Viscoelastic relaxation in a series of polyethylacrylates and poly- n -butylacrylates

Abstract

The viscoelastic properties of a series of four polyethylacrylates and of four poly- n -butylacrylates have been measured in alternating shear. All measurements were made above the glass transition temperature over the temperature range - 40 to 148 °C and at frequencies of 64 kHz, 15 MHz and 30 MHz. The steady-flow viscosity of each polymer follows a dependence upon temperature according to the equation log 10 ƞ = A + B /( T - T 0 ). The limiting high frequency elastic compliance, J ∞ , is found to be sensibly independent of molecular weight for each series of polymers and varies linearly with temperature. The results obtained are interpreted in terms of additive contributions to the complex shear modulus, G *, comprising: (1) a relaxation process at relatively high frequencies which follows the same form as that observed previously in supercooled non-polymeric liquids according to the model of Barlow, Erginsav & Lamb (1967 b ). (2) a limited summation of relaxation processes at lower frequencies arising from the modes of motion of the flexible polymer chain according to the extension of the Rouse (1953) theory made by Barlow, Harrison & Lamb (1964) for an undiluted polymer having a 'most probable’ distribution of molecular weight. For the polymers investigated the polymer modes (2) contribute a major part of the steady-flow viscosity while the high frequency relaxation process (1) is mainly responsible for the limiting high frequency shear modulus. Agreement is found between experimental results, covering a significant portion of the complete relaxation region, and the calculated behaviour based upon the sum of the above two separate relaxation processes.