K. Binder et al., Symmetric binary polymer blends confined in thin films between competing walls: Interplay between finite size and wetting behavior, PHYS CHEM P, 3(7), 2001, pp. 1160-1168
The phase behavior and structure of a symmetric binary polymer blend confin
ed between two hard impenetrable walls is studied, assuming also short rang
e forces between the monomers and the walls. Phenomenological consideration
s are elaborated with the self-consistent field theory of Gaussian chains,
and also some Monte Carlo simulations of the bond-fluctuation model. In the
case of "antisymmetric" walls (right wall attracts component A with the sa
me strength as does left wall component B) the phase diagram is symmetric a
round volume fraction phi =1/2. For very thin films (or very weak surface f
ields) one finds a single critical point at phi (c)=1/2, as in the bulk. Fo
r thicker films, or stronger surface fields, the phase diagram exhibits two
critical points and two concomitant coexistence regions, down to a triple
point T-t, while below the triple point there is a single coexistence regio
n. When the film thickness D --> infinity, the two coexistence regions for
T >T-t shrink into the prewetting lines, while T-t approaches the wetting t
ransition temperature. Asymmetric surface forces are also considered, study
ing the smooth crossover from "antisymmetric walls'' to "symmetric walls''
(that both attract the A component with the same strength). The resulting c
rossover between capillary-type behavior (i.e., a single critical point of
the thin film) and the above behavior with two critical points is analyzed.
Particular attention is paid to the behavior of interfaces between coexist
ing phases.