Polymer adsorbed layers compression forces

Surface Force Measurements. Another method to measure the thickness of adsorbed layers is by the surface force apparatus (SFA) (17). In this method two freshly cleaved mica sheets are glued to the surfaces of two crossed cylindrical lenses. Polymer chains are then allowed to adsorb on the mica sheets. In order to measure the thickness of the adsorbed layers the two cylinders are brought in contact and the force between them is measured as a fimction of separation. The onset of the repulsive force associated with compression of the adsorbed layer can be related to the thickness of the adsorbed layer. On the other hand, in the event of bridging between the adsorbed layers the force will be attractive. Recent advances in the instrument have made it possible to probe the effect adsorption has on the flow of fluid past a surface (18). 

In order to apply the above procedure to determine the conditions of phase separation, we have chosen the system of polyisobutene-stabilized silica particles with polystyrene as the free polymer dissolved in cyclohexane. The system temperature is chosen to be the 8 temperature for the polystyrene-cyclohexane system (34.5°C), corresponding to the experimental conditions of deHek and Vrij (1). The pertinent parameters required for the calculation of the contribution of the adsorbed layers to the total interaction potential are a = 48 nm, u, =0.18 nm3, 5 = 5 nm, Xi = 0.47(32), X2 = 0.10(32), v = 0.10, and up = 2.36 nm3. It can be seen from Fig. 2 that these forces are repulsive, with very large positive values for the potential energy at small distances of separation and falling off to zero at separation distances of the order of 25, where 6 is the thickness of the adsorbed layer. At the distance of separation 5, the expressions for the interpenetration domain and the interpenetration plus compression domain give the same value for the free energy, indicating a continuous transition from one domain to the other. 

Two extreme cases can be envisaged, one in which the polymer chains mix, while the other involves no mixing but the layers become compressed on close approach of the surfaces. Both involve a reduction in the configurational freedom of the adsorbed polymer molecules, which results in a repulsive force. One or both types of interaction could be present in any particular system, depending on the nature of the polymer and solvent, and the structure of the adsorbed layer. There has been considerable theoretical analysis of both cases, and the relative merits of the two approaches are debated in the literature -. ... 

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. 

Dynamic aspects of adsorbed layers of PS on oxidic surfaces have also been studied by surface-force measurements [73]. The effect of compression was measured for PS adsorbed on mica from cyclopentane near conditions. If compression was performed slowly the layers seemed to become irreversibly compressed as was concluded from the absence of long-range bridging attraction. Even after several days the polymer layers did not relax. 

Whether a polymer is grafted on the surface or simply adsorbed on the surface has a bearing on the force between two polymer-coated surfaces. Adsorption is a reversible process and can be affected by temperature, compression of the polymer layer, and the like. 

Two basic approaches are possible one Involves compression of a dispersion of (monodlsperse) particles canylng an adsorbed polymer layer and monitoring the pressure as a function of the volume fraction. In the second approach the force between two macroscopic surfaces with adsorbed pol5rmer layers is measured as a function of the surface separation. 

At the same time, a compression of the adsorbed macromoleeule ean proceed xmder the action of macromolecules arriving from solution into the adsorption layer, when the particle surface is covered by polymer. One can suggest that the rate of eompression of macromolecule is proportional to the fraction of the surface area covered by the polymer and the number of macromolecules that could stiU adsorb on the solid surface. That rate is inversely proportional to the concentration of monomer links of polymer in the adsorption layer, c . An increase of the concentration of monomer links in the adsorption layer increases the force that prevents macromolecule compression ... 

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