Interactions between polymer brushes: Varying the number of end-attaching groups

We use a surface force balance (SFB) to study the normal interactions between polymer brushes, which are self-assembled from solution. They consist of polystyrene (PS) chains in toluene (neutral chains in a good solvent) anchored on the interacting mica surfaces via sulfozwitterionic end groups. The properties of the brush depend on the length,N, of the chain, and the energy, alphak(B)T, with which the end group adsorbs on the surface. In contrast to earlier studies where N was varied, we attempt to vary the sticking energy by using polymer chains with one, two, and three zwitterions attached to their ends. We use the theory of Alexander and de Gennes to predict how the normal force profile should vary with a and N, finding, for example, that the brush height L-0 obeys L-0 = a . N-3/5. alpha(2/5). Surprisingly, our measurements show that the grafting density does not vary significantly between polymers with 1, 2, and 3 end groups. This could be attributed to dipole-dipole interactions between the zwitterions themselves. It is also possible that the effect could be kinetic: that the brush is unable to reach its equilibrium state because successive polymer chains are hindered from attaching to the surface by those already in the brush. Measurements on longer time scales will be necessary to determine whether kinetic effects are important.