, MNs have not only been applied to the skin but also to other tissues such as blood vessels, scleral tissue, and corneal tissue. In addition, the objective of the MN application has been diversified, ranging from drug delivery to wound closure and biosensing. However, since most MN fabrication methods are expensive and time-consuming, they are inappropriate to prototype MNs for various tissues that have different and complex anatomies. Although several drawing-based techniques have been introduced for rapid MN production, they fabricated MNs with limited shapes, such as thin MNs with wide bases. In this study, we propose a three-step thermal drawing for rapid, prototyping MNs that can have a variety of shapes and can be fabricated on curved surfaces. Based on the temperature control of polymer bridge formation during thermal drawing, the body profile and aspect ratios of MNs were conveniently controlled, and the effect of temperature control on the body profile of MNs was explained. Thermally drawn MNs with different shapes were fabricated both on flat and curved surfaces, and they were characterized in terms of their mechanical properties and insertion into vascular tissue to find an optimal shape for vascular tissue insertion.

Flocculation of a suspension of quartz in the presence of super flocculants of different charge

The influence of cationic and anionic superprocesses and their binary mixtures on the degree of aggregation and the charge of particles of a suspension of quartz. Varied the amount entered in the suspension of the specified reagents, the charge density of the flocculants, the order of introduction of mixture components in the system. The addition of anionic flocculant causes a twofold increase in the potential due to adsorption of negatively charged ions of polymer segments on the negatively charged surface of quartz particles due to non-Coulomb forces. It is shown that the addition of cationic flocculant leads to a significant decrease in the negative values of the particles and a change in the sign of their charge. In the presence of mixtures of anionic and cationic flocculants, regardless of their composition and the order of introduction of the components into the suspension, the particles acquire a negative potential characteristic of the particles adsorbing only anionic flocculant. With an increase in the amount of the added mixture, the flocculation rate and the size of the resulting floccules increase, while a further increase in the concentration of these mixtures causes a decrease in the size of the aggregates. For mixtures of charged anionic and cationic flocculants, the synergism of flocculant action is observed. This effect was observed even for mixtures with multiple excess of negatively charged segments compared to positively charged ones. The observed regularities are explained by the peculiarities of the structure of adsorption layers of flocculant mixtures by the coexistence of a thin layer of adsorbed cationic polymer chains with a large number of contacts with the surface and an extended layer of anionic flocculant (long loops and tails of macromolecules), in which a layer of cationic polymer is “hidden”. The electrokinetic potential and the ability of particles to aggregate (by the mechanism of formation of polymer bridges) in solutions of mixtures of anionic and cationic polyelectrolytes is determined, first of all, by the adsorption value of anionic polymer.

Electrokinetic potential and flocculation of bentonite suspensions in solutions of surfactants, polyelectrolytes and their mixtures

The effects of cationic and anionic surfactants and polyelectrolytes, as well as their binary mixtures, on the electrokinetic potential and the degree of aggregation/sizes of particle flocs in bentonite suspensions have been studied. The amounts of the above reagents, compositions and ratios of positive and negative charges in the mixtures, charge densities of the polyelectrolytes, and the order of the introduction of the components into systems have been varied. It has been shown that the addition of cationic surfactants or polyelectrolytes and mixtures thereof causes a substantial reduction of the negative ζ-potentials of the particles and reversal of the sign of their charge. The addition of anionic surfactants or polyelectrolytes leads to a twofold increase in the ζ-potential as a result of adsorption of negatively charged surfactant ions/polymer segments on negatively charged clay particles via non-Coulomb forces. In the presence of mixtures of anionic and cationic polyelectrolytes, irrespective of their composition and the order of the introduction of the components into a suspension, the particles acquire a negative ζ-potential, which is characteristic of particles that adsorb an anionic polyelectrolyte alone. Synergistic flocculating action is observed for mixtures of weakly and moderately charged anionic and cationic polyelectrolytes. This effect takes place even for mixtures with a manifold excess of negatively charged segments relative to positively charged ones. The observed regularities have been explained by the structural peculiarities of mixed adsorption layers of polyelectrolytes, i.e., the coexistence of a thin layer of adsorbed cationic polymer chains characterized by a large number of contacts with the surface and an extended layer of an anionic polyelectrolyte (long loops and tails of macromolecules), in which the cationic polymer layer is “drowned.” The electrokinetic potential and the ability of the particles to aggregation (via the formation of polymer bridges) in mixed solutions of anionic and cationic polyelectrolytes are primarily determined by the adsorption value of the anionic polymer.

Application of the zeta potential measurements to explanation of colloidal Cr2O3 stability mechanism in the presence of the ionic polyamino acids.

In the presented paper, the influence of the molecular weight and the type of polyamino acid functional groups on the electrokinetic properties and the stability of chromium (III) oxide suspension were examined. Analysis of the data obtained from the adsorption, potentiometric titration, zeta potential, and stability measurements allows to propose stabilization or destabilization mechanism of the studied systems. In the studies, there were used polyamino acids with different ionic characters: anionic polyaspartic acid and cationic polylysine. The measurements showed that the zeta potential depends on the concentration and molecular weight of the applied polymer. Stability of the chromium (III) oxide suspensions in the presence of ionic polyamino acids increases compared to the results obtained in the absence of polymers. The exception is LYS 4,900 at pH = 10. Under these conditions, the decrease in stability is observed due to formation of polymer bridges between the polymer chains adsorbed on different colloidal particles. Determination of the stabilization/destabilization mechanism of the polyamino acid/chromium (III) oxide system and examination of the effects of polymer molecular weight on the stabilization properties can contribute to a wider use of this group of compounds as potential stabilizers or flocculants in many industrial suspensions.

The Dynamics of Polymer Bridge Formation and Disruption and Its Effect on the Bulk Rheology of Suspensions

Bridge-flocculated colloidal gels are used in many important processes and products such as gel casting for advanced ceramics, precursor inks for 3D printing, and waste treatment strategies. An important aspect of polymer bridged gels that makes them excellent candidates for these applications is the precise control it affords for control of rheological properties. Recent studies have shown that adhesion between bridged surfaces increases with time as the number of polymer bridges formed grows. However, the consequences of the dynamics of these processes toward bulk rheological properties have not been studied. Here we investigate the link between the dynamics of polymer bridging and disruption and bulk rheology in dense colloidal silica particle suspensions flocculated by polyethylene oxide (PEO). Microscale pull-off force measurements using atomic force microscope (AFM) show that upon repeated disruption and establishment of bridged contact, the adhesion between the surfaces is reduced. During contact disruption, the polymer chains bridging the two surfaces are stretched leading to chain scission. On the re-establishment of contact, these fragmented polymer chains are unable to fully re-establish the adhesion. Macroscale measurements using oscillatory rheology show that this reduced adhesion results in reduction of both the storage modulus and the yield stress. If the slurry is subjected to high shear for long periods, polymer chain scission is amplified, and the fragmented polymer chains are unable to bridge the particles again, resulting in free-flowing slurries.

Effect of Cationic Polymers on Foam Rheological Properties

We study the effect of two cationic polymers, with trade names Jaguar C13s and Merquat 100, on the rheological properties of foams stabilized with a mixture of anionic and zwitterionic surfactants (sodium lauryloxyethylene sulfate and cocoamidopropyl betaine). A series of five cosurfactants are used to compare the effect of these polymers on foaming systems with high and low surface dilatational moduli. The experiments revealed that the addition of Jaguar to the foaming solutions leads to (1) a significant increase of the foam yield stress for all systems studied, (2) the presence of consecutive maximum and minimum in the stress vs shear rate rheological curve for foams stabilized by cosurfactants with a high surface modulus (these systems cannot be described by the Herschel-Bulkley model anymore), and (3) the presence of significant foam-wall yield stress for all foaming solutions. These effects are explained with the formation of polymer bridges between the neighboring bubbles in slowly sheared foams (for inside foam friction) and between the bubbles and the confining solid wall (for foam-wall friction). Upon addition of 150 mM NaCl, the effect of Jaguar disappears. The addition of Merquat does not noticeably affect any of the foam rheological properties studied. Optical observations of foam films, formed from all these systems, show a very good correlation between the polymer bridging of the foam film surfaces and the strong polymer effect on the foam rheological properties. The obtained results demonstrate that the bubble-bubble attraction can be used for efficient control of the foam yield stress and foam-wall yield stress, without significantly affecting the viscous friction in sheared foams.

Kinetics and mechanism of flocculation of bentonite and kaolin suspensions with polyelectrolytes and the strength of floccs

The effects of cationic (Zetag 7589, Zetag 8660, and SNF 528) and anionic (Magnafloc 1011) polyelectrolytes, as well as the regimes and intensity of suspension stirring, on the kinetics of formation, disruption, and subsequent regrowth of aggregates of bentonite and kaolin particles, are studied in a flow system. The optimum polymer doses and the stirring conditions providing the formation of largest and strongest floccules are found. Under comparable conditions, the effect of the adsorption of the above polymers on the electrokinetic potential and the degree of aggregation of particles is studied. It is shown that intense flocculation takes place long before the isoelectric point of particles is reached. This allows one to conclude that the formation of polymer bridges plays a significant role in the flocculation of the examined suspensions with polyelectrolytes. Different dependencies of flocculation on the dose of added polyelectrolytes have been found for diluted and concentrated suspensions. This is explained by different extents to which flocculants adsorbed on the particle surface approach the equilibrium state at different concentrations of dispersed phases.

Dynamics of polymer bridge formation and disruption

In this Rapid communication, we show, with colloidal probe atomic force microscopy(AFM) measurements, that the formation and subsequent disruption of polymer bridges between two solid surfaces is characterized by slow relaxation times. This is due to the retardation of polymer dynamics near a surface. For colloidal particles that are in constant (Brownian) motion kinetic aspects are key. To understand these effects, we develop a model of polymer bridging and bridge disruption that agrees quantitatively with our experiments.

Time and Distance Dependence of Reversible Polymer Bridging Followed by Single-Molecule Force Spectroscopy

Polymer bridging between surfaces plays an important role in a range of fundamental processes in the material and life sciences. Bridges formed by main-chain reversible polymers differ from their covalent analogs in that they can dynamically adjust their size and shape in response to external stimuli and have the potential to reform following bond scission. In this work, the time and distance dependence of main-chain reversible polymer bridge formation are studied using an atomic force microscope. The bridging process was studied using single-molecule force spectroscopy, and its dependence on the distance between surfaces and equilibration time was probed. The number of bridges formed decreases as the gap width increases, from approximately 2 bridges per 14 s equilibration at separations of 5-15 nm to approximately 0.5 bridges per 14 s equilibration at separations of 35-45 nm. The kinetics of bridge formation appear to be slightly faster at smaller separations.

Rheological Properties of UV-curable Flexographic Ink (II) Influence of Interfacial Interactions in Relation to Rheological Variables of Ink and its Printability

The rheological properties of UV flexographic (flexo) inks were investigated. UV flexo suspensions of carbon black in liquid medium containing various binding materials, such as acylate pre-polymers, di-/multi-functional monomers, and diluents, were used as sample inks. The inks were characterized on a rheometer in terms of steady and dynamic behaviors. The rheological properties of polymer-pigment systems are a function of some interfacial parameters that characterize the degree of affinity of pigment surfaces to the polymer medium. The addition of pigments into liquid medium introduces polymer-pigment junctions besides the polymer-polymer junctions due to entanglement. These newly created junctions arise from the sorption of pigments to the polymer chains. In this study, we found that the frequency dependence of storage modulus G' at a certain strain amplitude is proportional to the number of acryloyl groups in the multi-functional monomer (TMPTA<PETTA<<DPHA). The chemical difference due to chain length (TMPTA<GPTA) is another factor affecting the rheological properties of UV flexo ink. In addition, we surmise that a transient network structure is formed by mechanical interactions that are induced by the addition of high concentrations of small solid particles such as carbon black to a polymer or a multi-phase solution. In such a system, the particle chains form the network structure, although the formation of polymer bridges by adsorbing to adjacent particles is another possibility.

Conformations of Block Copolymers Terminally Adsorbed at the Solid-Liquid Interface

Abstract The conformations of end-adsorbed diblock and mblock copolymers of polystyrene (PS) and polyethylene oxide (PEO) adsorbed onto mica from toluene have been studied by the surface force method. The PS-PEO diblock copolymer system forms a semi-dilute brush with the non-adsorbing PS chains adopting an extended configuration away from the surface. Neutron reflectometry indicates that the volume fraction profile of such a system has a broadly parabolic shape, in agreement with theory. In contrast to PS-PEO diblock copolymers which adsorb only in a “tail” conformation, PEO-PS-PEO triblock chains form “loops” as well as “tails” on the mica substrate. Attractive forces are observed when an adsorbed layer of PEO-PS-PEO triblock copolymer is allowed to interact with a bare mica wall, due to the formation of polymer bridges between the two opposing surfaces.

Mechanical Strength of Clay Minerals as Influenced by an Adsorbed Polysaccharide

AbstractThe water‐stabilizing action of polysaccharides on soil aggregates has been ascribed to the binding together of soil particles by polymer bridges. This study was conducted to investigate the ability of polysaccharides to increase the strength of interparticle bonds by measuring the mechanical strength of clay‐polysaccharide complexes. Kaolinite, montmorillonite, and the polysaccharide scleroglucan were used. Cores were prepared from dilute suspensions of the complexes, air dried, and manufactured either into small briquets or into spherical aggregates to measure respectively the modulus of rupture and the tensile strength. The polysaccharide increased the mechanical strength of both clay minerals. The modulus of rupture of montmorillonite increased from 265 to 580 × 105 Pa at the maximum adsorption of scleroglucan, which was 140 g kg−1 clay. In the case of kaolinite, modulus of rupture measurements indicated that particle‐orientation effects were small and could be neglected. The tensile strength of kaolinite increased from 1 to 20.6 × 105 Pa at the maximum adsorption of scleroglucan (29.9 g kg−1 clay). In this case, the mechanical properties were not due to changes in the clay microorganization, since scleroglucan affected neither the fabric nor the porosity of the kaolinite. Increases in strength were thus intercepted as the formation of polymer bridges between the clay particles and were related to the progressive coverage of the surface area of the clay by the polysaccharide. We concluded that the aggregating action of polysaccharides is predominantly due to an increased strength of interparticle bonds.

Interactions between colloidal particles in simple shear flow

Recent progress in experimental and theoretical developments dealing with colloidal. interactions between two spheres in shear flow is reviewed. A systematic comparison is made between spheres suspended in simple electrolyte and in cationic polyelectrolye solutions. Microrheological observations, performed with the traveling microtube, make an in-depth investigation possible of the colloidal forces and the mechanisms of polymer bridge formation. Reasons are discussed for the often-observed aging of colloidal aggregates. Finally, coagulation rates are presented for systems with weak and strong Brownian motion. It is shown that the often-used assumption of additivity of the ortho- and perikinetic coagulation rates is incorrect.