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

“…SAXS and WAXS experiments on two-year-old spiropyranfunctionalized 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 37 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-crosslinked poly(AAm-co-DMAEMA-co-SP) hydrogels there is no evidence of a peak at 470 nm (Figure 2D).…”

“…No broad absorption at 470 nm is observed. 37 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.…”

“…No broad absorption at 470 nm is observed. 37 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. the focused beam was compared to the background to provide a control for the fluorescence dynamics in the absence of highintensity irradiation.…”

“…(D) Additional support is found in spiropyrancross-linked poly(AAm-co-DMAEMA) hydrogels. 37 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.…”

“…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 crosslinkers 37 or spiropyrans with modified linkages that sterically impede aggregation. 41 These design considerations are broadly important for the use of multiresponsive spiropyran-functionalized hydrogels for applications in a wide variety of fields, including microfluidics, 1−5 soft robotics, 6 drug delivery, 13 and nonlinear optics.…”