Eur. Phys. J. E 2, 91-101
Phase separation of polymer blends in solution near adsorbing surface
R. Cherrabi - A. Saout-Elhak - M. Benhamou
Laboratoire de Physique des Polymères et
Phénomènes Critiques, Faculté des Sciences Ben M'sik, Université Hassan
II-Mohammedia, B.P.
7955, Casablanca, Morocco
benhamou.mabrouk@caramail.com
Received 24 June 1999 and Received in final form 5 November 1999
Abstract
In this paper, we consider a mixture of two polymers A
and B of different chemical nature, dissolved in a common good
solvent, in contact with an interacting surface. We start from a
mixture of two incompatible homopolymers A and B in the molten
state, and assume that the surface adsorbs strongly one or both
polymer species at high temperature. It is assumed that this is a
strong adsorption, so that chains cannot desorb once they are
linked to the surface. This constrains the system to a quenched
composition on the surface. Once the adsorption process is
finished, a quantity of a good solvent is added to get a
semi-dilute solution. We assume that demixing transition in the
presence of solvent occurs at lower temperature. The purpose is to
discuss the influence of the quenched surface fluctuations on the
critical properties of the mixture. Within the framework of the
so-called blob model, we determine the exact shape of the
composition profile as a function of the distance z to the
surface, for any value of the relevant parameters, namely, the
temperature T, the molecular weight M, the monomer
concentration c and the surface composition x0. Our analysis
reveals a universal character of the composition profile for
,
where the characteristic size D is some
known length depending on the relevant parameters of the problem,
and not on temperature, and
is the thermal
correlation length. Near surface, for
(a is the
monomer size), the profile is no longer universal, and in
particular, it is sensitive to the boundary condition. Far from
the surface, that is
,
the profile tends
exponentially to its bulk value. We show that the length Dapproaches its lowest value as the surface composition reaches
its saturated value l. In this limit, we find that the profile
shape is a characteristic of critical adsorption in simple binary
fluid mixtures. Finally, this work must be regarded as a natural
extension of a previous one, which was concerned with the same
problem, but in the absence of solvent.
PACS
64.75.+g Solubility, segregation, and mixing; phase
separation - 68.45.-v Solid-fluid interfaces -
61.41.+e Polymers, elastomers, and plastics
Copyright EDP Sciences, Società Italiana di Fisica, Springer-Verlag
