Viscoelastic relaxation of supercooled liquids. I

Abstract
Complete information is presented for the viscoeleastic behaviour of n-propyl benzene, isopropyl benzene, sec. -butyl benzene, di(n-butyl) phthalate. di(isobutyl) phthalate and di(2-ethyl hexyl) phthalate. These liquids have been measured in the supercooled state using alternating shear stress of frequency f( = ω/2π), from the purely viscous range, through the region of viscoelastic relaxation into the purely elastic range. It has been established that the limiting shear modulus, G∞, of the liquids varies with temperature, T, according to the relation 1/G∞ = 1/G0 + C(T - T0), where C is a constant for a given liquid and G0 is the modulus at the temperature T0, at which temperature there is no free volume available for molecular translational motion in the equilibrium liquid (Barlow, Lamb & Matheson 1966). Normalized plots of the resistive component of the shear mechanical impedance versus effective frequency show a wide separation along the frequency axis. However, when this quantity is plotted against (ωη/G∞) there is a striking similarity of behaviour, the curves for five of the above liquids being indistinguishable within experimental error. This has led to further measurements being made on a range of other liquids, the results for which are given in the second paper (part II), and finally to a reappraisal of the basis upon which previous analyses of viscoelastic relaxation have been based. A new approach to the interpretation of this behaviour is given in part II.