Terence Cosgrove

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.

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.

Poly(NIPAM) microgel particle de-swelling: a light scattering and small-angle neutron scattering study

Small-angle neutron scattering (SANS) has been used to investigate structural changes during the de-swelling of poly(N-isopropylacrylamide) [poly(NIPAM)] microgel particles induced by temperature variation and the addition of free polymer [poly(ethylene oxide)]. The extent of particle de-swelling was characterized by photon correlation spectroscopy (PCS). Thermally-induced de-swelling of poly(NIPAM)/deuterated water dispersions occurred on increasing the temperature in the region of the lower critical solution temperature (LCST). The latter was found to be 34°C, which is 2°C higher than the value reported for poly(NIPAM) particles dispersed in water. The SANS data exhibit a Porod form (Q−4) of scattering in regard to the size of the colloidal particles. However, in the swollen state, the scattering measured at temperatures less than the LCST, also has a contribution from poly(NIPAM) chains in a solution-like environment (Ornstein–Zernike scattering). The trends observed in the SANS data as a function of temperature were simulated using a model comprised of Porod and Ornstein–Zernike scattering terms. The SANS data confirm earlier PCS measurements showing that addition of free polymer induces particle de-swelling. The SANS data obtained using added free polymer are the first examples of their type to be reported.

The configuration of sodium poly(styrene sulfonate) at polystyrene/solution interfaces

Abstract Adsorption isotherms, adsorbed layer hydrodynamic thicknesses (from photon correlation spectroscopy), and segment density profiles (from small angle neutron scattering) have been determined as a function of ionic strength for two, narrow-molecular-weight samples of sodium poly(styrene sulfonate) on polystyrene latex particles. Four latices were used, one having a high negative surface charge and three of varying positive surface charge density. The adsorbed amounts are strongly dependent on ionic strength, but not strongly dependent on the polyelectrolyte molecular weight. With increasing surface charge on the positive latex particles, the adsorbed amount decreases. The hydrodynamic thicknesses are relatively small, compared to the values generally found for adsorbed neutral polymers of comparable molecular weight and adsorbed amount. The one exception is with the very low charged (positive) surface, where more extended tails are thought to develop. The hydrodynamic thicknesses on the higher charged positive and negative surfaces are remarkably similar. The segment density profiles, in general, confirm the conclusions reached from the other studies here, that is of generally “flat” configurations on highly charged (positive or negative) surfaces, with relatively short tails, and possibly the development of some loops at high ionic strengths, where intersegmental electrostatic repulsion is suppressed and the polyelectrolyte chains begin to approach the behavior of neutral polymers.

Pluronic triblock copolymer systems and their interactions with ibuprofen

Small-angle neutron scattering and pulsed-field gradient stimulated-echo nuclear magnetic resonance (NMR) have been used to study the structural characteristics of aqueous Pluronic solutions. In particular, changes in the micellar structure upon addition of ibuprofen to the solutions and altering the temperature have been investigated. Increases in temperature and ibuprofen concentration both appear to favor micellization, with increases observed in the aggregation number, fraction of polymer micellized, and core radius of the micelle, along with a decrease in the volume fraction of the solvent in the core. At high drug concentrations and/or temperatures, micelles of the more hydrophobic Pluronics scatter neutrons strongly at low Q, indicating attractive interactions between micelles or a change in the shape of the aggregates. The addition of ibuprofen to Pluronic P104 has also been found to reduce the critical micellization temperature from approximately 20 degrees C to below 13 degrees C. The hydrophobicity of the Pluronic, quantity of ibuprofen present, and temperature of the solution all seem to have a strong influence on the extent and nature of aggregation observed.

Volume-fraction profiles of adsorbed polymers

The structure of adsorbed polymer layers has been studied extensively, both experimentally and theoretically, in a diverse range of systems. Although the form of the volume-fraction profile of polymer segments, normal to the interface has been predicted in detail from theory, few experimental methods can probe more than the first or second moments of this distribution. In this paper, the role of neutron scattering and magnetic resonance in such studies is surveyed. The results of these studies are compared with various theoretical models. The picture of the adsorbed layer that emerges is one that shows a strong dependence on the structure of the polymer, and the interactions between polymer segments, the interface and solvent(s).

Computer simulation of water molecules at kaolinite and silica surfaces

Molecular dynamics simulations of water molecules at the surface of kaolinite, and of amorphous silica, have been carried out. In contrast to previous work, clay and silica atoms are not kept fixed at their crystallographic positions; all atoms are allowed to move. In both cases water molecules at the surface show a marked decrease in the self-diffusion coefficient and an increase in the rotational correlation time. We consider the effects of increasing the ionic strength of the water. Comparison with experimental data is made by linking calculated diffusion and rotational correlation times to available pulse field gradient nuclear magnetic resonance spectroscopy and relaxation measurements.

Adsorption studies of polyethers: Part II: adsorption onto hydrophilic surfaces

Abstract The influences of copolymer molar mass and PEO/PPO block molar masses, anchor fraction and solution concentration on the adsorption characteristics of PEO/PPO/PEO triblock copolymers adsorbed onto model polystyrene latices have been investigated. The adsorbed amount was determined by the standard depletion method and the hydrodynamic thickness of the adsorbed layer by photon correlation spectroscopy. Model polystyrene latices prepared by the surfactant-free method were used as adsorbents. The adsorbed amount and the hydrodynamic layer thickness were found to be strongly dependent on the copolymer molar mass, PEO/PPO block masses and the copolymer solution concentration. The conformation of the adsorbed copolymer is determined by the surface-copolymer interaction and, principally, by interaction of the hydrophobic PPO block with the latex surface.

Polymer, particle, surfactant interactions

Abstract The effect of the surfactant sodium dodecyl sulphate (SDS) on the adsorption properties of the homopolymer poly(ethylene oxide) (PEO) at the solid–liquid interface has been studied using a variety of techniques: photon correlation spectroscopy (PCS), nuclear magnetic resonance (NMR) spectroscopy and small-angle neutron scattering (SANS). Physisorbed, grafted, and dendritic polymers have been studied. Across the range of systems studied, a remarkably consistent general picture has emerged. For the physisorbed system, the surfactant affected both the adsorption of the polymer and the extension of its adsorbed layer. For systems where the adsorbed amount was held at a constant value, the polymer structure was significantly perturbed by the addition of surfactant. It is proposed that the polymer layer structure is not governed by competitive adsorption for the interface, but by a competitive polymer–surfactant interaction.

FACTORS AFFECTING THE SWELLING OF POLY(N-ISOPROPYLACRYLAMIDE) MICROGEL PARTICLES : FUNDAMENTAL AND COMMERCIAL IMPLICATIONS

Abstract A microgel particle is a cross-linked latex particle which is swollen by a good solvent. Particle swelling is intrinsically related to the nature of the interaction between the polymer and continuous phase. Microgel particles based on PNP [PNP=poly( N -isopropylacrylamide)] are particularly interesting since the parent homopolymer undergoes a coil-to-globule transition in water when the temperature increases above 32°C. In this work, PCS (photon correlation spectroscopy) and SANS (small-angle neutron scattering) are employed in a complementary manner to study the environmentally induced de-swelling of PNP particles. Further, we show that particle de-swelling may be induced at room temperature by addition of alcohols or excluded free polymer (i.e. non-adsorbing free polymer) to the continuous phase. (The extents of particle de-swelling observed using these additives are similar to those achieved by heating the pure microgel particles in water above 32°C.) Particle de-swelling in the presence of added alcohol or free polymer arises from “co-non-solvency” and osmotic de-swelling effects, respectively. Copolymerization of N -isopropylacrylamide (NP) with acrylic acid yields microgel particles whose diameters are sensitive to both pH and temperature. These particles adsorb Pb II ions from solution in a reversible manner. The latter property has potential application in water purification.