Plasticizing agents are known to shift the material functions of polymer melts like raised temperatures to shorter times and to higher frequencies. The influence of dioctyl sebacate, dioctyl phthalate and bibenzyl on the viscoelastic properties of polystyrene melts is sized by shift factors which can be evaluated very exactly from the dynamic moduli in the rubberlike liquid region. Besides, these shift possibilities enable us to enlarge the experimental window to the low frequency region for some decades. The typical diluent effect caused by the additives is quite similar to the one caused by not entangled polymer molecules. The lowering of the plateau modulus, i.e., the shift on the modul axis, is found to be equal to the square of the weight fraction of the polymer. The reduction of the relaxation times, i. e., the shift on the time or frequency axis, mainly is due to the increased fraction of free volume accounted for by A/&&,,, where A rates the fraction created by one mole additive and @, , is the number-average of the molar masses of polymer and additive. The influence of the free volume on the relaxation times is calculated with the aid of a Doolittle equation. The concentration dependence of the time-temperature shift factors is also accounted for by A/&?,,,. Therefore, besides the parameter A, only the three parameters used in (and known by) the analysis of the time-temperature superposition principle, i. e., the fraction of free volume of the pure polymer 0, the thermal expansion coefficient o f f (af) and the Doolittle parameter B, are needed for the calculation of the temperature-dependent timeconcentration shift factors and of the concentration-dependent time-temperature shift factors as well. 0 1995,
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