Don't cry for me Charlie Brown, or with compliance comes comprehension

The different linear viscoelastic functions are, in theory, equivalent to e ach other, although the information that they emphasize is different. Howev er, since viscoelastic measurements are never available over the entire tim e or frequency scale, and the results are not known with infinite precision , it is worthwhile to look at various of the otherwise equivalent viscoelas tic functions. We present experimental data which show what appears to be a n entanglement network plateau in the modulus, in nonentangled samples of s elenium and a low molecular weight polystyrene with a narrow distribution o f molecular weights. We show how the confusion can be easily avoided by ins pecting the compliances and the retardation spectrum. We also present data on the molecular weight dependence of the length of the plateau as determin ed from the dynamic storage modulus and from the retardation spectrum. We f ind that the retardation spectrum identifies the presence of an entanglemen t network of polymer chains, whereas at low molecular weights the storage m odulus does not. The origin of the false entanglement plateaus is shown to arise from the reciprocal of steady-state recoverable creep compliance J(s) , which is convoluted in the modulus functions but is additive in the compl iances. (C) 2000 The Society of Rheology. [S0148-6055(00)01704-1].