Supramolecular structures in monohydroxy alcohols: Insights from shear-mechanical studies of a systematic series of octanol structural isomers

TL;DR

This study uses shear-mechanical spectroscopy to investigate supramolecular structures in a series of octanol isomers, revealing minimal structural influence on mechanical signatures despite dielectric differences, and suggesting consistent size of structures responsible for the Debye process.

Contribution

It provides the first systematic shear-mechanical analysis of octanol isomers, highlighting the weak structural influence on mechanical Debye signatures and consistent supramolecular structure sizes.

Findings

Mechanical Debye signatures show minimal variation across isomers.

Structural differences have a large impact on dielectric but not on mechanical responses.

Supramolecular structure size remains nearly unchanged in the series.

Abstract

A recent study [Gainaru et al. PRL., 112, 098301 (2014)] of two supercooled monohydroxy alcohols close to the glass-transition temperature showed that the Debye peak, thus far mainly observed in the electrical response, also has a mechanical signature. In this work, we apply broadband shear-mechanical spectroscopy to a systematic series of octanol structural isomers, x-methyl-3-heptanol (with x ranging from 2 to 6). We find that the characteristics of the mechanical signature overall follow the systematic behavior observed in dielectric spectroscopy. However, the influence from the molecular structure is strikingly small in mechanics (compared to roughly a factor 100 increase in dielectric strength) and one isomer clearly does not conform to the general ordering. Finally, the mechanical data surprisingly indicate that the size of the supramolecular structures responsible for the Debye process is nearly unchanged in the series.