SLIDING PLATE RHEOMETER STUDIES OF CONCENTRATED POLYSTYRENE SOLUTIONS- LARGE-AMPLITUDE OSCILLATORY SHEAR OF A VERY HIGH-MOLECULAR-WEIGHT POLYMER IN DIETHYL PHTHALATE

A very high molecular weight, narrow molecular weight distribution pol ystyrene, having a molecular weight of 8.42x10(6), was investigated us ing a sliding plate rheometer. The solvent was diethyl phthalate. The solution exhibited wall slip during steady shear, even at very low she ar rates and exhibited a marked normal-stress-driven secondary flow at high shear rates. However, there was no evidence of slip during the o scillatory shear tests. Using fast Fourier transform analysis, the res ults of the oscillatory shear tests on this solution are presented in terms of response surfaces in a space based on a Pipkin diagram (strai n-rate amplitude versus frequency). These reveal the linear and nonlin ear regimes and the approach to a purely elastic regime at high freque ncies. Wagner's constitutive equation predicts the major trends in the experimental data but does not provide quantitative predictions over the entire range of experimental parameters. (C) 1996 Society of Rheol ogy.