From Appendage to Crosslinker – Unusual Swelling Behavior in Spiropyran-Modified Hydrogels

Abstract

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, we study a stimuli-responsive platform consisting of polyacrylamide-based hydrogels with well-known multi-responsive spiropyrans covalently bound as pendant groups or ´non-innocent´ crosslinkers. Surprisingly, as compared to their appended counterparts, spiropyran crosslinkers cause up to two-fold larger hydrogel swelling in methylenebisacrylamide-crosslinked poly(acrylamide-co-acrylic acid) hydrogels, despite their increased relative crosslinking density. We seek the origin of this unexpected behavior by employing nanoindentation, swelling studies, and UV-vis spectroscopy to study changes in mechanical properties and in spiropyran isomer distribution as a function of solution pH, co-monomer chemistry, and swelling-induced polymer strain. We then estimate the osmotic counterion pressures as a function of spiropyran isomer distribution but find that such pressures alone are insufficient to explain the observed behavior. Charge complexation, cooperative effects between the hydrogel´s mechanics and chemistry, and aggregate formation may all be invoked to explain features of the observed ´non-innocence´ of spiropyran crosslinkers. Taken together, these insights will aid rational implementation of such responsive crosslinkers in materials design and extend the functionality of existing polymeric materials towards more complex and better tunable behaviors.

Spiropyran Photoisomerization Dynamics in Multiresponsive Hydrogels

“…The two main peaks observed in poly­(AAm- co -AAc- co -SP) polymers are replaced by a single peak at about 460–470 nm, even under acidic conditions. (D) Additional support is found in spiropyran-cross-linked poly­(AAm- co -DMAEMA) hydrogels . Such hydrogels show absorption bands at λ max ∼ 400 nm and λ max ∼ 530 nm corresponding to the MCH + and MC-species, respectively, when immersed in phosphate buffers (0.05 M).…”

“…Such hydrogels show absorption bands at λ max ∼ 400 nm and λ max ∼ 530 nm corresponding to the MCH + and MC-species, respectively, when immersed in phosphate buffers (0.05 M). No broad absorption at 470 nm is observed . In the context of possible H-MC aggregates, we propose that lack of a λ max ∼ 470 nm absorption band is due to the tethering constraints inhibiting the aggregation of MC.…”

“…SAXS and WAXS experiments on two-year-old spiropyran-functionalized poly­(AAm- co -DMAEMA) gels show sharp peaks suggestive of crystallinity (Supporting Information section S6), providing further support for the slow formation of aggregates. In addition, our own work on bis-acrylated spiropyran cross-linkers provides more circumstantial support for the H-MC hypothesis: a bis-acrylated spiropyran acting as a cross-linker would not be expected to form aggregates due to the steric constraints on both ends of the chromophore. Indeed, in UV–vis spectra of spiropyran-cross-linked poly­(AAm- co -DMAEMA- co -SP) hydrogels there is no evidence of a peak at 470 nm (Figure D).…”

“…While aggregation adds further complexity and responsivity to spiropyran-functionalized gel systems, which may be particularly interesting for the design of multistimuli-responsive and self-regulated smart actuators, it may be undesirable in simple actuators where long-term switching effects would interfere with actuator operation. In such cases, modified spiropyran moieties that do not aggregate should be used, such as doubly acrylated spiropyran cross-linkers or spiropyrans with modified linkages that sterically impede aggregation . These design considerations are broadly important for the use of multiresponsive spiropyran-functionalized hydrogels for applications in a wide variety of fields, including microfluidics, − soft robotics, drug delivery, and nonlinear optics. , …”

“…While significant work has been done to understand the effects of the spiropyran molecular structure on its photochemistry in solution ,,,− and on the effects of incorporation into a dense polymer matrix, less work has been done to explore the effects that co-monomers in an aqueous hydrogel environment have on spiropyran dynamics. , Additionally, the high-intensity, focused laser light conditions relevant to the nonlinear optics of spiropyran-functionalized hydrogels , constitute an environment in which spiropyran switching kinetics have rarely been studied. As part of our ongoing efforts to codify design principles for hydrogels that achieve self-regulation via opto-chemo-mechanical feedback behaviors, ,,− we used laser scanning confocal fluorescence microscopy and UV–vis spectroscopy to probe the effects of different co-monomers on the dynamics of light-driven and thermal isomerization of the spiropyran moieties within four different commonly used hydrogel materials: temperature-responsive pNIPAAm gels, pH-responsive pAAc and poly­(2-(dimethylamino)­methacrylate) (pDMAEMA) gels, and nonresponsive polyacrylamide (pAAm) gels. These gels differ significantly in terms of charge, acidity, and hydrophobicity and thus provide a good platform to study spiropyran behavior in a wide variety of practical hydrogel environments.…”