Dynamic Mechanical Properties of Cross-Linked Rubbers. II. Effects of Cross-Link Spacing and Initial Molecular Weight in Polybutadiene1

Abstract The complex shear compliances of eight samples of polybutadiene crosslinked by cumyl peroxide and four samples crosslinked by sulfur have been measured over a frequency range from 0.2 to 2 cps at temperatures from − 6 to 45° C by a torsion pendulum. On four of the samples, measurements were extended by the Fitzgerald transducer from 45 to 600 cps at temperatures from − 71 to 55°. The vulcanizates had been prepared from polymers of two different molecular weights (180,000 and 510,000) with sharp molecular weight distribution; the physical crosslink density ranged from 0.57 to 2.68×10−4 mole/cm3, and the chemical crosslink density calculated following Kraus ranged from 0.22 to 1.49×10−4 mole/cm3. The mechanical data were all reduced to T0=298° K by shift factors calculated from the equation log aT=−3.64(T−T0)/(186.5+T−T0). In the transition zone of frequencies, the viscoelastic functions of the cumyl peroxide vulcanizates were closely similar, except for a shift toward lower frequencies with increa...