Fakultät Physik, Technische Universität Dortmund, 44221 Dortmund, Germany.
Soft Matter. 2019 May 29;15(21):4334-4345. doi: 10.1039/c9sm00434c.
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.
超冷液态二级酰胺显示出电吸收峰，其特征为几乎类似于德拜的形状，表明其极化响应接近指数。这种被认为是超分子性质的响应强度非常大，以至于酰胺的结构过程仅对总弛豫强度贡献了几个百分点，使用标准介电谱法很难可靠地解析。为了克服这个问题，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法，接近 N-甲基甲酰胺和 N-乙基乙酰胺混合物的量热玻璃化转变温度。无需依赖繁琐的解卷积程序，证明了超分子响应比结构弛豫慢 6 倍。相反，在线性流变实验中只能解析结构弛豫，而不能解析超分子弛豫。然而，在 160 K 下进行的中等幅度振荡剪切实验确实揭示了超分子过程。因此，线性和非线性力学测量的组合证实了两个过程在介电方面的光谱分离。
