Instability of Surface-Grafted Weak Polyacid Brushes on Flat Substrates

Abstract

We study the stability of weak polyacid brush (WPAB) gradients in aqueous media covering a range in grafting density (σ) spanning 0.05–0.5 chains/nm² using two analogous surface-anchored bromoisobutyrate-based initiators for atom transfer radical polymerization (ATRP) bearing either an ester or amide linker. Variations in dry thickness of ester-based WPABs as a function of time and pH are consistent with WPAB degrafting via linker hydrolysis catalyzed by mechanical tension in the grafted chains. Sources of tension considered include high σ, as well as swelling and electrostatic repulsion associated with increasing degree of deprotonation (α) of repeat units in the WPAB. Normalized thickness of the WPAB decreases by a maximum amount at intermediate σ between ∼0.05−0.15 chains/nm², implying that contributions to tension by α are counterbalanced by charge regulation in the WPAB at high σ. Amide-based WPABs are more stable up to 264 h incubation, suggesting that commonly used ester-bearing ATRP initiators are more susceptible to hydrolysis over the time scales examined