LAUSR

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 r...