Evolution of the Dynamics in 1,4-Polyisoprene from a Nearly Constant Loss to a Johari−Goldstein β-Relaxation to the α-Relaxation

Abstract

The Johari-Goldstein (JG) secondary relaxation, presumed to be universal, has never been seen in 1,4-polyisoprene (PI) by dielectric spectroscopy, despite very many measurements extending over the past half-century. By using a high-resolution capacitance bridge, we are able to show the existence of a secondary relaxation in PI that has the properties of the JG process. Measurements were also carried out at lower temperatures, which probe the dynamics of chain units "caged" by neighboring segments comprising the local structure. The caged dynamics precede by decades of time the JG relaxation and, from general physical principles, are also expected to be a property of all glass-forming materials. Collectively, the caged dynamics and JG relaxation serve as precursors to structural relaxation (i.e., the glass transition) and thus are of central importance to understanding vitrification. The present data show that the dynamics of caged PI repeat units are manifested as a nearly constant loss (NCL). This NCL has been found in other glass-formers, both molecular and polymeric, and its temperature dependence in PI is similar to that for other materials. The experimental results are consistent with the predictions from the coupling model.

Johari–Goldstein relaxation as the origin of the excess wing observed in metallic glasses

“…1). It was suggested before that the process preceding the JG relaxation (assuming there is no secondary relaxation other than the JG relaxation) is the anharmonic dynamics of caged molecules, which are manifested as the nearly constant loss (NCL) [8][9][10]. Possibly, the NCL arises from the fluctuations of cages.…”

“…Another indication of a connection is the empirical finding from many glass-formers that log 10 (s a /s JG ) at constant s a is proportional to n [2,14]. Furthermore, s JG is approximately the same as the primitive relaxation time s 0 of the coupling model (CM) at any temperature T and pressure P in the supercooled liquid state of many glass-formers [8,9,15,16], i.e.,…”

“…These include the relaxation while all relaxing entities are caged and manifested as the 'nearly constant loss' in the frequency spectrum [8][9][10], and the secondary relaxations. Among secondary relaxations there are some that have dynamic properties similar to or correlated with that of the a-relaxation.…”

“…Thus, this study validates the assumption of Rö sner et al that the excess wing in Zr 65 Al 7.5 Cu 27.5 is indeed the trace of the hidden JG secondary relaxation and its relaxation time satisfies Eq. (4), like molecular and polymeric glass-formers and plastic crystalline materials [8,9,13,15,16,21].…”

New sample cell configuration for wide-frequency dielectric spectroscopy: DC to radio frequencies

“…1,5,6 Generally, the former is used in the HF region, such as radio frequencies, [7][8][9][10] and the latter is used in the LF region. [10][11][12] For coaxial cells, it is straightforward to keep a coaxial structure up to the end of the whole circuit and the parasitic inductance can be minimized at an extremely low level; typically, less than 1 nH.…”