Gerard J. Fleer

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.

Interaction between hairy surfaces and the effect of free polymer

Abstract In this paper, the interaction between surfaces coated with grafted polymer is studied. The influence of free, nonadsorbed polymer on the interaction is described in some detail. A self-consistent field theory, based on the model for homopolymer adsorption of 19. , 20. , is used to take into account all possible conformations of terminally anchored and free polymer and to calculate the interaction between the two surfaces. For hard surfaces in the presence of free polymer, the attraction is determined only by the osmotic effect. In the case of hairy surfaces, the mixing of the grafted polymer layers gives an additional attractive contribution, because the hairs mix more easily with other hairs than with free polymer; this attraction is strongest at plate separations just below twice the hydrodynamic layer thickness. At small separations, the interaction between hairs becomes repulsive. The free energy of interaction shows a minimum as a function of the bulk volume fraction of free polymer. For infinitely large plates, the attraction is always stronger for higher chain lengths of free polymer. Increasing the mixing energy of grafted and free polymer (Xgf) increases the attraction drastically, whereas varying their solvency (Xf0, Xg0) has a much less pronounced effect. If no free polymer is present, the interaction becomes attractive when the solvency of the grafted chains becomes worse than the Θ condition.

Adsorption of Graft Copolymers onto Silica and Titania

The adsorption of graft copolymers of poly(acrylamide) (PAAm, backbone) and poly(ethylene oxide) (PEO, side chains) from aqueous solution onto silica and titania was studied with reflectometry. Two high-molar-mass copolymers were used with different PEO graft densities (10 and 18% w/w PEO in copolymers G10 and G18, respectively). On titania only the PAAm backbone adsorbs and the PEO does not. This results in adsorbed amounts of 0.83 and 0.85 mg m-2, respectively, which is about the same as that for a PAAm homopolymer. On silica the situation is reversed:  now the PEO side chains adsorb and the PAAm backbone does not. The adsorption as a function of time shows a maximum, before the stable plateau is reached. The adsorbed amount on silica is much higher than that on titania:  in the final plateau it is 1.35 and 1.2 mg m-2 for G18 and G10, respectively. On silica the polymers form longer loops and tails so that more molecules can be accommodated at the surface. The overshoot on silica depends on the polymer ...

Chain stiffness and bond correlations in polymer brushes

We have incorporated chain stiffness and correlations between neighboring bonds into a self‐consistent field (SCF) lattice model for end‐attached polymer layers (commonly known as ‘‘brushes’’). An increase in the chain stiffness leads to an increasing brush height. This increase is directly related to the change of the length of a Kuhn segment in the polymer chain. Introducing correlations between neighboring bonds gives a higher density of the brush, corresponding to a decrease of the brush height. For not too stiff chains these two effects virtually compensate each other. Hence, the volume fraction profile of ‘‘real’’ grafted chains is nearly identical to that of a polymer brush consisting of freely jointed chains.

Terminally-Attached Chains

The interfacial properties of terminally-attached chains differ from those of a homopolymer because one end of the polymer chain is irreversibly held at an interface. This surface bond may be either physical or chemical in nature. In the former case the chain must possess one end group which is capable of forming a strong physical bond with the surface. Strictly speaking, this is a special case of a block copolymer with a very strong anchoring group; it is treated in this chapter because of the similarity with chemically attached chains. The latter type of system may be prepared by a chemical grafting reaction between an end group and the surface.

Adsorption of Copolymers

In this chapter we discuss the adsorption of copolymers, i.e., polymers consisting of more than one type of structural unit. These may be arranged in blocks along the molecule, or be distributed statistically over the chain. We treat the former situation in sec 6.1, and the second case in sec. 6.2.

Polymers in Solution

In a dilute polymer solution the solute molecules are on average so far apart that their mutual interactions are of no concern, and we can largely understand the solution properties by simply considering individual molecules surrounded by solvent. We will do this in some detail in this section, before turning our attention to interacting polymer molecules in sections 1.1.2–4.