Polymer Solutions and Dispersions Consider a polymer in solution, the solution being in contact with a solid. First, the polymer may be attracted to the surface, see Figure 1. In this model, the mers in contact with the surface are called trains. These are adsorbed on the surface, usually by physical forces. The loops have no contact with the surface, but connect two trains, and the tails are non-adsorbed chain ends.
People refer to the bound fraction as that fraction of chains forming trains.
When a surface exerts no net attraction on the polymer, the case of negative adsorption arises. The cause is the reduced entropy of polymer chains near the surface, in turn caused by the reduced number of conformations possible in chains near a surface, or in any confined circumstance. Hence, the fluid immediately adjacent to the interface will have a lower mer concentration than in the solution. The depletion phenomenon will always exist when a polymer solution is near a surface. However, if the attractive forces are strong enough, the enthalpy gained overcomes the entropy lost, leading to the model in Figure 1.