Abstract
It is occasionally proposed by Leblanc (1976) and GOttfert (1994) that information about the viscoelasticity of molten polymers can be obtained by recording the pressure as a function of time when the piston driving a capillary or slit rheometer is suddenly stopped. In the cited reference by G6ttfert (1994), a single exponential is fitted to the pressure decay curve in order to estimate a measure of the melt's relaxation time, while in the other work cited by Leblanc (1976) a "relaxation spectrum" is inferred from the pressure data. The purpose of this note is to demonstrate that the pressure decay depends primarily on the viscosity and compressibility of the melt and that the contribution of melt elasticity is too small to be measured using a standard capillary rheometer. Hatzikiriakos and Dealy (1992, 1994) have modelled the pressure transients in a capillary rheometer, and it will be convenient to build on their analysis. For purposes of the present demonstration, I consider the case of a fluid with constant viscosity, as this makes it possible to obtain an analytical solution. As an initial condition, we consider that the piston speed, Vp, the reservoir pressure, P, and the flow rate, Q, are at steady state. At t = 0, the piston is suddenly stopped, and we wish to calculate P ( t ) and see how it is related to the properties of the fluid. We make the following simplifying assumptions:
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