Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 368, 69120 Heidelberg (Germany).
Angew Chem Int Ed Engl. 2016 Jan 11;55(2):811-4. doi: 10.1002/anie.201508706. Epub 2015 Dec 2.
Long polymer chains inevitably get tangled into knots. Like macroscopic ropes, polymer chains are substantially weakened by knots and the rupture point is always located at the "entry" or "exit" of the knot. However, these phenomena are only poorly understood at a molecular level. Here we show that when a knotted polyethylene chain is tightened, most of the stress energy is stored in torsions around the curved part of the chain. The torsions act as "work funnels" that effectively localize mechanical stress in the immediate vicinity of the knot. As a result, the knot "chokes" the chain at its entry or exit, thus leading to bond rupture at much lower forces than those needed to break a linear, unknotted chain. Our work not only explains the weakening of the polymer chain and the position of the rupture point, but more generally demonstrates that chemical bonds do not have to be extensively stretched to be broken.
长聚合物链不可避免地会纠缠成结。就像宏观绳索一样，聚合物链由于结而大大减弱，断裂点总是位于结的“入口”或“出口”处。然而，这些现象在分子水平上还没有得到很好的理解。在这里，我们表明，当一个缠结的聚乙烯链被拉紧时，大部分的应力能都储存在链的弯曲部分周围的扭转中。扭转作用作为“功漏斗”，有效地将机械应力集中在结的附近。结果，结在其入口或出口处“阻塞”了链，从而导致键断裂所需的力比断裂线性、无结链所需的力小得多。我们的工作不仅解释了聚合物链的弱化和断裂点的位置，而且更普遍地表明，化学键不必被广泛拉伸就可以断裂。
