(83e) Reactivity You Can Feel: Substituent Effects and Network Fracture

The fracture of rubbery covalent polymer networks leads to critical failure of the material, limiting the lifetime of products such as biomedical implants, contact lenses, and even tough consumer products like tires. While these fracture events are usually thought of on a macroscopic scale, understanding the microscopic processes involved in fracture is critical to avoid or prolong failure of a material. The classical Lake-Thomas theory predicts the tearing energy of a network based on the number of elastically effective chains per cross-sectional area of the crack plane multiplied by the energy required to break a chain. At a very simplistic level, adjusting the strength of the bridging chain should therefore moderate the macroscopic tearing energy of the network. At a slightly more complex level, fracture of rubbery polymer networks involves a series of molecular events, beginning with conformational changes along the polymer backbone and culminating with a chain scission reaction, all of which contribute to the overall fracture behavior.

Here, we report covalent polymer gels in which the macroscopic fracture “reaction” is controlled by mechanophores of varying strength embedded within mechanically active network strands. We synthesized poly(ethylene glycol) (PEG) gels through the end-linking of azide-terminated tetra-arm PEG with bisalkyne linkers. Networks were formed under identical conditions, except that the bis-alkyne was varied to include either a cis-diaryl (1) or cis-dialkyl (2) linked cyclobutane mechanophore that acts as a mechanochemical “weak link” through a force-coupled cycloreversion. A control network featuring a bis-alkyne without cyclobutane (3) was also synthesized. The networks show the same linear elasticity (G′ = 23−24 kPa, 0.1−100 Hz) and equilibrium mass swelling ratios (Q = 10−11 in tetrahydrofuran), but they exhibit tearing energies that span a factor of 8 (3.4 J, 10.6, and 27.1 J·m^−2 for networks with 1, 2, and 3, respectively). The difference in fracture energy is well-aligned with the force-coupled scission kinetics of the mechanophores observed in single-molecule force spectroscopy experiments, implicating local resonance stabilization of a diradical transition state in the cycloreversion of 1 as a key determinant of the relative ease with which its network is torn. This system allows for further exploration into a recently proposed micro-network fracture theory, which treats the crack tip as a region instead of a plane. This hypothesis was tested by varying the proportion of strong to weak linkers, leading to a better fundamental understanding of crack propagation behavior at the crack tip. The connection between macroscopic fracture and a small-molecule reaction mechanism suggests opportunities for molecular understanding and optimization of polymer network behavior.