Tuning the Volume Phase Transition Temperature of Microgels by Light

Abstract

Temperature‐responsive microgels find widespread applications as soft materials for designing actuators in microfluidic systems, as carriers for drug delivery or catalysts, as functional coatings, and as adaptable sensors. The key property is their volume phase transition temperature, which allows for thermally induced reversible swelling/deswelling. It is determined by the gel's chemical structure as well as network topology and cannot be varied easily within one system. Here a paradigm change of this notion by facilitating a light‐triggered reversible switching of the microgel volume in the range between 32 and 82 °C is suggested. Photo‐sensitivity is introduced by photosensitive azobenzene containing surfactant, which forms a complex with microgels consisting of poly(N‐isopropylacrylamide‐co‐acrylic acid) (PNIPAM‐AAc) chains when assuming a hydrophobic trans‐state, and prefers to leave the gel matrix in its cis‐state. Using a similar strategy, it is demonstrated that at a fixed temperature, for example, 37 °C, one can reversibly change the microgel radius by a factor of 3 (7–21 µm) by irradiating either with UV (collapsed state) or green light (swollen state). It is envisaged that the possibility to deploy a swift external means of adapting the swelling behavior of microgels may impact and redefine the latter's application across all fields.

Adsorption Kinetics of a Photosensitive Surfactant Inside Microgels

“…(a) Microgel ( c PAA = 20 mol %, measured VPTT of 80 °C collapse under illumination with blue light ( I = 2 mW cm –2 )) at different temperatures. (b) Temperature-dependent size of the microgel at equilibrium (at 120 s from (a)) normalized over the initial diameter.…”

Photo- and thermo-responsive microgels with supramolecular crosslinks for wavelength tunability of the volume phase transition temperature

“…Notably, the cis-trans isomerization is fully reversible and switching can be controlled by light of appropriate wavelength. Furthermore, fine-tuning of the VPTT of microgels 39,40 and, in general, polymers 41,42 can be achieved by the introduction of functional groups or different monomers. We demonstrate that the configuration of our crosslinks significantly influences the VPTT since the apolar azobenzene in its trans form is shielded by the polar, embedding bCD (crosslink closed) whereas the cis form (crosslink open) exhibits a non-shielded apolar surrounding.…”

Thermosensitive P(AAc‐co‐NIPAm) Hydrogels Display Enhanced Toughness and Self‐Healing via Ion–Ligand Interactions

“…We then demonstrate that the incorporation of thermosensitive P(NIPAm) induces temperature-sensitive equilibrium swelling response and imbues these gels with a volumetric phase transition above the critical transition temperature of 32 °C. [32,33] Furthermore, we show that the inclusion of reversible bonds imparts these gels with excellent recovery of mechanical strength during cyclic loading, and the ability to self-heal. Finally, we exhibit these gels' excellent adsorption properties.…”

“…Since P(NIPAm) constitutes a large fraction of the copolymer networks, we hypothesized that the mechanical response of the emergent hydrogels would be highly sensitive to temperature. Indeed, when subjected to uniaxial tension at 36 ○ C, which is above the lower critical solution temperatures (i.e., volume transformation temperature) of 32 °C for P(NIPAm), [32,33] D h-0.033-2 hydrogels exhibit a significant decrease in average tensile strength from 365 ± 31 kPa to 183 ± 19 kPa, and a reduction in the corresponding average fracture strain from 1754 ± 87% to 1056 ± 57%, as compared to D h-0.033-2 hydrogels loaded at 25 ○ C (Figure S8, Supporting Information). Together, these result in an ≈71% decrease in toughness from 2.99 ± 0.26 MJ m −3 to 0.86 ± 0.11 MJ m −3 .…”