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 energy dissipation 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.

Here, I will discuss the use of chain transfer agents and catalysts 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 tests, and mechanochemistry. I will show that delayed percolation of polymer networks 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 reversible elasticity and fracture toughness at high temperatures and solvent concentrations.