Molecular Transport in Thin Thermoresponsive Poly(N-isopropylacrylamide) Brushes with Varying Grafting Density

Abstract

The effect of the grafting density on the molecular transport through thermoresponsive brushes of poly­(N-isopropylacrylamide) (PNIPAM) grafted onto flat gold substrates was investigated using voltammetry and impedance spectroscopy. PNIPAM brush layers were synthesized at four different grafting densities using surface-initiated atom transfer radical polymerization (SI-ATRP) from mixed self-assembled monolayers of ω-mercaptoundecyl bromoisobutyrate and undecanethiol chemisorbed on gold surfaces. Tethered PNIPAM layers with grafting densities resulting from initiator concentrations lower than 25% in the thiol monolayer show the same transport properties as the initial self-assembled monolayer before brush synthesis. For higher grafting densities, the diffusion coefficients, D, of the K3[Fe­(CN)6]/K4[Fe­(CN)6] redox probe is 7 orders of magnitude smaller than those typically measured in aqueous solutions and independent of whether the brush is collapsed or swollen. The collapse of the PNIPAM brush drives a hydrophilic/hydrophobic transition in addition to structural/conformational transformations of the grafted layers, resulting in still smaller values of D. However, these changes do not lead to a blocking effect on the active area of the gold surface, which is only determined by pinholes or discontinuities in the thiol initiator monolayer. These results are only observed for thin PNIPAM brush layers.

Molecule motion at polymer brush interfaces from single‐molecule experimental perspectives

“…Regarding the diffusion of the K 3 [Fe(CN) 6 ]/K 4 [Fe(CN) 6 ] probe in neutral PNIPAM brushes, Gervasi et al observed a strong grafting density dependence: in the PNIPAM brushes of high grafting densities, the measured D of the redox probe was 7 orders of magnitude smaller than that in the bulk solution. Yet in the PNIPAM brushes of low grafting densities, the measured D was the same as that on the self‐assembled monolayer of initiators before brush synthesis . It seemed that the strong dependence of D on grafting density originated from the blocking effect of active sites on Au electrode surfaces for the impedance measurements.…”

Layer-by-Layer Assembled Microgels Can Combine Conflicting Properties: Switchable Stiffness and Wettability without Affecting Permeability

“…They observed that an increase in modulus was unambiguously correlated with a decrease in the permeability to iodide ion. 14 The same correlation was also observed in PNIPAM brushes 15 and hydrogels 16 ; a thermo-induced increase in stiffness or hydrophobicity was associated with a concomitant decrease in permeability. In principle, the permeability and mechanical properties of water-swollen systems such as polyelectrolyte multilayers, polymer brushes or hydrogels are expected to be strongly correlated because softer, more hydrated materials also tend to be more permeable.…”

Molecular Design of Solid‐State Nanopores: Fundamental Concepts and Applications

“…Polymer brushes are assemblies of macromolecules that are tethered by one end to a surface. These macromolecular architectures bearing different functional groups along their backbones can be grafted on different surfaces, and even in constrained environments . There are two main strategies for generating polymer brushes on solid surfaces: “grafting to” and “grafting from.” In the “grafting‐to” technique, presynthesized polymers are anchored to a surface from solution.…”