The viscoelastic properties of three samples of a random styrene-butadiene co-polymer with 23.5 wt-% styrene, cross-linked by dicumyl peroxide to different extents, have been studied by dynamic shear and shear creep measurements, and the unvulcanized precursor has been studied in creep. The frequency and temperature ranges were 0.2 to 3600 cps and −30 to 55°C. The creep data were converted to the corresponding dynamic viscoelastic functions at very low frequencies. All data were reduced to T0=298°K by shift factors calculated from the equation log aT=−4.57(T−T0)/(113.6+T−T0). By fitting the creep to a modification of the empirical equation of Thirion and Chasset, values of the equilibrium compliance and relaxation parameters were obtained in good agreement with results of those authors from stress relaxation in extension. All the viscoelastic functions displayed two principal regions of frequency dependence as found for other rubberlike polymers. The high-frequency dispersion, near 105 on the radian frequency scale, is affected very little by increasing cross-linking and corresponds to a compliance increment of about 1.1×10−7 cm2/dyne for all samples. It is attributed to the entanglement network originally present before vulcanization. The low-frequency dispersion, at 1 radian/sec and below, corresponds to a compliance increment which increases rapidly with decreasing chemical cross-linking, while the dispersion broadens and shifts to lower frequencies.
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Mancke et al. (1968) studied this question.