Reversibly Cross-Linking Polymer Brushes Using Interchain Disulfide Bonds

Abstract

The introduction of interchain cross-links in surface-grafted polymer brushes increases the robustness and mechanical properties of these thin polymer films. In most cases, cross-linked polymer brushes contain permanent interchain cross-links. The use of reversible interchain cross-links, in contrast, provides opportunities to dynamically modulate the cross-link density and properties of surface-grafted polymer brushes. This study explores the use of disulfide bonds to reversibly cross-link poly(2-(dimethylamino)ethyl methacrylate) copolymer brushes. These brushes were prepared via surface-initiated atom transfer radical copolymerization of 2-(dimethylamino)ethyl methacrylate and an azide-containing comonomer, 3-azido-2-hydroxypropyl methacrylate, followed by copper(I)-catalyzed azide−alkyne cycloaddition of S-propargyl thioacetate and subsequent deprotection of the thioacetate moieties. Heating the polymer brushes to 60 °C under air for 2 h resulted in the formation of disulfide cross-links, which could be reduced to generate the corresponding free thiol groups upon brief exposure to tris(2-carboxyethyl)phosphine hydrochloride. The formation and cleavage of the interchain disulfide cross-links has a profound influence on the swelling and viscoelastic properties of the brushes. Cross-linking leads to a decrease in swelling ratio and a concomitant dehydration and loss in dissipative properties of the brush film. These changes were observed using ellipsometry and quartz crystal microbalance with dissipation monitoring experiments by exposing the polymer brushes to a sequence of successive cross-linking and uncross-linking steps. These experiments indicated that while cross-linking and uncross-linking were fully reversible during the first few cycles, the response of the brush films became less pronounced upon prolonged oxidation/reduction, which was attributed to the oxidation of thiol side-chain functional groups and a concomitant reduction in the cross-link density of the polymer brushes. The results presented in this study show that the incorporation of disulfide interchain cross-links allows access to polymer brush films that can be reversibly cross-linked and uncrosslinked over many cycles.

Thiol- and Disulfide-Based Stimulus-Responsive Soft Materials and Self-Assembling Systems

“…Mocny and coworkers examined redox-reversible disulfide crosslinking of thiol-containing polymer brushes [ 86 ] that are thin films formed by end-grafting linear polymer chains. Unlike the sol-gel examples above, polymer brushes are grafted to a substrate, so the polymer “gels” in this case are swollen thin films attached to a substrate, and the reduction of the disulfide crosslinks merely changes the brush density, instead of forming a sol.…”

“… ( a ) Cartoon illustrating the reversible formation of disulfide crosslinks between polymer brush chains. ( b ) The composition of the polymer brushes studied by Mocny et al Reprinted with permission from [ 86 ]. Copyright (2020) American Chemical Society.…”

Post‐polymerization modification of polyetherimide by Friedel‐Crafts acylation: Physical–chemical characterization and performance as gas separation membrane

“…[1][2][3] Compared to direct synthesis, postpolymerization modification has the advantage of allowing the straightforward establishment of structure/property relations, since different derivatives can be prepared with the same molar mass and molar mass distribution, and this is a suitable route for the preparation of polymers whose monomers are not stable under the polymerization conditions. [4][5][6] The recent literature shows an increasing interest in the application of this strategy for the preparation of copolymers, 7,8 polymer bottlebrushes, 9 and polymer brushes 10 as well as other polymeric materials such as ion exchange membranes, 11 microporous organic polymers for CO 2 absorption, 12 digital polymers for information storage, 13 and biomedical devices for drug delivery. 2,14 Despite the increasing interest, the post-polymerization modification of high-performance aromatic polymers, such as polyetherimide and polysulfones, has not received much attention.…”

Light‐Gated Control of Conformational Changes in Polymer Brushes

“…[ 16 ] Recently, Mocny and Klok explored the reversible crosslinking of polymer brushes using interchain disulfide bonds. [ 17 ] The crosslinking of the polymer brushes was achieved by heating the polymer brushes to 60 °C under air, whereas the decrosslinking was achieved by exposing the initially formed disulfide bridges to a chemical trigger, tris(2‐carboxyethyl)phosphine hydrochloride, releasing free thiol groups. Other examples of conformation changes in polymer brush structures, and their properties, via crosslinks, are based on the complexation of ions or low molecular guests, [ 18 ] the addition of bis(Pd II ‐pincer) complexes [ 19 ] or urea, [ 20 ] depending on the functional moieties attached to the polymer.…”

“…[16] Recently, Mocny and Klok explored the reversible crosslinking of polymer brushes using interchain disulfide bonds. [17] The crosslinking of the polymer brushes was achieved by heating the polymer brushes to 60 °C under air, whereas the decrosslinking was achieved by exposing the initially formed disulfide bridges to…”