Mechanism Dictates Mechanics: A Molecular Substituent Effect in the Macroscopic Fracture of a Covalent Polymer Network
Mechanism Dictates Mechanics: A Molecular Substituent Effect in the Macroscopic Fracture of a Covalent Polymer Network
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机制决定力学：共价聚合物网络宏观断裂中的分子取代效应
DOI：
10.1021/jacs.1c00265
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发表时间：
2021
影响因子：
15
通讯作者：
Craig, Stephen L.
中科院分区：
文献类型：
--
作者：
Wang, Shu;Beech, Haley K.;Bowser, Brandon H.;Kouznetsova, Tatiana B.;Olsen, Bradley D.;Rubinstein, Michael;Craig, Stephen L.
橡胶状聚合物网络的断裂涉及一系列分子事件，从沿着聚合物主链的构象变化开始，并以断链反应结束。在这里，我们报告共价聚合物凝胶中的宏观断裂“反应”是由机械活性网络链内嵌入的机械载体控制。我们合成了聚（乙二醇）（PEG）凝胶通过末端连接的叠氮封端的四臂PEG（Mn= 5 kDa）与双炔连接。在相同的条件下形成网络，不同的是双炔被改变以包括通过力偶联的环化逆转作为机械化学“弱连接”的顺式-二芳基（1）或顺式-二烷基（2）连接的环丁烷机械载体。还合成了以不含环丁烷的双炔（3）为特征的控制网络。网络表现出相同的线弹性（G′ = 23-24 kPa，0.1-100 Hz）和平衡质量溶胀比（Q= 10-11），但它们的撕裂能跨度为8倍（1、2和3的网络分别为3.4 J、10.6和27.1 J·m-2）。断裂能的差异是一致的力耦合断裂动力学的单分子力谱实验中观察到的mechanores，暗示当地的共振稳定的双自由基过渡态的cycloreversal of 1作为一个关键的决定因素的相对容易，它的网络被撕裂。宏观断裂和小分子反应机制之间的联系为分子理解和优化聚合物网络行为提供了机会。
The fracture of rubbery polymer networks involves a series of molecular events, beginning with conformational changes along the polymer backbone and culminating with a chain scission reaction. Here, we report covalent polymer gels in which the macroscopic fracture “reaction” is controlled by mechanophores embedded within mechanically active network strands. We synthesized poly(ethylene glycol) (PEG) gels through the end-linking of azide-terminated tetra-arm PEG (Mn= 5 kDa) with bis-alkyne linkers. Networks were formed under identical conditions, except that the bis-alkyne was varied to include either acis-diaryl (1) orcis-dialkyl (2) linked cyclobutane mechanophore that acts as a mechanochemical “weak link” through a force-coupled cycloreversion. A control network featuring a bis-alkyne without cyclobutane (3) was also synthesized. The networks show the same linear elasticity (G′ = 23–24 kPa, 0.1–100 Hz) and equilibrium mass swelling ratios (Q= 10–11 in tetrahydrofuran), but they exhibit tearing energies that span a factor of 8 (3.4 J, 10.6, and 27.1 J·m–2for networks with1,2, and3, respectively). The difference in fracture energy is well-aligned with the force-coupled scission kinetics of the mechanophores observed in single-molecule force spectroscopy experiments, implicating local resonance stabilization of a diradical transition state in the cycloreversion of1as a key determinant of the relative ease with which its network is torn. The connection between macroscopic fracture and a small-molecule reaction mechanism suggests opportunities for molecular understanding and optimization of polymer network behavior.