A fluorescence study on the local environment of hydrogels: Double-network hydrogels having extraordinarily high mechanical strength and its constituent single-network hydrogels

We studied the local environment of a novel double-network (DN) hydrogel and its constituent hydrogels, poly-(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) and poly-acrylamide (PAAm) gel, by fluorescence spectroscopy using a fluorescent probe. The steady-state spectra and lifetimes of the probe fluorescence in the three hydrogels were similar to those in bulk water, indicating that almost all the probe molecules reside in water pools that have local polarity similar to bulk water. The fluorescence anisotropy decay in PAMPS gel exhibits a bulk-like single exponential decay, indicating that the probe exists in sufficiently large water pools where the probe freely rotates, while it shows bi-exponential decays in PAAm and DN gels, suggesting that a part of the probe reside in confined water pools. This observation indicates that PAAm is entangled with PAMPS gel in DN gel realizing high mechanical strength, and PAAm plays the dominant role in forming the local water environment in DN gel.

Highlights► Fluorescence study on local environments, polarity and viscosity in the hydrogels. ► The probe molecules in the hydrogels experience water-like high local polarity. ► There are large water pools in all the hydrogels. ► There are also small confined water pools in the PAAM and DN gel. ► Association of constituent polymers generates high mechanical strength of DN gel.