Poly brushes repulsion forces

The extremely low friction achieved on PLL(20)-g[2.9]-PEG(5) films in SFA experiments is not a unique property of this specific copolymer but has also been observed for other grafted poly(ethylene glycol) films as well as for other polymer-brush systems in a good solvent, such as grafted polystyrene in toluene. The osmotic pressure, which leads to strong repulsive forces as the polymer brushes are compressed, effectively prevents the direct contact of the solid surfaces. At comparable solid surface separations, thin films of water or aqueous salt solutions have been shown to retain a shear fluidity characteristic of the bulk liquid. In a control experiment, this was also observed in 10 mM HEPES buffer solution (data not shown). 

Surface force profiles between these polyelectrolyte brush layers have consisted of a long-range electrostatic repulsion and a short-range steric repulsion, as described earlier. Short-range steric repulsion has been analyzed quantitatively to provide the compressibility modulus per unit area (T) of the poly electrolyte brushes as a function of chain density (F) (Fig. 12a). The modulus F decreases linearly with a decrease in the chain density F, and suddenly increases beyond the critical density. The maximum value lies at F = 0.13 chain/nm. When we have decreased the chain density further, the modulus again linearly decreased relative to the chain density, which is natural for chains in the same state. The linear dependence of Y on F in both the low- and the high-density regions indicates that the jump in the compressibility modulus should be correlated with a kind of transition between the two different states. 

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