For highly entangled cis-polyisoprene (PI) star polymers having more than 10 entanglements in each arm, dielectric and viscoelastic properties were examined within a context of the generalized tube model incorporating the dynamic tube dilation (DTD) mechanism. The star PI had the type A dipoles parallel along the arm backbone, and the global motion results in the viscoelastic as well as dielectric relaxation. The DTD relationship between the dielectric and viscoelastic relaxation functions Φ(t) and μ(t), μ(t) ≅ [Φ(t)]2 (derived under an assumption of random displacement of the entanglement segment in the dilated tube edge), was not valid for the star PI. Furthermore, the DTD model (Milner−McLeish model) excellently described the viscoelastic data, but considerable differences were found for the dielectric data, even if an effect of the segment displacement in the tube edge was considered in the model. These results indicated a failure of the DTD molecular picture for a few entanglement segments near the branching point. Thus, these segments near the branching point appeared to fully relax via the constraint release (CR) mechanism before the expected tube dilation was completed. On the basis of this result, the DTD model was modified by explicitly incorporating this CR process (though in a crude way). This modification moderately improved the model prediction, suggesting a possible direction of further refinement of the model.

cis-Polyisoprene (PI) has the type A dipole parallel along the chain backbone so that the end-to-end fluctuation of PI chains results in slow dielectric relaxation. Comparison of dielectric and viscoelastic data of PI has revealed several interesting features related to the entanglement dynamics, for example, success and failure of the full dynamic tube dilation (DTD) picture for monodisperse linear and star PI, respectively [see a review: Watanabe, H. Polym. J. 2009, 41, 929, for example]. For monodisperse linear PI, recent modeling [Glomann et al. Macromolecules 2011, 44, 7430] and single-chain slip-link simulation [Pilyugina et al. Macromolecules 2012, 45, 5728] suggest that the constraint release (CR) mechanism has negligible influence on the dielectric relaxation time τε in the entangled regime, which appears to disagree with the previous data. Thus, we revisited the classical problem: CR contribution to the dielectric relaxation of PI. Specifically, we made dielectric and viscoelastic measurements for PI/PI blends in a wide range of the molecular weights of long and short components, M 2 = 1.1M and M 1 = 21K–179K, and with a small volume fraction of the short component, υ1 = 0.1 and/or 0.2, to examine the CR contribution in the experimentally clearest way. It turned out that τε of the short component was longer in the blends than in respective monodisperse bulk even for M 1 = 179K. Furthermore, the viscoelastic and dielectric data of the short components (M 1 ≤ 43K) in the blend exhibited identical mode distribution and relaxation time, which confirmed that the CR mechanism was fully suppressed for these components in the blends. These results demonstrate that the CR mechanism does contribute/accelerate the dielectric relaxation in monodisperse bulk PI systems even in the highly entangled regime (M 1/M e = 36 for M 1 = 179K). This CR-induced acceleration was found to be consistent with the empirical equations for the terminal relaxation time and CR time of monodisperse PI available in the literature, as noted from a simple DTD analysis of the terminal relaxation process (reptation along a partially dilated tube that wriggles in a fully dilated tube).

Viscoelastic and dielectric experiments were conducted for entangled binary blends of linear cis-polyisoprenes (PI) having widely separated molecular weights, M 1 = 21.4 × 103 ≅ 4M e and M 2 = 308 × 103 ≅ 62M e with M e (≅5 × 103) being the entanglement molecular weight. The PI chain had type-A dipoles and its global motion (end-to-end vector fluctuation) was dielectrically active. The volume fraction of the high-M chain, υ2, was varied from 0.005 to 1. The Struglinski-Graessley parameter for the blends, M 2 M e 2/M 1 3 = 0.79, was larger than a threshold value ≅0.5 necessary for the thermal constraint release (CR) mechanism to dominate the relaxation of the dilute high-M chains. Indeed, the Rouse-like CR relaxation was experimentally detected for the blends with small υ2 ≤ 0.01. For large υ2 (≥0.05), the high-M chains were entangled with each other (as well as with the low-M chains) and exhibited solution-like relaxation behavior at long times in their terminal regime. Comparison of viscoelastic and dielectric data suggested that the molecular picture of the dynamic tube dilation (DTD; with the dilation exponent d ≅ 1.3) incorporating the tube-edge fluctuation effect was valid in this regime. However, at intermediate times that were still longer than the relaxation time of the low-M chains, the moduli of the high-M chains were larger than the DTD prediction. In this intermediate regime, the CR equilibration of the entanglement segments in a dilated tube segment (the prerequisite of DTD) could not occur in time, thereby resulting in this failure of the DTD picture. In addition, the viscoelastic mode distribution of the high-M chains in the blends (with υ2 ≥ 0.05) agreed with that in the corresponding solutions only in time scales longer than the time required for the CR equilibration over the whole contour of the chain. These results demonstrated the importance of the CR equilibration in the entanglement dynamics.