Cauchy relation in relaxing liquids.

The Cauchy-like relation M(infinity) = A + BG(infinity) has recently been found to hold for the high frequency limit values of the longitudinal modulus M(infinity) and transverse modulus G(infinity) of viscoelastic liquids, with B approximately 3 in all the investigated systems. The Brillouin scattering results here reported for curing epoxy systems and thermal glass formers give evidence for the validity of a Cauchy-like relation M(') = A + BG(') for the real part of the elastic moduli measured at finite frequencies. Our results suggest as well the validity of a pure Cauchy relation DeltaM = 3 DeltaG for the relaxation strengths of longitudinal and shear moduli in relaxing liquids.