Surface-Anchored Poly(N-isopropylacrylamide) Orthogonal Gradient Networks

Abstract

We present a versatile synthetic route leading toward generating surface-attached polyacrylamide gels, in which the cross-link density varies continuously and gradually across the substrate in two orthogonal directions. We employ free radical polymerization to synthesize random copolymers comprising ~5% of photoactive methacrylyloxybenzophenone (MABP), ~5% of thermally active styrene sulfonyl azide (SSAz), and ~90% of N-isopropylacrylamide (NIPAAm) units. The presence of MABP and SSAz in the copolymer facilitates control over the cross-link density of the gel in an orthogonal manner using photoactivated and thermally activated cross-linking chemistries, respectively. Spectroscopic ellipsometry is employed to determine the degree of swelling of the gel in water and methanol as a function of position on the substrate. Network swelling varies continuously and gradually across the substrate and is high in regions of low gel fractions and low in regions of high gel fractions.

Thermally Activated One-Pot, Simultaneous Radical and Condensation Reactions Generate Surface-Anchored Network Layers from Common Polymers

“…Such reactions occur readily under ambient conditions due to the presence of a minute amount of water (see the Supporting Information). We reason that 6-ASHTES forms complexes featuring two (or more) 6-ASHTES molecules when the mixture is first formed while the sulfonyl azide (SAz) groups react with the polymer to form sulfonamide bonds (and it is independent of subsequent annealing). − We also note that there will be an upper limit for the concentration of 6-ASHTES above which the current method will not work. This is determined by the miscibility of the silane and the polymer in the solution that is used to cast the film.…”

“…Substrate-anchored polymer coatings have been conventionally generated by adopting either the grafting-to or the grafting-from , strategies. Polymer networks on substrates have been formed by employing a precursor copolymer incorporating a cross-linkable unit, i.e., a photoactive (e.g., benzophenone) or thermally active (e.g., sulfonylazide) moieties, and grafting them to surfaces, which contain precoated anchoring chemical motifs . Irradiation with UV-light or exposure to heat triggers the cross-linking and surface-attachment reactions, thus resulting in surface-anchored polymer networks.…”

“…Figure a). We then expose the sample to an elevated temperature, which triggers cross-linking and concurrent attachment of the network to the substrate . Upon heating the sample to temperatures >100 °C, the azide groups release nitrogen gas and form singlet nitrenes, which are in equilibrium with the triplet state .…”

UV‐ and Thermally‐Active Bifunctional Gelators Create Surface‐Anchored Polymer Networks

“…[ 6–9 ] Upon UV irradiation or annealing, the layers undergo simultaneous crosslinking and surface‐attachment reactions that result in surface‐attached polymer networks. [ 10 ] Such an approach involves a complex synthesis of crosslinkers and surface‐anchors, demanding at least two fabrication steps that limit their application. We recently developed a robust and cost‐effective one‐step, one‐pot synthesis for depositing surface‐attached networks using 6‐azidosulfonylhexyltriethoxysilane (6‐ASHTES).…”

Surface‐Bound Microgels for Separation, Sensing, and Biomedical Applications

“…[22][23][24] The swelling ratio, defined as the relative weight fraction of water within the microparticle, [25] can also be adjusted either during preparation, [26][27][28] or in real-time by leveraging environmentally-responsive monomers and additives. [29][30][31][32][33] Reversible swelling behavior is arguably the most often exploited feature of microgel coatings, as it provides a means to control both morphological and physicochemical properties of the coated surface. Specifically, controlled swelling has been leveraged to release physically bound molecules, [20,[34][35][36] adjust the surface stiffness, [5,25,[37][38][39] and enable sensing activity.…”