Relationship between the primary and secondary dielectric relaxation processes in propylene glycol and its oligomers

Abstract

Dielectric relaxation measurements were performed on propylene glycol (PG) and oligomers having different number of repeat units (N=2, 3, and 69). The primary α-relaxation had the Kohlrausch–Williams–Watts (KWW) form, with a stretch exponent (1−n) which decreased with increasing N. The temperature dependence of the α-relaxation time, as reflected in the fragility index, increased with N. A broad, rather symmetric secondary β-relaxation was observed at higher frequencies in the dielectric loss spectrum for all samples with N>1. This is the first observation of the β-relaxation peak in dipropylene glycol (N=2) and tripropylene glycol (N=3). The separation between the α- and β-relaxations increased with increasing N. This trend indicates that the separation is minimal in PG, which makes it difficult to resolve the β-relaxation from the more intense α-relaxation. This, together with the fact that the strength of the β-relaxation decreases with the molecular weight of PPG, as found by Johari and coworkers, explains the absence of an observable β- peak or shoulder in isothermal or isochronal dielectric measurement on PG. It is proposed that the deviation of the dielectric loss peak for PG from either the KWW or the Cole–Davidson functions at higher frequencies is due to a β-relaxation masked by the α-process. The same conclusion was reached by Johari and co-workers, based on evidence from their “difference isochrone method.” Finally, we show that the data from oligomers of propylene glycol support the recently proposed correlation of τβ(Tg) with (1−n) and the rough agreement between τβ(Tg) and the primitive relaxation time of the coupling model at temperatures above Tg.