Critical point wetting for binary two-phase polymer-solvent mixtures on solid interfaces

Cahn argued that the wetting temperature, T_w, is always below the critical temperature, T_c, of a binary solvent mixture. A self-consistent field theory is used to show that this phenomenon is expected to be best observable for low molecular weight compounds. In polymer-solvent mixtures in contact with a rigid surface made of the same material as the polymer units, the difference T_w -T_c becomes minimal for an intermediate degree of polymerization N = N** (∼65). The polymer wets the surface for N < N**, whereas for larger N the solvent is at the wall. Critical wetting is the rule; first-order wetting is only found for relatively short chains, 2 ≤ N ≤ 16.