Critical Salt Effects in the Swelling Behavior of a Weak Polybasic Brush

Abstract

The swelling behavior of poly(2-(diethylamino)ethyl methacrylate) (PDEA) brushes in response to changes in solution pH and ionic strength has been investigated. The brushes were synthesized by ARGET ATRP methodology at the silica-aqueous solution interface via two different surface-bound initiator approaches: electrostatically adsorbed cationic macroinitiator and covalently anchored silane-based ATRP initiator moieties. The pH-response of these brushes is studied as a function of the solvated brush thickness in a constant flow regime that elucidates the intrinsic behavior of polymer brushes. In situ ellipsometry equilibrium measurements show the pH-induced brush swelling and collapse transitions are hysteretic in nature. Furthermore, high temporal resolution kinetic studies demonstrate that protonation and solvent ingress during swelling occur much faster than the brush charge neutralization and solvent expulsion during collapse. This hysteresis is attributed to the formation of a dense outer region or skin during collapse that retards solvent egress. Moreover, at a constant pH below its pKa, the PDEA brush exhibited a critical conformational change in the range 0.5-1 mM electrolyte, a range much narrower than predicted by the theory of the osmotic brush regime. This behavior is attributed to the hydrophobicity of the collapsed brush. The swelling and collapse kinetics for this salt-induced transition are nearly identical. This is in contrast to the asymmetry in the rate of the pH-induced response, suggesting an alternative mechanism for the two processes dependent on the nature of the environmental trigger.

Stimuli-sensitive intrinsically disordered protein brushes

“…7) 22,23 , which predicts that the brush should collapse with increasing salt concentration (C), but only as a relatively weak power law (C À 1/3 ). We see a comparatively steeper ionic strength-dependence, which has been previously observed in synthetic polymer systems [24][25][26][27][28][29] and may arise from the local modulation of protein charge-based interactions due to added salt. At pH 10.9, below a critical ionic strength (B2 mM), the system enters an 'osmotic brush' regime in which the brush height increases with ionic strength.…”

“…As a consequence, the pH within the brush increases, thereby increasing the degree of deprotonation of the protein chains. This results in greater chain stretching due to counterion-induced osmotic swelling and increased inter-and intra-molecular electrostatic repulsion associated with chain ionization, as has been observed with synthetic polyelectrolyte and polyampholyte brush systems 24,28,29 . Above a critical ionic strength, this exchange mechanism saturates and the system enters the 'salted brush' regime, in which further addition of salt collapses the brush due to electrostatic screening ( Supplementary Figs 6 and 7).…”

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.…”

Anion-Specific Effects on the Behavior of pH-Sensitive Polybasic Brushes

“…28 Our previous work has shown that the ideal dry thickness for the brushes to be investigated by ellipsometry is ~20 nm. 26,28,30 Polymerization times were selected accordingly and measured brush thickness values are given in Table 1. For the QCM-D measurements similar dry thickness brushes were used for poly(DEA) and poly(DPA).…”

“…Several publications have experimentally investigated the effects of salt on a weak polyelectrolyte brush in order to compare with theoretical scaling laws. 28,53,54,55,56,57 Significantly, three of these studies report weaker scaling exponents for brush thickness with increasing ionic strength for the salted brush regime. 28,55,57 Furthermore, our previous work on a poly(2-diethylamino)ethyl methacrylate brush showed that in the osmotic brush regime the brush transitions from a collapsed to swollen conformation over a much narrower concentration range than predicted theoretically.…”