Amphiphilic polymer brush in a mixture of incompatible liquids. Numerical self-consistent-field calculations

We studied a polymer brush composed of homodisperse end-grafted chains in a binary A-B solvent mixture by means of numerical self-consistent-field calculations. The focus is on the case that the solvents have a solubility gap in the bulk phase behavior, and we investigated the system near the bulk binodal. We assume that both solvents are good solvents for the polymer: the monomers of the chains have amphiphilic properties. When the minority solvent B is the better solvent, it is possible to find a preferential uptake of the solvent B. This solvent uptake can either occur in a continuous manner or in a first-order transition. From a wetting perspective, such a stepwise increase in B uptake may be identified as a prewetting step. In this case, however, the step is not necessarily caused by specific interactions with the solid substrate, but it results from an instability in the structure of the polymer brush at intermediate compositions of A and B in the brush. It is not always true that at coexistence the substrate is completely wet by the minority solvent, even when there is a prewetting step. We examine the post-transition solvent uptake up to and beyond the bulk binodal (in the case of partial wetting). The numerical SCF results complement a recent analysis of the same problem by a model of the Alexander de Gennes type. Both in the numerical and in the analytical models, it is observed that the first-order phase transition is unusual: the polymer chains absorb the better solvent and then suddenly collapse to a very dense sublayer when there is only a small amount of the wetting component.