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Tube Dilation in Isofrictional Polymer Blends Based on Polyisoprene with Different Topologies: Combination of Dielectric and Rheological Spectroscopy, Pulsed-Field-Gradient NMR, and Neutron Spin Echo (NSE) Techniques

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journal contribution
posted on 08.07.2020, 19:33 by Paula Malo de Molina, Angel Alegría, Jürgen Allgaier, Margarita Kruteva, Ingo Hoffmann, Sylvain Prévost, Michael Monkenbusch, Dieter Richter, Arantxa Arbe, Juan Colmenero
We address the dynamics of isofrictional bimodal polyisoprene (PI) blends and emphasize the effect of concentration and topology of the short component on the dynamics of the long-chain component. The experiments were performed on blends of long well-entangled linear chains with shorter linear and star-branched chains varying systematically the concentration of the short component (additive). Small-angle neutron scattering showed that the conformation of the long chains does not change either with concentration or with the additive topology. Applying different spectroscopic techniques, we studied the terminal times of both the long chain and the additives. Thereby, the dielectric and viscoelastic terminal times for the long-chain dynamics, as well as the diffusion times deduced from pulsed-field-gradient (PFG)-NMR measurements, follow the same scaling behavior as a function of the volume fraction of the long chains (ϕL). For the case of the linear additive, the scaling τL ∼ ϕL is observed in the full concentration range covered (0.1 ≤ ϕL ≤ 1). In the case of the star additives, deviations from this scaling law are evident at ϕL ≲ 0.4. The zero-shear viscosity scales as η0 ∼ ϕL3 for ϕL ≳ 0.4. Deviations are observed at lower values of ϕL for both additive topologies. On the other hand, the terminal relaxation time of the short chain or the arm retraction time for the stars obtained from dielectric spectroscopy (DS) is only weakly affected by blending and stays nearly constant over the full concentration range. However, the diffusion times of both types of additives depend significantly on ϕL. Finally, measuring the dynamic structure factor by neutron spin echo (NSE), we directly observe the process of constraint removal at the molecular scale. These results are discussed in terms of the different theoretical approaches available. Thereby, the nearly quantitative agreement of the rheological results with the Read extension of the Viovy theory for long-chain volume fractions ϕL ≥ 0.5 is emphasized. On the other hand, the low ϕL results cannot be accounted for. Also, predictions of the tension equilibration mechanism leading to an enhanced ϕL-dependence are not supported by our data.