标题： 出乎意料的大的熵势垒控制了动态玻璃中的键重排 显示英文标题

标题： Unexpectedly large entropic barrier controls bond rearrangements in vitrimers

摘要： 动态共价键的可逆聚合物材料是一类相对较新的高分子材料，它们含有可逆的共价动态键，使其具有设计上的可回收性。 然而，控制其粘弹性特性的基本机制仍然知之甚少。 我们对模型动态共价键聚合物的弛豫动力学和粘弹性行为的详细研究显示，动态共价交联密度对链动力学没有影响（除了玻璃化转变温度的微弱变化），但它强烈影响动态共价键聚合物的线性粘弹性。 增加交联密度会引发溶胶-凝胶转变，这与经典凝胶理论的预测一致，证明了该理论在动态共价键聚合物中的适用性。 值得注意的是，对键重排时间的温度依赖性分析揭示了过渡态中出人意料的大负活化熵，尽管其活化焓相对较低，但这种负活化熵显著减缓了键交换过程。 这一发现解释了动态共价键聚合物中键重排的异常长的时间尺度，并强调了熵在控制动态共价网络粘弹性中的重要性。 显示英文摘要

摘要： Vitrimers are a relatively new class of polymeric materials containing associative covalent dynamic bonds that make them recyclable by design. However, the fundamental mechanisms controlling their viscoelastic properties remain poorly understood. Our detailed studies of relaxation dynamics and viscoelastic behavior of model vitrimers revealed that the density of dynamic covalent crosslinks has no influence on chain dynamics (beyond a weak change in the glass transition temperature), yet it strongly affects the linear viscoelasticity of vitrimers. Increasing the crosslink density induces a sol-gel transition consistent with predictions of classical gelation theory, demonstrating its applicability to vitrimers. Remarkably, the temperature-dependent analysis of the bond rearrangement time reveals an unexpectedly large negative activation entropy in the transition state that strongly slows down the bond exchange process despite its relatively low activation enthalpy. This insight explains the unusual long timescale for bond rearrangement in vitrimers and highlights the significance of entropy in controlling the viscoelasticity of dynamic covalent networks.