(540g) Living Copolymerizations As a Tool to Tailor the Architecture and Fracture Properties of Polymer Networks

Soft materials are irreplaceable in applications that require large reversible deformations such as elastomers in rubber tires, dampers, and seals; as well as hydrogels in contact lenses, superabsorbent diapers, and artificial prosthetics. Despite finding widespread use in our daily lives, soft materials still suffer from excessive brittleness at high temperatures or solvent concentrations. The reason is rather simple. Soft materials are constituted of polymer networks and typically rely on chain friction to dissipate strain energy in the vicinity of cracks. At high temperatures or solvent concentrations, chain friction is negligible and the strain energy is readily dissipated by chain scission or fracture.

Early in the 21^st century, Gong and co-workers introduced a network architecture that could help solve this problem, interpenetrating a stiff and brittle poly(2-acrylamido-2-methylpropanesulfonic acid) filler network into a soft and extensible poly(acrylamide) matrix network. The resulting materials, referred to as multiple-networks, dissipate considerable strain energy by scission of filler network bonds and, as a result, are remarkably tough at high temperatures or solvent concentrations. However, how the molecular architecture of the constituent filler and matrix networks ultimately impacts the mesoscopic scission of filler network bonds and the macroscopic resistance to fracture still remains unknown.

We address this question by considering two families of multiple-networks with filler networks (i) synthesized either by RAFT or free radical copolymerizations and (ii) labeled with mechanophores. By fracturing these materials over a range of temperatures and stretch rates, and quantifying their damage by laser scanning confocal microscopy, we demonstrate that more homogeneous filler networks afford tougher multiple-networks not only due to a notable increase in the size of the dissipation zone, but also because of considerable chain friction in the vicinity of cracks as chains untangle and stretch near their limiting extensibility. In other words, we demonstrate that homogeneous filler networks amplify the energetic cost of breaking a filler network bond, providing molecular rationale for leveraging living copolymerizations to fine tune the mechanisms responsible for dissipating strain energy and to design soft materials that resist fracture at high temperatures or solvent concentrations.