Effect of Cross-Linking Density on Horizontal and Vertical Shift Factors in Linear Viscoelastic Functions of Epoxy Resins
Effect of Cross-Linking Density on Horizontal and Vertical Shift Factors in Linear Viscoelastic Functions of Epoxy Resins
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DOI：
10.1021/acs.macromol.1c01293
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发表时间：
2021-10
期刊：
影响因子：
5.5
通讯作者：
A. Shundo;Mika Aoki;Satoru Yamamoto;Keiji Tanaka
中科院分区：
文献类型：
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作者：
A. Shundo;Mika Aoki;Satoru Yamamoto;Keiji Tanaka
环氧树脂是一类重要的热固性树脂，其网络结构是由环氧树脂和胺类化合物固化而成，对材料性能起着重要的作用。我们在这里重新审视的时间-温度叠加（TTS）的原则适用于动态粘弹性功能的环氧树脂，其中网络定义良好，并系统地改变的基础上的长度的n-烷基二胺。在频域中的等温曲线的叠加不仅需要水平位移，而且需要垂直位移，而与正烷基二胺的长度无关。水平位移因子aT的温度依赖性可以用Williams-Landel-Ferry方程很好地表示。被称为C2的拟合参数随着二胺烷基链长度的减小而增加，这意味着由于更大的交联密度，自由体积的热膨胀被抑制。这是定性证实了全原子分子动力学（MD）模拟。同时，垂直位移因子bT随温度的升高而增大，且随交联密度的增大而减小。这可以用熵对高于玻璃化转变温度的温度区域中的模量的贡献来解释。从分子动力学模拟的等压摩尔热容估计的熵变强烈支持这一假设。这里获得的知识应该是有用的热固性聚合物的更好的设计，以及为长期耐久性的预测。
Epoxy resins are an important class of thermosetting resins, and their network structure, obtained by the curing reaction of epoxy and amine compounds, plays an important role in the material properties. We here revisited a time–temperature superposition (TTS) principle applied to the dynamic viscoelastic functions of epoxy resins, in which the network was well defined and systematically varied on the basis of the length ofn-alkyl diamine. The superimposition of isothermal curves in the frequency domain required not only a horizontal shift but also a vertical shift, regardless of the length ofn-alkyl diamine. The temperature dependence of the horizontal shift factor,aT, could be well expressed by the Williams–Landel–Ferry equation. The fitting parameter, calledC2, increased with decreasing alkyl chain length of the diamine, meaning that the thermal expansion of the free volume was suppressed due to greater cross-linking density. This was qualitatively confirmed by a full-atomistic molecular dynamics (MD) simulation. Meanwhile, the vertical shift factor,bT, increased with increasing temperature, and the extent was smaller with increasing cross-linking density. This can be explained in terms of the entropic contribution to the modulus in the temperature region above the glass transition temperature. The entropy change estimated using the isobaric molar heat capacity from the MD simulation strongly supported this hypothesis. The knowledge here obtained should be useful for a better design of thermosetting polymers as well as for the prediction of long-term durability.