Structure of a diblock copolymer adsorbed at the hydrophobic solid/aqueousinterface: Effects of charge density on a weak polyelectrolyte brush

We have used neutron reflection to study the effects of pH and added electr olyte on the structure of a weak polyelectrolyte brush anchored by means of a hydrophobic block to a hydrophobic surface. The copolymer was poly(2-(di methylamino)ethyl methacrylate-block-methyl methacrylate) (poly(DMAEMA-b-MM A)) containing 70% DMAEMA and having a molecular weight of 10000. The MMA b lock and the hydrophobic octadecyltrichlorosilane (OTS) layer were deuterat ed in order to highlight the DMAEMA fraction of the layer. The MMA block wa s found to bind strongly and irreversibly to a self-assembled monolayer of octadecyl trichlorosilane (OTS) on silica. The copolymer distribution could be described in terms of two uniform layers, a thin layer next to the surf ace containing all the MMA block and variable amounts of the DMAEMA residue s with a copolymer volume fraction of about 0.8 and a more diffuse block co rresponding to the polyelectrolyte brush. The presence of DMAEMA residues i n the thin layer and the variation of this amount with pH and added electro lyte show that, near the anchor layer of the MMA block, the DMAEMA block is not ionized, in agreement with the model of a grafted weak polyelectrolyte proposed by Israels et al.