| Abstract: | Viscoelastic properties of four linear and three very lightly crosslinked polybutadienes (microstructure about 50% trans) were studied. Of the latter, two had not reached the gel point, and their molecular weight distributions were determined by sedimentation velocity analysis; the third was crosslinked just past the gel point, with only 32% gel fraction present. The crosslinking agent was sulfur. Complex shear compliances were measured over a frequency range from 0.1 to 1000 cps at temperatures from ?70 to 30°C. with a Fitzgerald transducer and a Plazek torsion pendulum; and torsional creep measurements were made over time periods up to about three days. The creep data were converted to the corresponding dynamic viscoelastic functions at very low frequencies by conventional approximation methods. All data were reduced to 25°C. by shift factors calculated from a previously adopted equation of the WLF form. In the transition zone, the viscoelastic properties of linear samples were almost independent of molecular weight. The entanglement spacing, derived from the minimum in the loss tangent and the inflection in the storage compliance, was 130 to 160 chain atoms. The maximum in the retardation spectrum attributable to motions of individual network strands was closely similar to the corresponding maxima for more highly crosslinked vulcanizates previously studied, showing that even in the latter it is associated with entanglement network strands rather than strands between chemical crosslinks. For a linear sample with molecular weight 180,000, the retardation processes disappear at times beyond about 10 sec. at 25°C. With crosslinking short of the gel point (i.e., branching) the slow retardation processes are enormously increased and prolonged to longer times. With further crosslinking through the gel point and beyond, the slow retardation processes decrease progressively in magnitude. Qualitatively, this behavior resembles the sharp maximum in content of highly branched and aggregated molecular species which is predicted at the gel point by crosslinking statistics; but the slow processes (or low-frequency losses) persist farther past the gel point than would be expected on this basis. The steady-state compliances of the linear samples were smaller, but for a sample crosslinked short of the gel point were much larger, than the prediction of the Rouse theory modified for molecular weight distribution. |
