Surface roughening in polymer blend thin films by lateral phase separation: a thermodynamic mechanism.

We use a phase portrait approach to study the phase equilibria of a symmetric binary polymer blend confined between an attracting wall and a neutral wall. We find multiple solutions for this wall regime where only one solution exists for antisymmetric walls. We also argue that, when one wall is a free surface, roughening of the free surface upon lateral phase separation is expected, since thermodynamics alone implies that the coexisting phases should exist at different heights to minimise energy. We focus on the graphical insight provided by this phase portrait approach.