Stress relaxation of branched polymers: Effects of polymer architectures

The stress relaxation dynamics of well characterized branched polymers and blends of branched polymers with highly entangled linear polymers were studied using small-amplitude oscillatory shear experiments. The tube model predictions of the measured relaxation responses were also preformed to understand the effect of polymer architecture on the relaxation mechanisms quantitatively.; Dynamic moduli of 1,4-polybutadiene star/linear blends with different linear polymers and low star polymer concentrations were measured to investigate the effect of linear polymers on overall dynamics of branched molecules. A transition from dilution dynamics to Rouse-like constraint release dynamics of star polymers was observed on increasing the molecular weights of matrix linear polymers. A scaling model, in which polymer chains attempt to drag other chains entangled with them, was proposed to describe the observed dynamics. This scaling approach provided several relaxation modes equivalent to the physics of the tube model and some plausible relaxation modes not considered in the tube model. The quantitative tube model predictions of these star/linear blends under the dynamic dilution assumption helped to identify when the constraint release relaxation of star arm is completed. In order to probe the effect of branched polymers on the relaxation of linear polymers, 1,4-polybutadiene four-arm and 1,4-polyisoprene three-arm star polymers with short arm molecular weights were also blended with well-entangled long linear polymers. Comparisons between the measured moduli and the tube model predictions with possible relaxation modes suggested the branched molecules exert a long-live influence on the tube equilibration process of long linear polymers.; A series of 1,4-polyisoprene asymmetric three-arm star polymers, which have two equal-length long arms with different molecular weights and one short arm with a fixed molecular weight, were synthesized via anionic polymerization. Relaxation dynamics of these asymmetric star polymers were investigated experimentally and theoretically to determine how the motion of branch point affects the terminal relaxation. A relationship between the mobility of branch point and long arms with different molecular weights was found from the tube model predictions of the experimental data. This relationship indicates the effective entanglement resistance from the unrelaxed parts of long arm retards the motion of branch point. This relationship was extended to other branched polymers to predict the effect of polymer architectures on the relaxation dynamics and the generalized model showed good agreements with the measured relaxation responses of H-shaped, multiarm and comb polymers.