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 attractive because of the strong steric repulsion between the polymer chains. To circumvent this difficulty we graft polymer layers in a poor solvent to exploit attractive polymer-polymer interactions which largely nullify the repulsive steric interactions. This simple strategy gives 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 °C) where the chains are chemically attached to the surface utilizing a trichlorosilane end-group. Polished silicon wafers were then exposed to the reactive polymer solutions for a series of “induction times” $\tau_{\rm I}$ and the evolving layer was characterized by X-ray reflectivity and atomic force microscopy. Distinct morphologies were found depending on $\tau_{\rm I}$. For short $\tau_{\rm I}$, corresponding to a grafting density less than 5 mg/m$^2$, the grafted layer forms an inhomogeneous island-like structure. At intermediate $\tau_{\rm I}$, where the coverage becomes percolating, a surface pattern develops which appears similar to spinodal decomposition in bulk solution. Finally, after sufficiently long $\tau_{\rm I}$, a dense and nearly homogeneous layer with a sharp interface is formed which does not exhibit surface pattern formation. The stages of brush growth are discussed qualitatively in terms of a random deposition model