Spontaneous degrafting of polyelectrolyte brushes from solid substrates

Polymer brushes grafted covalently to solid substrates may degraft in some instances. We study the stability of strongly and weakly charged polymeric grafts derived from poly(2-dimethyl aminoethyl methacrylate) (PDMAEMA). We employ surface-initiated atom transfer radical polymerization to prepare polymer assemblies featuring gradients of grafting density and molecular weight. The degree of permanent charge in strong electrolytes is adjusted by reacting PDMAEMA with methyl iodide to a given extent. We interrogate the stability of those surface-grafted polymers under various pH at ionic strength values in solution. Swelling of the brush due to electrostatic charging in aqueous media (adjusted by varying pH and degree of quaternization) generates tension along the grafted macromolecular backbone. Such tension focuses at the bottom-most section of the polymer brush close to the substrate. This lowers the activation energy for breaking labile chemical bonds either in the initiator itself or the head-group chemistry of the initiator that links the initiator to the underlying substrate. Weak polyelectrolyte brushes are more stable than strong polyelectrolyte grafts. The stability of brushes decreases with increasing pH and decreasing ionic strength of surrounding solution.