Phase separation in confined polymer blends

We consider a binary blend of two polymers of different chemical nature, confined between two adsorptive parallel surfaces separated by a distance L. We assume that the two surfaces adsorb strongly one or both species at high temperature. This implies quenched compositions on surfaces. We assume that phase separation takes place at lower temperature. Two independent effects are present and must be taken into account, the quenched surface fluctuations and finite-size effect due to the finite thickness L. Using a systematic and analytical method, we first investigate the variation of the composition profile with the distance z from one surface taken as origin. The profile shape depends on the monomer fractions on the surfaces, the temperature, and the size L. We find that the profile may have three distinct behaviors depending on the value of the surface composition: it may (i) exhibit one extremum between the two plates, (ii) have a new, surprising, oscillatory character, or (iii) be strictly monotonous. We then evaluate the interaction forces between the walls due to density fluctuations. We show that these forces may be either repulsive or attractive, and we estimate in their dependence in the separation distance L for all cases. Such a system may play the role of a force sensor. (C) 1999 American Institute of Physics. [S0021- 9606(99)50238-9].