M.A. Cohen Stuart

Polymers at interfaces

Preface. Polymers in solution. General features of polymers at interfaces. Experimental methods. Theoretical methods. Homopolymer adsorption. Adsorption of copolymers. Electrostatic effects: charged surfaces and polyelectrolyte adsorption. Terminally-attached chains. Fluid interfaces. Depletion. Interactions in the presence of polymers. Appendices. Glossary of symbols. Key word index.

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.

Experimental aspects of polymer adsorption at solid/solution interfaces.

Several recent review articles have been concerned with the topic of polymers at interfaces from the theoretical standpoint. This reflects the extensive effort made in this area over the last 10 – 15 years. However, new experimental techniques for studying polymers at interfaces have also begun to appear in recent years; so have better defined model systems. This article is therefore directed more to a survey of these experimental aspects of the subject. However, a short review of the current state of the theory is given first as background and to define concepts. In the following chapter, details of the modern experimental methods are given. The last chapter comprises an extensive comparative review of results obtained using these techniques with model systems, covering homopolymers, copolymers and polyelectrolytes.

Kinetics of polymer adsorption in stagnation point flow

Abstract The kinetics of adsorption of poly(ethylene oxide) from water onto silica in stagnation point flow are studied using a reflectometric technique. It is shown that with reflectometry an absolute and continuous determination of the adsorbed mass per unit area can be obtained. For the stagnation point flow, the maximum rate of mass transfer to the surface is theoretically calculated. This rate is compared to the observed adsorption rate and it is concluded that mass transfer is the rate-determining step up to 75% or more saturation, depending on the molecular weight. Plateau adsorbed amounts obtained are in good agreement with those found by others. The molecular weight dependence of the diffusion constant agrees with scaling theory and the plateau adsorbed mass is in accordance with the theory for polymer adsorption from good solvents.

Reflectometry as a tool for adsorption studies

Abstract We discuss optical reflectometry as a tool for studying adsorption from dilute solutions. In particular, we elaborate on the kind of reflectometry where the reflecting substrate carries a thin dielectric film on which the adsorption takes place. The role of this film is to enhance the sensitivity of the method by producing a suitable phase shift in the reflected beam. The experimental set-up, calibration procedure and some conditions for successful determination of the surface excess are described. By means of an optical model we then calculate sensitivity and linearity of the technique for various solvents and adsorbents and we determine the optimum thickness of the dielectric film and the most appropriate angle of incidence. We find that, provided such optimum conditions are chosen, the technique allows sensitive and accurate detection of the adsorbed mass per unit area in all cases where the refractive index of the adsorbate film is sufficiently different from that of the solvent. For a typical example (organic solutes in water), adsorbed amounts as low as 0.01 mg m 2 can be detected.

Displacement of polymers. I. Theory. Segmental adsorption energy from polymer desorption in binary solvents

Abstract The complete removal of high molecular weight polymers from adsorbent surfaces is proved to be possible. A novel method to desorb polymers through displacement by a low molecular weight component, a so called displacer, is shown. A critical displacer concentration may be defined, above which the polymer desorption is complete. This desorption method may be important for a number of practical applications. Theoretical analysis is employed to determine which parameters govern the desorption process. A simplified model leads to an analytical expression for the critical displacer concentration. Additional information on the displacement process (i.e., effects of polymer concentration, molecular weight, and solvent quality) is obtained from more elaborate model calculations. It is shown that it is possible to determine the segmental adsorption energy if displacement data are combined with adsorption data for the corresponding displacer, provided solvency effects are properly taken into account.

Hydrophobicity measurements of microfiltration and ultrafiltration membranes

Abstract A method for the determination of the hydrophobicity of membrane materials is developed. The advantage of this method over existing methods is that it is not influenced by the presence of the pores. A piece of the membrane material is submerged horizontally in a liquid with surface tension γ L . Hydrophobicity is expressed in terms of γ d , the surface tension at which an air bubble brought into contact with the top surface of the membrane has a 50% chance of detaching from the surface. Values of γ d are expected to be 2-4 mN/m higher than critical surface tension (γ c ) values found in the literature. For polypropylene, PTFE and polydimethylsiloxane membranes, a good agreement was found between γ d and γ c values. Poly (vinylidene fluoride), polysulfone and polyethersulfone membranes appeared to be more hydrophilic than was expected on the basis of the literature γ c values for the polymers. Using X-ray photoelectron spectroscopy, constituents that are not present in the pure polymer have been found in the surface of some membranes. These constituents and the production techniques are shown to influence the hydrophobicity of the membranes investigated.

Displacement of polymers. II. Experiment. Determination of segmental adsorption energy of poly(vinylpyrrolidone) on silica

The displacement of poly(vinylpyrrolidone) (PVP) from silica in two different solvents (water and dioxane) by a number of low molecular weight organic displacers was studied. The experimental data compare favorably with a theory developed in a previous paper. Using this theory we determined the segmental adsorption energy for PVP to be 4 kT, both in dioxane.and in water. This result is in good agreement with inferences from calorimetric and spectroscopic-bound fraction studies.

The Adsorption of Poly (vinyl-pyrrolidone) onto Silica. I. Adsorbed Amount

The adsorption of the flexible, linear, and nonionic homopolymer poly(vinyl pyrrolidone) from water and from 1,4-dioxane onto pyrogenic silica was studied. Results are reported for the adsorbed amount as a function of adsorption time, molecular mass, and molecular mass distribution (polydispersity). It is found that the adsorption of fractionated samples can be qualitatively explained by the recent theory of Scheutjens and Fleer. However, the influence of the solvent type is larger than predicted by this theory, and an extension of the model to account for this influence is suggested. The polydispersity effects encountered in adsorption isotherms are satisfactorily accounted for by a theory published by Cohen Stuart, Scheutjens, and Fleer.

Isolation and physical characterization of an exocellular polysaccharide.

The physical properties of a polysaccharide produced by the lactic acid bacterium Lactococcus lactis subsp. cremoris strain NIZO B40 were investigated. Separation of the polysaccharide from most low molar mass compounds in the culture broth was performed by filtration processes. Residual proteins and peptides were removed by washing with a mixture of formic acid, ethanol, and water. Gel permeation chromatography (GPC) was used to size fractionate the polysaccharide. Fractions were analyzed by multiangle static light scattering in aqueous 0.10 M NaNO3 solutions from which a number- (Mn) and weight-averaged (Mw) molar mass of (1.47 +/- 0.06).10(3) and (1.62 +/- 0.07).10(3) kg/mol, respectively, were calculated so that Mw/Mn approximately 1.13. The number-averaged radius of gyration was found to be 86 +/- 2 nm. From dynamic light scattering an apparent z-averaged diffusion coefficient was obtained. Upon correcting for the contributions from intramolecular modes by extrapolating to zero wave vector a hydrodynamic radius of 86 +/- 4 nm was calculated. Theoretical models for random coil polymers show that this z-averaged hydrodynamic radius is consistent with the z-averaged radius of gyration, 97 +/- 3 nm, as found with GPC.