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.

超声处理与自由基进攻均被证实可促进聚合物降解。量子化学模型揭示了二者协同作用的机制：自由基进攻可降低材料的有效抗拉强度。以聚丙烯酸（Poly(acrylic acid), PAA）为模型聚合物，研究发现聚丙烯酸分子中的氢原子夺取反应可优化化学键断裂的热力学与动力学过程。经估算，化学键断裂所需的作用力从4.7纳牛降至2.5纳牛。该现象源于氢原子夺取反应显著改变了易断裂键的势能面。此外，自由基活化还可削弱外加作用力引发的键断裂几何结构与能量变化幅度。尽管自由基进攻整体上有利于机械键断裂，但经活化的聚合物相较于未活化聚合物，其对外加作用力的响应性有所降低。该发现为自由基进攻与外加作用力之间的协同效应的效能设定了上限。同时，本研究也凸显了反应路径优化的重要性：与COGEF方法对比后发现，后者无法定性描述自由基活化后的链断裂过程。