Biomedical subjects

M A Cohen Stuart

Publications and source records attributed to M A Cohen Stuart.

15 recordsLinked to original sources

PubMed2007

Rheology of a reversible supramolecular polymer studied by comparison of the effects of temperature and chain stoppers.

The rheology of a reversible supramolecular polymer is studied by comparing the effects of an increase in temperature and the addition of chain stoppers. The dependence of the zero-shear viscosity and the terminal relaxation time on temperature is exponential, and the activation energy for viscous flow can be calculated. Above a critical stopper fraction, power laws describe the stopper dependence of the viscosity and relaxation time. A simple model for the effect of the addition of chain stoppers on the average degree of polymerization adequately describes the results. A comparison of flow curves at several temperatures and stopper fractions reveals considerable differences between solutions with the same zero-shear viscosity. These are mainly associated with differences in the terminal relaxation time. A mechanism of shear-induced alignment and subsequent elongation of chains is proposed, with which the experimental results are consistent.

Biopolymers↗

PubMed2006

On the binding of calcium by micelles composed of carboxy-modified pluronics measured by means of differential potentiometric titration and modeled with a self-consistent-field theory.

We perform differential potentiometric titration measurements for the binding of Ca2+ ions to micelles composed of the carboxylic acid end-standing Pluronic P85 block copolymer (i.e., CAE-85 (COOH-(EO)26-(PO)39-(EO)26-COOH)). Two different ion-selective electrodes (ISEs) are used to detect the free calcium concentration; the first ISE is an indicator electrode, and the second is a reference electrode. The titration is done by adding the block copolymers to a known solution of Ca2+ at neutral pH and high enough temperature (above the critical micellization temperature CMT) and various amount of added monovalent salt. By measuring the difference in the electromotive force between the two ISEs, the amount of Ca2+ that is bound by the micelles is calculated. This is then used to determine the binding constant of Ca2+ with the micelles, which is a missing parameter needed to perform molecular realistic self-consistent-field (SCF) calculations. It turns out that the micelles from block copolymer CAE-85 bind Ca2+ ions both electrostatically and specifically. The specific binding between Ca2+ and carboxylic groups in the corona of the micelles is modeled through the reaction equilibrium -COOCa+ <==> -COO- + Ca2+ with pKCa = 1.7 +/- 0.06.

Journal Article↗

PubMed2006

Long-range depletion forces induced by associating small molecules.

This is the first report of experimental observations of depletion interactions in solutions of a (hydrogen-bonded) reversible supramolecular polymer. Depletion forces were measured directly by colloidal probe atomic force microscopy. The range of the depletion force is consistent with existing independent experimental data. The interaction can be tuned by adding monofunctional chain stoppers to the solution, a possibility which is unique to supramolecular polymers. The depletion force is shown to be strong enough to induce phase separation in a colloidal suspension.

Colloids↗

PubMed2006

Thinning of wetting films formed from aqueous solutions of non-ionic surfactant.

We investigated the thinning of wetting films formed from aqueous solution of non-ionic triblock copolymer Pluronic F127 on the surface of silica using a home-made thin film balance and time-resolved ellipsometry. Imaging ellipsometry was used to visualize the film structures at subsequent stages of their development. The results unambiguously show that the time required for the formation of steady films strongly depends on the electrolyte concentration. When increasing the latter from 10(-4) to 0.1 M, this time typically increases with several orders of magnitude, from a few minutes to several hours. Moreover, for sufficiently large amounts of salt, two characteristic relaxation regimes can be clearly identified. After initial quick thinning, further thinning slows down enormously. These typical kinetic regimes are thought to result from the coupled dependencies of the bulk and interfacial properties of F127 on salt concentration. Possible explanations of the phenomenon are discussed.

Journal Article↗

PubMed2006

Why surfaces modified by flexible polymers often have a finite contact angle for good solvents.

Polymers adsorbing from a dilute solution onto the solvent-vapor interface generate a nonzero surface pressure. When the same polymers are end-grafted onto a surface such that a so-called polymer brush is formed, one will find that the solvent wets this compound interface partially. The partial wetting and the finite surface pressure are intimately linked properties of the polymer-solvent-vapor combination. It is shown that the spreading parameter in the wetting problem is proportional to the surface pressure in the adsorption case. Complete wetting is only possible when this surface pressure is nonpositive. The wetting characteristics are hardly influenced by the grafting density and chain length characterizing the brush. We argue that the grafted polymer chains can bridge to the solvent-vapor interface, thereby preventing the wetting film to become macroscopically thick. We present experimental data underpinning our self-consistent field analysis. Indeed, finite contact angles should be expected in various systems in which bridging attraction contributes to the disjoining pressure in wetting films.

Journal Article↗

PubMed2006

Self-consistent-field analysis of the micellization of carboxy-modified poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymers.

The micellization properties of carboxy-modified Pluronics P85 (poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers) are investigated by means of a molecularly realistic self-consistent-field theory. We consider the, so-called, carboxylic acid end-standing P85 (CAE-85) case where the carboxylic group is located at the end of both PEO parts and the carboxylic acid center-standing P85 (CAC-85) case where each of the carboxylic group presents between the PEO and PPO blocks. The micellization of these copolymers depends on the pH, the added electrolyte concentration phis, and the temperature. It is shown that the aggregation number (Nagg) decreases, whereas the critical micellization concentration (CMC) increases with pH. For the case of increasing phis, the Nagg increases and the CMC decreases. The critical micellization temperature (CMT) and cloud point temperature (CPT) increase with pH at low phis and decrease at increasing phis. The changing from CAE-85 to CAC-85 leads to increasing CMC and CMT, but lower CPT.

Journal Article↗

PubMed2005

BSA adsorption on bimodal PEO brushes.

BSA adsorption onto bimodal PEO brushes at a solid surface was measured using optical reflectometry. Bimodal brushes consist of long (N=770) and short (N=48) PEO chains and were prepared on PS surfaces, applying mixtures of PS(29)-PEO(48) and PS(37)-PEO(770) block copolymers and using the Langmuir-Blodgett technique. Pi-A isotherms of (mixtures of) the block copolymers were measured to establish the brush regime. The isotherms of PS(29)-PEO(48) show hysteresis between compression and expansion cycles, indicating aggregation of the PS(29)-PEO(48) upon compression. Mixtures of PS(29)-PEO(48) and PS(37)-PEO(770) demonstrate a similar hysteresis effect, which eventually vanishes when the ratio of PS(37)-PEO(770) to PS(29)-PEO(48) is increased. The adsorption of BSA was determined at brushes for which the grafting density of the long PEO chains was varied, while the total grafting density was kept constant. BSA adsorption onto monomodal PEO(48) and PEO(770) brushes was determined for comparison. The BSA adsorption behavior of the bimodal brushes is similar to the adsorption of BSA at PEO(770) monomodal brushes. The maximum of BSA adsorption at low grafting density of PEO(770) can be explained by ternary adsorption, implying an attraction between BSA and PEO. The contribution of primary adsorption to the total adsorbed amount is negligible.

Adsorption↗

PubMed2005

Influence of NaCl on the behavior of PEO-PPO-PEO triblock copolymers in solution, at interfaces, and in asymmetric liquid films.

The solution behavior of the polymeric surfactant Pluronic F127 (PEO(99)PPO(65)PEO(99)) and its adsorption behavior on aqueous-silica and aqueous-air interfaces, as well as the disjoining pressure isotherms of asymmetric films (silica/aqueous film/air) containing F127, are studied. The interfacial properties of adsorbed F127 layers (the adsorbed amount Gamma and the thickness h) as well as the aqueous wetting film properties [film thickness (h) and refractive indexes] were studied via ellipsometry. The solution properties of F127 were investigated using surface tensiometry and light scattering. The interactions between the air-water and silica-water interfaces were measured with a thin film pressure balance technique (TFB) and interpreted in terms of disjoining pressure as a function of the film thickness. The relations between the behaviors of the asymmetric films, adsorption at aqueous air, and aqueous silica interfaces and the solution behavior of the polymeric surfactant are discussed. Special attention is paid to the influence of the concentrations of F127 and NaCl. Addition of electrolyte lowers the critical micelle concentration, diminishes adsorption on silica, and increases the thickness of the asymmetric film.

Journal Article↗

PubMed2004

Charged lipid vesicles: effects of salts on bending rigidity, stability, and size.

The swelling behavior of charged phospholipids in pure water is completely different from that of neutral or isoelectric phospholipids. It was therefore suggested in the past that, instead of multilamellar phases, vesicles represent the stable structures of charged lipids in excess water. In this article, we show that this might indeed be the case for dioleoylphosphatidylglycerol and even for dioleoylphosphatidylcholine in certain salts. The size of the vesicles formed by these lipids depends on the phospholipid concentration in a way that has been predicted in the literature for vesicles of which the curvature energy is compensated for by translational entropy and a renormalization of the bending moduli (entropic stabilization). Self-consistent field calculations on charged bilayers show that the mean bending modulus kc and the Gaussian bending modulus k have opposite sign and /k/>kc, especially at low ionic strength. This has the implication that the energy needed to curve the bilayer into a closed vesicle Eves=4pi(2kc+k) is much less than one would expect based on the value of kc alone. As a result, Eves can relatively easily be entropically compensated. The radii of vesicles that are stabilized by entropy are expected to depend on the membrane persistence length and thus on kc. Experiments in which the vesicle size is studied as a function of the salt and the salt concentration correlate well with self-consistent field predictions of kc as a function of ionic strength.

Computer Simulation↗

PubMed2003

Brownian particles in supramolecular polymer solutions.

The Brownian motion of colloidal particles embedded in solutions of hydrogen-bonded supramolecular polymers has been studied using dynamic light scattering. At short times, the motion of the probe particles is diffusive with a diffusion coefficient equal to that in pure solvent. At intermediate time scales the particles are slowed down as a result of trapping in elastic cages formed by the polymer chains, while at longer times the motion is diffusive again, but with a much smaller diffusion coefficient. The influence of particle size and polymer concentration was investigated. The experimental data are compared to a theoretical expression for the mean-square displacement of an embedded particle in a viscoelastic medium, in which the solvent is explicitly taken into account. Differences between the friction and elastic forces experienced by the particle and the macroscopic viscosity and elasticity are explained by the inhomogeneity of the medium on the length scale of the particle size.

Journal Article↗

PubMed2003

Wetting in a colloidal liquid-gas system.

We present first observations of wetting phenomena in depletion interaction driven, phase separated colloidal dispersions (coated silica-cyclohexane-polydimethylsiloxane). The contact angle of the colloidal liquid-gas interface at a solid substrate (coated glass) was determined for a series of compositions. Upon approach to the critical point, a transition occurs from partial to complete wetting.

Journal Article↗

PubMed2003

Tethered polymer chains: surface chemistry and their impact on colloidal and surface properties.

In this review the grafting of polymer chains to solid supports or interfaces and the subsequent impact on colloidal properties is examined. We start by examining theoretical models for densely grafted polymers (brushes), experimental techniques for their preparation and the properties of the ensuing structures. Our aim is to present a broad overview of the state of the art in this field, rather than an in-depth study. In the second section the interactions of surfaces with tethered polymers with the surrounding environment and the impact on colloidal properties are considered. Various theoretical models for such interactions are discussed. We then review the properties of colloids with tethered polymer chains, interactions between planar brushes and nanocolloids, interactions between brushes and biocolloids and the impact of grafted polymers on wetting properties of surfaces, using the ideas presented in the first section. The review closes with an outlook to possible new directions of research.

Colloids↗

PubMed2002

First-order wetting transition at finite contact angle.

The wetting of a polymer brush by a melt of similar chains can have a window of complete wetting with a classical allophobic wetting transition at low grafting density sigma and an autophobic one at high sigma. However, when the melt chains are much longer than the brush chains, the contact angle alpha goes through a nonzero minimum where partial differential alpha/ partial differential sigma has a jump. A self-consistent-field analysis and experimental observations indicate a double-well disjoining pressure curve, consistent with a first-order wetting transition at finite alpha. The metastable contact angle can become zero.

Journal Article↗

PubMed2002

Monitoring entering and spreading of emulsion droplets at an expanding air/water interface: a novel technique.

The entering and spreading of emulsion droplets at quiescent and expanding air/water interfaces was studied using a new apparatus consisting of a modified Langmuir trough in which the air/water interface can be continuously expanded by means of rollers in the place of traditional barriers. When sodium caseinate and whey protein isolate-stabilized emulsion droplets were injected under the surface of sodium caseinate and whey protein isolate solutions, respectively, it appeared that the droplets entered the air/water interface only if the air/water surface pressure did not exceed a threshold value of approximately 15 mN/m. This condition was satisfied either under quiescent conditions for low protein concentrations or by continuous expansion of the interface at higher protein concentrations. According to equilibrium thermodynamics, entering of the droplets and the formation of lenses should occur for all the systems investigated, but this was not observed. At surface pressures higher than approximately 15 mN/m, immersed emulsion droplets were metastable. This is probably due to a kinetic barrier caused by the formation of a thin water film bounded by protein adsorption layers between the emulsion droplet and the air/water interface.

Journal Article↗

PubMed1999

Growth of a single-domain smectic phase in a thin liquid-crystalline polymer film.

The ordering process and kinetics in thin films (200-800-nm thick) of a thermotropic side-chain liquid-crystalline polymer have been investigated vertically and laterally, respectively, by x-ray reflectivity and atomic-force microscopy. The original smooth and amorphous spin-coated films initially become corrugated upon annealing in the smectic mesophase. The roughening of the surface results from the formation of randomly oriented microcrystalline domains in the film. At the same time, however, a laterally macroscopic crystal starts to grow from the substrate surface in the direction of the polymer-air interface at the expense of these domain structures. Finally, a nicely ordered single crystal with parallel-ordered bilayers is formed in the film as well as at the polymer-air interface. This one-dimensional crystallization, actually recrystallization, depends strongly on the temperature due to viscosity effects. At low temperatures, just above the glass-transition temperature, the ordering is very slow, but with increasing temperature the crystal growth is faster. An Arrhenius-type plot gives an activation energy of 122 kJ/mol, which we ascribe to the expected reorientations of the mesogenic groups during the recrystallization process.

Journal Article↗