The effect of pressure on the viscoelastic properties of liquids

Abstract

The viscoelastic behaviour of di(2-ethylhexyl) phthalate and bis(m-(m-phenoxy phenoxy)- phenyl) ether has been determined as a function of pressure under conditions of alternating shear. The shear mechanical resistance has been measured at frequencies of 10 and 30 MHz. For di(2-ethylhexyl) phthalate the pressure range of measurement extended to 1400 MN/m 2 and to 300 MN/m 2 for bis(m-(m-phenoxy phenoxy)phenyl) ether. Associated measurements have also been made of the density, p, and steady-flow viscosity, η, of each liquid as a function of pressure, the viscosity results being confined to the range 0.001 to 300 N s/m 2 . The limiting shear modulus, G ∞ , of each liquid has been found to vary linearly with pressure whilst, within experimental error, the viscoelastic relaxation can be represented by an equation put forward by Barlow, Erginsav & Lamb (1967 a ), J *(jw) = J ∞ [1+1/jwr m ]+2J∞/jwr m )1/2 J *(jw) is the complex compliance measured at angular frequency w, J ∞ (= 1/(G ∞ ) is the limiting high-frequency shear compliance and r m is the ‘Maxwell relaxation time’ equal to n/J ∞ . This equation gives an adequate description of the relaxational behaviour when due account is taken of the variation of viscosity, density and shear modulus with pressure and temperature.