(718b) Detecting Bond Breakage and Fracture in Tough Hydrogels

Synthetic
hydrogels are
soft materials composed of three-dimensional polymer networks swollen with
water. They are promising synthetic analogues of tissues but their excessive
brittleness remains an important performance limitation. Although there have
been advances in the design[1]–[3] and
characterization[4] of polymer
networks with a range of structures and topologies that provide hydrogels with
remarkable properties (e.g., high toughness and self-healing), it is not yet
possible to quantitatively predict toughness from molecular design. A promising
strategy to gain molecular insight on bond scission in polymers prior to
failure is the incorporation of mechanoluminescent cross-linkers.[5] Upon
force-induced bond breakage, emission of visible light allows for
spatio-temporal mapping of molecular bond breaking during crack propagation. 
Despite recent demonstrations in elastomers, no mechanophore has been
successfully incorporated in a hydrogel to visualize bond scission upon
deformation, yield, and fracture.

We have synthesized and incorporated a water-soluble
mechanoluminescent probe based on bis(adamantyl)-1,2-dioxetane into a model
double-network hydrogel composed of stiff
poly(2-acrylamido-2-methylpropanesulfonic acid) and soft poly(acrylamide). Due
to the low quantum yield of the emitter ketone resulting from force-induced
ring-opening, we explored a range of FRET agents to detect covalent bond
scission during deformation. This information serves not only to establish a
direct relation between polymer network structure and hydrogel fracture energy,
but also to develop new multi-scale physical models of macroscopic fracture of
soft materials.

Figure 1: Real-time mapping of covalent bond
scission during compression using mechanoluminescent probes. (A) Schematic of the light emission
obtained during compression of a double-network hydrogel containing
mechanoluminescent cross-linkers.  (B) Mechanoluminescent reaction where
force-induced bon breakage of a 1,2-dioxetane cross-linker results in light
emission.

References

^[1]
Gong, J. P.; et. al. Adv. Mater. 2003, 15
(14), 1155–1158. ^[2] Sun, J.-Y.; Zhao, et. al.Nature 2012,
489 (7414),

133–136. ^[3] Ducrot, E.; et.
al. Science. 2014, 344 (6180), 186–189.^[4] Baumberger,
T.; et al. Nat. Mater. 2006, 5

(7), 552–555. ^[5] Chen, Y.; et.
al.Nat. Chem. 2012, 4 (7), 559–562.