Reactivity-Guided Depercolation Processes Determine Fracture Behavior in End-Linked Polymer Networks

Area of Science:

• Materials Science
• Polymer Chemistry
• Mechanochemistry

Background:

• The mechanical failure of polymer networks is complex, with molecular-level processes determining fracture limits.
• Understanding how strand reactivity influences network fracture is crucial for designing robust materials.

Purpose of the Study:

• To investigate reactivity-guided fracture in polymer networks by varying the composition of strands with different mechanochemical reactivities.
• To elucidate the molecular mechanisms underlying the mechanical limits of polymer networks.

Main Methods:

• Synthesized end-linked polymer networks with varying ratios of 'strong' (less reactive) and 'weak' (more reactive) strands.
• Measured fracture energy and determined the percolation threshold of strong strands.
• Performed coarse-grained fracture simulations to model network behavior.

Main Results:

• Fracture energy remained low until strong strand content reached ~45%, then sharply increased with higher strong strand concentrations.
• The sharp increase in fracture energy correlated with the strong strand percolation threshold of 48 ± 3%.
• Simulations showed weak strand scissions dominate failure until strong strands approach percolation, and mixtures experience twice as many strand breaks as pure networks.

Conclusions:

• Network depercolation, or the loss of a connected structure of strong strands, is a necessary condition for crack propagation.
• The study reveals a critical molecular-level mechanism controlling polymer network fracture toughness.
• Computational models validated experimental observations, highlighting the role of weak strand failure and increased strand breakage in mixed-reactivity networks.