Polymers on Wet Adhesion

Polymers in solution have an enormous effect on the adhesion between surfaces. Such polymers are used as lubricants, as thickeners, as colloid stabilisers, as binders, glues, and inks. Also they are ubiquitous in biological systems. Their practical significance is large but understanding their effects remains to be explored to a great extent. This section describes some experimental observations of the adhesion forces and draws a schematic theoretical picture of the effects. 

There are three kinds of polymer to be found dispersed in liquids the first is the insoluble polymer colloid found in a latex dispersion, for example polyvinyl acetate used in latex glues. Fig. 6.22(a). This is essentially a suspension of plastic beads of very small size. 

An example of this effect was described by Klein who, in 1980, used the Israelachvili apparatus to measure polystyrene molecules at mica surfaces. At a temperature of 24°C where the polymer was not very soluble, the results shown in Fig. 6.23 were seen. 

The high molecular weight polystyrene started to show an attraction between the surfaces just above one molecular diamete. As the molecules were compressed together a repulsion was then observed at a gap of about 20 nm. The lower molecular weight material behaved in somewhat similar fashion but with a repulsion at 8 nm gap. When the temperature was raised to 35°C, to give improved solvation of the polystyrene, the attraction was reduced by half. 

The results can be understood in terms of the forces between rubbery swollen polystyrene. As the layers approach, there is a van der Waals attraction, but the adsorbed particles then press into each other to give the repulsion (Fig. 6.24). The polymer is too strongly adherent for the adsorbed layer to be squeezed out by the compressive force. 

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