How does gelation impact the mechanical properties of polymer networks? Insights from polymer mechanochemistry

Polymer networks that sustain large reversible deformations are widespread in engineering, biomedical, and electronic applications. At high temperatures or solvent concentrations, these materials are excessively brittle due to negligible dissipation of strain energy in the vicinity of cracks. Recently, this issue was resolved by Gong and co-workers by embedding a stiff, and pre-stretched polymer filler network within a soft and extensible polymer matrix network. Yet, how the molecular architecture of these filler and matrix networks ultimately dictates the macroscopic fracture toughness remains unknown.

In this talk, I will discuss the use of chain transfer agents and catalysts as a tool to control the concentration of chain ends during gelation, the percolation threshold, the static heterogeneities, and the small- and large-strain mechanical properties of polymer networks. I will consider three (3) networks with similar densities of elastically active chains and evaluate their architecture and mechanical properties through confocal microscopy, tensile testing, and mechanochemistry. I will show that delayed percolation results in nucleation of static inhomogeneities near the gel point, and lower chain extensibilities and delocalized stresses ahead of the crack front. Finally, I will provide rationale for controlling gelation to design networks with an optimal combination of high-temperature elasticity and fracture toughness.