Nonlinear electrical and rheological spectroscopies identify structural and supramolecular relaxations in a model peptide.
Nonlinear electrical and rheological spectroscopies identify structural and supramolecular relaxations in a model peptide.
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DOI：
10.1039/c9sm00434c
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发表时间：
2019-05
期刊：
影响因子：
3.4
通讯作者：
Gabriel Honorio;S. Bierwirth;C. Gainaru;R. Böhmer
中科院分区：
文献类型：
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作者：
Gabriel Honorio;S. Bierwirth;C. Gainaru;R. Böhmer
过冷液体仲酰胺显示出以几乎德拜形状为特征的电吸收峰，表明接近指数的偏振响应。这种反应被认为是超分子性质的，但其强度如此之大，以至于酰胺的结构过程仅占总弛豫强度的百分之几，很难使用标准介电谱来可靠地解析。为了克服这个问题，在 N-甲基甲酰胺和 N-乙基乙酰胺混合物的量热玻璃化转变附近应用了涉及场诱导结构恢复和温度诱导物理老化的非线性介电光谱。无需依赖繁琐的反卷积程序，因此证明超分子响应比结构弛豫慢六倍。相反，在线性流变实验中只能解决结构松弛问题，而不能解决超分子问题。然而，在 160 K 下进行的中振幅振荡剪切实验确实揭示了超分子过程。因此，线性和非线性机械测量的组合证实了两个过程的介电未覆盖光谱分离。
Supercooled liquid secondary amides display an electrical absorption peak characterized by an almost Debye-like shape, indicative of a close-to-exponential polarization response. This response, believed to be supramolecular in nature, is so enormously intense that the amide's structural process, contributing only a few percent to the total relaxation strength, is hard to resolve reliably using standard dielectric spectroscopy. To overcome this issue, nonlinear dielectric spectroscopy involving field-induced structural recovery and temperature-induced physical aging, was applied near the calorimetric glass transition of a mixture of N-methylformamide and N-ethylacetamide. Without the need to rely on cumbersome deconvolution procedures, it is thus demonstrated that the supramolecular response is by a factor of 6 slower than the structural relaxation. Conversely, in linear rheological experiments only the structural relaxation could be resolved, but not the supramolecular one. However, medium-amplitude oscillatory shear experiments carried out at 160 K do reveal the supramolecular process. Hence, the combination of linear and nonlinear mechanical measurements corroborates the dielectrically uncovered spectral separation of the two processes.