Cooperativity and Heterogeneity in Plastic Crystals Studied by Nonlinear Dielectric Spectroscopy

TL;DR

This study investigates the glassy dynamics of plastic crystals using nonlinear dielectric spectroscopy, revealing unique cooperativity and heterogeneity behaviors distinct from canonical glass formers, and suggesting intrinsic non-exponential relaxation.

Contribution

It provides new insights into the molecular relaxation mechanisms in plastic crystals, highlighting differences from supercooled liquids and challenging existing heterogeneity models.

Findings

Molecular cooperativity influences glassy freezing but causes less slowdown than in supercooled liquids.

Nonlinear effects cannot be fully explained by heterogeneity scenarios used for canonical glass formers.

No nonlinear effects observed for secondary processes in cyclo-octanol.

Abstract

The glassy dynamics of plastic-crystalline cyclo-octanol and ortho-carborane, where only the molecular reorientational degrees of freedom freeze without long-range order, is investigated by nonlinear dielectric spectroscopy. Marked differences to canonical glass formers show up: While molecular cooperativity governs the glassy freezing, it leads to a much weaker slowing down of molecular dynamics than in supercooled liquids. Moreover, the observed nonlinear effects cannot be explained with the same heterogeneity scenario recently applied to canonical glass formers. This supports ideas that molecular relaxation in plastic crystals may be intrinsically non-exponential. Finally, no nonlinear effects were detected for the secondary processes in cyclo-octanol.