Interplay Between Applied Force and Radical Attack in the Mechanochemical Chain Scission of Poly(acrylic acid)

Sonication and radical attack are both known to contribute to breaking down polymers. Quantum chemical models show how the two can operate together, where radical attack is shown to reduce the effective tensile strength of the material. Using poly(acrylic acid) (PAA) as a model, hydrogen atom abstraction in PAA was found to improve the thermodynamics and kinetics of bond scission. The force needed for bond rupture was estimated to decrease from 4.7 to 2.5 nN. This occurs because hydrogen atom abstraction drastically alters the potential energy surface of the scissile bond. Bond activation was also found to decrease the magnitude of the changes in bond scission geometries and energetics in response to the applied force. While radical abstraction is overall beneficial for mechanical bond scission, the polymer also becomes less responsive to force than the unactivated polymer. This finding places upper limits on the efficacy of the synergy between radical attack and applied force. In addition, the importance of reaction pathway optimization is also shown, where comparisons to the COGEF method show the latter to be qualitatively incapable of describing chain scission after radical activation.