Articles you may be interested in Effects of attractive colloids on the phase separation behaviors of binary polymer blendsWe 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 ͑i͒ exhibit one extremum between the two plates, ͑ii͒ have a new, surprising, oscillatory character, or ͑iii͒ 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.

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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 x 0. Our analysis reveals a universal character of the composition profile for D z ξt, where the characteristic size D is some known length depending on the relevant parameters of the problem, and not on temperature, and ξt is the thermal correlation length. Near surface, for a z D (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 z ξt, the profile tends exponentially to its bulk value. We show that the length D approaches 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.

We consider a mixture of two incompatible polymers A and B, confined between two parallel surfaces of the same chemical nature, separated by a distance L. It is assumed that both surfaces strongly adsorb one of the species (A) at high temperature. It is also assumed that a demixing transition occurs at a critical temperature T(c) below the adsorption temperature T(a). The strong adsorption implies that the composition of species A on surfaces is quenched even when the temperature is lowered. The presence of strong density fluctuations near the critical point induces an interaction between the surfaces. We reexamine this attractive force and determine its dependence with the thickness L, when the latter is smaller than the thermal correlation length. We find that, in the vicinity of the critical point, this force decreases with distance as L-4. We show that the corresponding amplitude is a universal number, independent of the value of the composition on surfaces, and we give its exact expression. Finally, we note that the present system may be considered as a typical model enabling one to understand qualitatively and quantitatively the flocculation of colloids embedded in critical binary polymer blends.