Competing Supramolecular Structures: Dielectric and Rheological Spectroscopy on Glycerol/Propanol Mixtures

TL;DR

The work addresses how mixing a chain-forming alcohol ($\text{propanol}$) with a network-forming polyol ($\text{glycerol}$) reshapes hydrogen-bonded supramolecular structures and their dynamics. By combining dielectric spectroscopy, shear rheology, and calorimetry, the authors reveal persistent Debye-type cross-correlations tied to chain and network motifs, and observe two distinct glass transitions near $\chi_{gly}=0.3$ with a percolation-like stiffening around $\chi_{gly}\approx0.5$–$0.7$. The study shows that dielectric and mechanical responses track different relaxation channels: dielectric spectra are dominated by cross-correlated hydrogen-bonded structures (Debye and related processes), while the mechanical response reflects the evolving network connectivity through $G_{\infty}$ and $J''(\omega)$. These findings highlight strong dynamical heterogeneity and a competition between chain-like and network-like supramolecular assemblies, with potential implications for tuning the rheological and dielectric properties of hydrogen-bonded liquids by composition. All mathematical relationships are expressed with explicit $...$ notation to preserve clarity of the underlying physics, including $\chi_{gly}$, $T_g$, $T_m$, $G_{\infty}$, $\tau$, and related parameters.

Abstract

Significant progress has been made in recent years in understanding the dynamics of pure hydrogen-bonded systems by analyzing the spectral shape of various susceptibilities. Monohydroxy- and polyalcohols are currently considered to form transient supramolecular hydrogen-bonded structures in the form of chains, rings, and networks. This complex dynamic behavior has been identified in network-forming glycerol and chain-forming propanol by combining dielectric and light-scattering spectra. We apply these concepts to study the combined dielectric and shear rheological spectral shape of glycerol/propanol mixtures. Glycerol differs from propanol by having two additional hydroxy groups, which leads to significant differences in melting temperatures($ΔT_{\textbf{m}}$\,=\,291\,K\,-\,147\,K\,=\,143\,K) and glass transition temperatures ($ΔT_{\textbf{g}}$\,=\,190\,K-\,98\,K\,=\,92\,K). The strong difference results in two distinct calorimetric glass transitions at a molar glycerol concentration of $χ_{gly}=0.3$, as well as a change in the shear modulus $G_{\infty}$ between $χ_{gly}=0.5$ and 0.7. Performing a comprehensive analysis of the three applied experimental techniques leads to the conclusion that dielectric spectroscopy monitors the evolution of supramolecular chain and network structures and that the mechanical properties depend heavily on the formed hydrogen-bonded network. A strong dynamical heterogeneity is observed and manifests itself in two distinguishable glass transitions in dielectric spectroscopy and calorimetry. The presented chain/network mixture is dynamically highly heterogeneous when compared to the rather narrow dynamical heterogeneity in the network/network mixture Water/Glycerol.

Figures (34)

• Figure 1: (a) Propanol Gabriel:2017a and (b) glycerol Gabriel2020a data from dielectric and light scattering show Debye, $\alpha$, and secondary relaxation processes. (c-e) Schematic representations illustrate the hydrogen-bonded chain and network structures in the propanol/glycerol mixtures.
• Figure 1: Viscosity representation - illustrates how close the measurements come to reaching a terminal flow.
• Figure 2: Selection of calorimetric measurements of glycerol/propanol mixtures showing two calorimetric features at 50% mol. The inset shows the obtained onset glass transition temperatures $T_{\textbf{g}}$ and melting temperatures of the pure substances.
• Figure 2: Scaled Viscosity representation - illustrates how close the measurements come to reaching a terminal flow - details bierwirth2018communication.
• Figure 3: Concentration-dependent panels show dielectric loss spectra (left) and mechanical shear modulus (right) between 110K and 230K. Dielectric spectra are cut at the frequency where the DC conductivity starts.