Polyelectrolyte brush pH-response at the silica–aqueous solution interface: a kinetic and equilibrium investigation

Abstract

Polymer brushes are commonly used to modify the properties of solid surfaces. Here a family of polybasic poly(2-(diethylamino)ethyl methacrylate) brushes have been grown using ARGET ATRP from a cationic macroinitiator adsorbed on two types of silica surfaces: QCM crystals and oxidised silicon wafers. The pH-response of these brushes is investigated as a function of brush thickness in a constant flow environment in order to focus on the intrinsic dynamics of the polymer brushes. Independent QCM-D and in situ ellipsometry kinetic studies demonstrate the swelling process of protonation and solvent uptake is typically eight times faster than the corresponding neutralisation and solvent expulsion from the collapsing brush. However, the maximum rate of these processes is independent of brush thickness. The initial pH response of the brushes is hysteretic due to brush entanglement, which once overcome allows highly reversible pH-induced conformational changes. Multiple pH cycles demonstrate that the viscoelastic nature of the swollen state relative to the collapsed brush is independent of brush thickness.

Huge Differences in the Kinetics of Swelling Enhancement and De‐enhancement of Permanently Charged Polyelectrolyte Brushes

“…Taking the NaPSS brush with more polymer chains ( σ =0.15 chain nm −2 ) as an example, the time for the brushes to recover to their initial state (the state under salt‐free conditions) was about seven times longer than the process of swelling enhancement. This difference is very similar to what was observed in the weak polyelectrolyte brushes systems, although its physical mechanism differs from the current permanently charged system. Secondly, the kinetics of de‐enhancement depend on the grafting density: The brushes with lower grafting densities recovered much faster than the brushes with higher grafting densities.…”

Specific ion modulated thermoresponse of poly(N-isopropylacrylamide) brushes

“…suggest that brushes grown on the QCM sensors have similar dry brush thickness values to brushes grown on wafers under identical polymerization conditions (the 'sister' wafer). 1 Therefore, the dry thickness value reported here for the brush-modified QCM sensor was determined from the sister wafer.…”

“…Specific ion effects, which cannot be not wholly explained by classical electrolyte theories, have been extensively investigated ever since they were first observed by Franz Hofmeister in the late 19 th century. [1][2][3][4][5] Initially identified through salt induced variations in the precipitation of egg white protein in aqueous solution, specific ion effects have been found to affect a wide range of phenomena including the stability of colloidal dispersions, 6 bubbles and foams, 7 protein solubility, 8 the response of thermoresponsive polymers (cloud points), 9 solution properties like viscosity (Dole-Jones Equation), 10 and interfacial properties such as surface tension and ion partitioning. 11 Such effects are generally more pronounced for anions than cations due to their greater range in polarisability and follow an approximate trend often referred to as the Hofmeister series.…”

Uptake of pH-Sensitive Gold Nanoparticles in Strong Polyelectrolyte Brushes

“…Polymer brushes represent a class of thin films where the polymer chains are chemically end-grafted to the substrate. The chain functionality can be tailored by chemical composition and allows the brushes responding to outer stimuli like temperature [1,2,3,4], pH [5,6,7,8,9], ionic strength [10,11,12,13,14] or solvent [15,16,17,18]. Due to their responsive nature, brushes are highly applicable for their use as “smart” surfaces.…”