Unusual Swelling Behavior of Hydrogels Modified with Spiropyran as Appendage or Crosslinker

Stimuli‐responsive materials typically contain responsive molecular units that couple an external trigger to a defined macroscale response. Ongoing efforts to boost the versatility and complexity of these responses increasingly focus on multi‐stimuli‐responsive molecular units and crosslinkers, as these bear the potential to impart self‐regulatory behaviors building on cooperative effects and feedback mechanisms. Herein, such a stimuli‐responsive platform consisting of polyacrylamide‐based hydrogels with spiropyrans covalently bound as pendant groups or ‘non‐innocent’ crosslinkers is studied. Surprisingly, as compared to methylenebisacrylamide‐crosslinked poly(acrylamide‐ co ‐acrylic acid) gels incorporating monofunctional appended spiropyran, gels with bifunctional spiropyran as a secondary crosslinker swell up to twofold larger despite their increased crosslinking density. To uncover the origin of this unexpected behavior, we employ nanoindentation, swelling analysis, and UV–vis spectroscopy to study changes in mechanical properties and in spiropyran isomer equilibrium distribution as a function of solution pH, co‐monomer chemistry, and swelling‐induced polymer strain. The osmotic counterion pressures as a function of spiropyran isomer distribution were estimated but found to be insufficient to explain the observed behavior. The combined effects of (i) charge complexation, (ii) cooperativity between the hydrogel's mechanics and chemistry, and (iii) formation of aggregates may all be invoked to explain features of the observed ‘non‐innocence’ of spiropyran crosslinkers. Taken together, these insights aid rational implementation of such responsive crosslinkers in materials design and extend the functionality of existing polymeric materials toward more complex tunable behaviors, useful in soft robotics, microfluidics, drug delivery, or self‐regulated materials.