Force between polymer coated surfaces

Already the ancient Egyptians knew that one can keep soot particles dispersed in water when they were incubated with gum arabicum, an exudate from the stems of acaia trees, or egg white. In this way ink was made. The reason for the stabilizing effect is the steric repulsive force cause by adsorbed polymers. In the first case these are a mixture of polysaccharide and plycoprotein, in the second case it is mainly the protein albumin. Steric stabilisation of dispersions is very important in many industrial applications. Direct quantitative measurements were 

The force between surfaces which are coated with polymers is mainly determined by two factors. The first one is the quality of the solvent. In good solvents the force tends to be repulsive, in bad solvents attractive. Moreover, in good solvents polymer tends to remain in solution rather than adsorbing to surfaces. 

There is no simple, comprehensive theory and steric forces are complex and difficult to describe. Different components contribute to the force, and depending upon the situation, dominate the total force. The most important interaction is repulsive and of entropic origin. It is caused by the reduced configuration entropy of the polymer chains. If the thermal movement of a polymer chain at a surface is limited by the approach of another surface, then the entropy of the individual polymer chain decreases. In addition, the concentration of monomers in the gap increases. This leads to an increased osmotic pressure. 

This repulsion was calculated by different authors [201,202], For a low grafting density (T 1 /R2g), the repulsive force per unit area in a good solvent and between two polymer coated surfaces is [200] 

For the other extreme, at high grafting density, the force per unit area can be approximated by [201] 

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FIG. 13.16 Interaction forces between polymer-coated surfaces and their dependence on the type of layers and quality of the solvents. (Redrawn with permission from Israelachvili 1991.)... 

If two such sparsely coated layers are brought into close contact, the pinned micelles from each surface interact and form novel structures. Little is known of the interactions in systems that involve both solvophobic and solvophilic (solvent-compatible) chains at low grafting densities [19,20,28]. Probing the structure and energies between these layers can yield insight into the nature of the adhesive forces between polymer-coated substrates and provide guidelines for tailoring the interactions between the interfaces. 

The adhesion force between two objects can arise from a combination of different contributions such as the van der Waals force, electrostatic force, chemical bonding, and hydrogen bonding forces, capillary forces, and others (e.g., bridging or steric forces on polymer-coated surfaces) ... 

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. 

Force measurements between polymer-coated mica surfaces exhibit many of the features predicted by the SF model. The force measurement technique characterizes the interaction between polymer layers and provides an indirect... 

The applications of the SFM include force measurement between surfaces in liquid and vapor, adhesion between similar or dissimilar materials, contact deformation, wetting and capillary condensation, viscosity in thin films, forces between surfactant and polymer-coated surfaces, and surface chemistry. Fluid-electrolyte interactions between conductive surfaces can also be measured [Smith, et. al., 1988]. A typical microforce of 10 nN can be detected over separation distances to a resolution of 0.1 nm with optical interoferometry between reflective surfaces. With electrostatic forces, relatively large separation are measured 1-100 nm, whereas, short range forces such as van der Waals forces take place over distances of less than 3.0 nm. Ultrasmooth and electrically conductive surfaces can be formed by the deposition of a metal film (40 nm thickness) such as Pt on a smooth substrate of mica [Smith, et. al., 1988]. The separation distance between the two surfaces is controlled by a... 

Klein and Luckham were able to estimate the slopes of the force-distance curves in the two limiting regions. Just when interaction between the two polymer-coated surfaces begins, the slope should be large and negative, while at close approach (< 100 A separation) they obtain... 

Figure 12.24 Using the Surface Forces Apparatus, the repulsive forces between polymer chains on both crossed cylinders, left, and one coated surface and one clean mica sheet, right, can be measured. Poly(ethylene oxide) anchors the nonadsorbing polystyrene chains to the mica, with toluene as the solvent.

In bad solvent, the polymer-coated surfaces attract each other [1386, 1387]. One reason is the absence of steric repulsion. Van der Waals forces can become dominant again and lead to aggregation. Napper [1311] formulated the rule of thumb that the onset of dispersion flocculation coincides with the creation of conditions in the solvent medium. Exceptions to this rule are encountered when either the polymeric stabilizers are very short or the particles become very small. In addition to van der Waals forces, the polymer itself induces attraction. In a bad solvent, the polymer usually has a strong tendency to adsorb to any surface available. For that reason, we do not have to distinguish all possible cases. Attraction between surfaces is due to intersegment forces. The intersegment force is caused by the direct interaction between segments of polymers with each other. Since in a bad solvent the interaction... 

Electric double layer forces between polyelectrolyte and non-polymer surfaces in aqueous media have also been studied very intensively [371,394,400-402]. The adhesion between polyelectrolyte surfaces could be reduced considerably by increasing the ionic strength of the medium [400]. Using an electrochemical cell and a gold coated tip, the adhesion between electroactive layer of p oly( vinyl-ferrocene) was controlled through the selective oxidation or reduction of the polymer films [401]. 

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