SELF-ORGANIZATION OF POLYMER BRUSH LAYERS IN A POOR SOLVENT

Synthesis of densely grafted polymer brushes from good solvent polymer solutions is difficult when the surface interaction is only weakly at tractive because of the strong steric repulsion between the polymer ch ains. To circumvent this difficulty we graft polymer layers in a poor solvent to exploit attractive polymer-polymer interactions which large ly nullify the repulsive steric interactions. This simple strategy giv es rise to densely grafted and homogeneous polymer brush layers. Model end-grafted polystyrene chains (M(w) = 105, 000) are prepared in the poor solvent cyclohexane (9.5 degrees C) where the chains are chemical ly attached to the surface utilizing a trichlorosilane end-group. Poli shed silicon wafers were then exposed to the reactive polymer solution s for a series of ''induction times'' tau(I) and the evolving layer wa s characterized by X-ray reflectivity and atomic force microscopy. Dis tinct morphologies were found depending on tau(I). For short tau(I), c orresponding to a grafting density less than 5 mg/m(2), the grafted la yer forms an inhomogeneous island-like structure. At intermediate tau( I), where the coverage becomes percolating, a surface pattern develops which appears similar to spinodal decomposition in bulk solution. Fin ally, after sufficiently long tau(I), a dense and nearly homogeneous l ayer with a sharp interface is formed which does not exhibit surface p attern formation. The stages of brush growth are discussed qualitative ly in terms of a random deposition model.