Relaxation dynamics in plastic crystals

TLDR

The study investigates whether the excess wing of the α‑relaxation peak exists in plastic crystals. Dielectric spectroscopy was performed on six plastic crystalline materials over 14 decades up to 20 GHz to characterize their relaxation behavior. The measurements show non‑Debye α‑relaxation, low fragility, β‑relaxations in some samples, and that any excess wing is absent or attributable to submerged β‑relaxations, providing a comprehensive database of plastic crystal dynamics comparable to structural glasses.

Abstract

We report dielectric data on six different plastic crystalline materials, namely 1-cyanoadamantane, adamantanone, pentachloronitrobenzene, cyclo-hexanol, ethanol, and meta-carborane, covering a frequency range of up to 14 decades and up to 20 GHz. Information on phase transitions, the α-relaxation, and relaxation processes beyond the α-relaxation are provided. The α-relaxation shows clear non-Debye behavior and varying degrees of deviation from thermally activated behavior. Our results reveal a generally rather low fragility of plastic crystals. In some of the investigated materials evidence for Johari–Goldstein type β-relaxations is obtained. In addition, the question of the so-called excess wing of the α-relaxation peak is addressed in detail. In all cases, it is either absent or can be ascribed to a β-relaxation submerged under the α-peak. Overall, the present work provides a broad database on the dielectric behavior of plastic crystals, and may be taken as a review of the dynamic phenomena occurring in these materials, many of them being observed also in structural glass formers.