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.