Verifying the Approximate Coinvariance of the α and Johari–Goldstein β Relaxation Times to Variations of Pressure and Temperature in Polyisoprene

Abstract

From dielectric spectroscopy measurements on polyisoprene performed at ambient and elevated pressures, we show that the secondary relaxation has properties of the supposedly universal Johari–Goldstein β-relaxation. These properties include approximate (i.e., in order of magnitude only) invariance of the ratio of its relaxation time τβ(T,P) to the structural α-relaxation time τα(T,P) to temperature T and pressure P variations provided τα(T,P) is kept constant. Under the same condition, the frequency dispersion of the α-relaxation or the fractional exponent βKWW(T,P) of the Kohlrausch–Williams–Watts correlation function is also unchanged. Thus, τα(T,P) and βKWW(T,P) characterizing the α-relaxation dynamics of polyisoprene are approximately coinvariant with τβ(T,P) in order of magnitude to variations of T and P at constant τα(T,P). Although this approximate coinvariance property was found before by dielectric relaxation of polar small molecules in mixtures with apolar small molecules or polystyrene, it was rarely found in amorphous polymers. The present finding of the property in polyisoprene extends its generality to polymers.