Molecular-Level Elucidation of a Fracture Process in Slide-Ring Gels via Coarse-Grained Molecular Dynamics Simulations
Molecular-Level Elucidation of a Fracture Process in Slide-Ring Gels via Coarse-Grained Molecular Dynamics Simulations
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通过粗粒度分子动力学模拟在分子水平上阐明滑环凝胶的断裂过程
DOI：
10.1021/acs.macromol.1c01981
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发表时间：
2022
期刊：
影响因子：
5.5
通讯作者：
Kubo Momoji
中科院分区：
文献类型：
--
作者：
Uehara Shuichi;Wang Yang;Ootani Yusuke;Ozawa Nobuki;Kubo Momoji
具有可滑动的交联环状分子的滑环（SR）凝胶比常规的固定交联（FC）凝胶表现出相当高的断裂韧性。然而，SR凝胶的机械性能仍然不令人满意，因此，这些凝胶不能实际应用。因此，需要从分子尺度上深入研究SR凝胶的断裂机理，以提高其力学性能。本研究利用粗粒分子动力学（CGMD）方法对FC和SR凝胶进行了拉伸应变模拟，以阐明SR凝胶的断裂机制。CGMD模拟表明，SR凝胶表现出更高的断裂韧性比FC凝胶。在FC凝胶中，轴链在低应变下容易断裂，因为固定的交联约束聚合物链，导致轴链上的应力集中。相反，在SR凝胶中，直到施加高应变，轴链才断裂，因为交联的环状分子滑动而不累积应力集中。对SR凝胶断裂机理的分析表明，链端是SR凝胶断裂的根源。交联和未交联的环状分子在轴链上滑动并围绕链端堆叠。堆叠的交联环状分子在链端周围引起应力集中。接下来，我们研究了覆盖对SR凝胶的影响。我们发现，低覆盖率SR凝胶表现出更高的断裂韧性和极限强度比高覆盖率SR凝胶，因为堆叠的未交联的环状分子阻止交联的环状分子从滑动，导致低断裂韧性所造成的应力集中在链端。具有高断裂韧性的低覆盖度SR凝胶的轴链向应变方向变形，直到高应变。因此，低覆盖率SR凝胶显示出朝向应变方向的高取向。高取向有助于链上机械应力的均匀分布，从而导致高极限强度。虽然SR凝胶通常表现出较低的极限强度，但具有5%的低覆盖率的SR凝胶表现出比FC凝胶更高的极限强度。最后，我们建议，与高覆盖率的SR凝胶相比，低覆盖率的SR凝胶表现出更高的断裂韧性和极限强度，基于原子的洞察力有关的机制，其中低覆盖率的SR凝胶具有自由空间的交联环状分子移动到链端，直到施加高应变。
Slide-ring (SR) gels with slidable cross-linked cyclic molecules exhibit considerably higher fracture toughness than conventional fixed cross-link (FC) gels. However, the mechanical properties of SR gels are still unsatisfactory, and thus, these gels cannot be practically applied. Therefore, molecular scale insights into the fracture mechanism of SR gels are required to improve their mechanical properties. This study conducted tensile strain simulations of FC and SR gels using a coarse-grained molecular dynamics (CGMD) method to elucidate the fracture mechanism of SR gels. The CGMD simulations showed that the SR gels exhibited higher fracture toughness than the FC gels. In the FC gels, the axis chains easily broke under a low strain because fixed cross-links constrain the polymer chain, resulting in stress concentration on the axis chains. On the contrary, in the SR gels, the axis chains did not break until a high strain was applied because the cross-linked cyclic molecules slide not to accumulate stress concentration. The analysis of the fracture mechanism of the SR gels revealed that the chain-ends are the origin of the fracture in the SR gels. The cross-linked and un-cross-linked cyclic molecules slide on the axis chains and are stacked around the chain-ends. The stacked cross-linked cyclic molecules induced stress concentrations around the chain-ends. Next, we investigated the effect of coverage on the SR gels. We found that low-coverage SR gels exhibited higher fracture toughness and ultimate strength than high-coverage SR gels because the stacked un-cross-linked cyclic molecules prevent cross-linked cyclic molecules from sliding, leading to a low fracture toughness caused by the stress concentration at the chain-ends. The axis chains of the low-coverage SR gels with a high fracture toughness deformed to the strain direction until a high strain. Thus, the low-coverage SR gels showed a high orientation toward the strain direction. A high orientation contributes to the uniform distribution of mechanical stress on the chains, leading to a high ultimate strength. Although the SR gels generally exhibit a low ultimate strength, SR gels with a low coverage ratio of 5% showed a higher ultimate strength than FC gels. Finally, we propose that, compared with the high-coverage SR gels, the low-coverage SR gels exhibit both higher fracture toughness and ultimate strength, based on the atomistic insight pertaining to the mechanism by which the low-coverage SR gels possess free space for cross-linked cyclic molecules to move to the chain-ends until a high strain is applied.