Ultraslow Relaxation Dynamics In Complex Glass-forming Hydrogen-bonded Liquids

The ultraslow relaxation dynamics studied in this paper is Debye relaxation in the dielectric spectra of glass-forming monoalcohols. The structural and secondary relaxations are also studied.A complete interpretation of the physical origin of Debye relaxation is still absent, although the Debye relaxation has been intensively studied nearly for one hundred years and a number of unique dynamics characters have been presented against the structural relaxation. The Debye relaxations are generally identified to be associated with the hydrogen-bond associations in monoalcohols. The hydrogen-bond associations generally exist in biophysical macromolecules. The researches on Debye relaxation not only help to clarify the hydrogen-bond cluster structures and the physical origin of Debye relaxation in monoalcohols but also promote the structure and function of biophysical macromolecules.Including monoalcohols, the relaxation dynamics in the substituted monoalcohols, 3-methylthio-1-hexanol and six alkoxy alcohols, are studied by dielectric and calorimetric measurements at temperatures around the glass transition. The temperature dependence of Debye relaxation and the coupling with the structural relaxation are systematically investigated. The physical origin of Debye relaxation in monoalcohols is discussed.The dielectric relaxations of two long-chain glass forming monoalcohols, 2-butyl-1-octanol(2B1O) and 2-hexyl-1-decanol(2H1D), are studied at low temperature. Remarkable broadening from the pure Debye relaxation is identified for the slowest dynamics, differing from the dielectric spectra of the short-chain alcohols such as 2-ethyl-1-hexanol(2E1H). The broadening of the Debye-like relaxation in the two monoalcohols develops as temperature increases, and the approaching of the Debye-like and structural relaxation widths is shown. Similar results are observed in the dielectric spectra of dilute 2E1 H in either 2H1 D or squalane. The results of the liquids and mixtures reveal a correlation between the broadening and the Debye-like relaxation strength. The physical origin of Debye relaxation and molecular associations in monoalcohols are discussed with the modification of the Debye relaxation.The dielectric relaxation of a substituted monoalcohol, 3-methylthio-1-hexanol, is studied in the highly viscous regime near the glass transition. The Debye relaxation is detected in the dielectric spectra showing the slowest and strongest relaxation dynamics. The calorimetric and dielectric measurements of the liquid and the mixtures with a Debye liquid(2-ethyl-1-hexanol, 2E1H) and a non-Debye liquid(2-ethylhexylamine) reproduce the dynamic characters of the relaxations in monoalcohols. The Debye relaxation strength and time of 3MT1 H do not change much compared with 2E1 H, while the structural relaxation strength show a considerable enhancement accompanied a reduction in the dynamic separation between the Debye and structural relaxations. It is proved by experiments that the orientations of alkyl chain involved in both the H-bonded chains and free(unbonded) molecules in monoalcohols contribute to the structural relaxation. The experimental results test the structural models proposed for the Debye relaxation.Considering Debye relaxation detected in 3MT1 H, while oxygen and sulfur located at the same group in the periodical table of elements, the dielectric relaxations of six alkoxy alcohols with varying structure are further studied at temperatures around glass transition. The dielectric spectra of alkoxy alcohols do not show a resolved Debye relaxation, indicate of no continuity in the dielectric spectra from monoalcohols, sulfur substituted monoalcohols, to alkoxy alcohols. Unique dynamic features are identified in the alkoxy alcohols, uncovering the minimum width in the temperature dependence of the apparent main relaxation dispersions. The stretching exponents for the alkoxy alcohols determined by the dielectric relaxations are found not to follow the empirical correlations with other dynamics quantities established for generic liquids. If the ?- relaxations are introduced to explain the minimum behavior, a direct conclusion can be reached that the ?- relaxation does not disappear at high temperatures where the structural relaxation dynamics is quite fast with the relaxation time being around the nano-second time scale.