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 cal culations. 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 b inodal. We assume that both solvents are good solvents for the polymer: the monomers of the chains have amphiphilic properties. When the minority solv ent B is the better solvent, it is possible to find a preferential uptake o f the solvent B. This solvent uptake can either occur in a continuous manne r or in a first-order transition. From a wetting perspective, such a stepwi se increase in B uptake may be identified as a prewetting step. In this cas e, however, the step is not necessarily caused by specific interactions wit h 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 b y the minority solvent, even when there is a prewetting step. We examine th e post-transition solvent uptake up to and beyond the bulk binodal (in the case of partial wetting). The numerical SCF results complement a recent ana lysis 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 firs t-order phase transition is unusual: the polymer chains absorb the better s olvent and then suddenly collapse to a very dense sublayer when there is on ly a small amount of the wetting component.