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 /2 π ), 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/ G 0 + C ( T - T 0 ), where C is a constant for a given liquid and G 0 is the modulus at the temperature T 0 , 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.