Association Behavior of End-Functionalized Polymers. 2. Melt Rheology of Polyisoprenes with Carboxylate, Amine, and Zwitterion End Groups

Viscoelastic behavior in the melt state was investigated for polyisoprenes with associating groups on the ends of the chain and compared with the behavior of their nonfunctionalized counterparts. Narrow distribution samples were prepared with a range of molecular weights by anionic polymerization; functional groups such as tertiary amine, zwitterion, carboxylic acid, and sodium carboxylate were introduced through the initiator as well as by appropriate postpolymerization reactions. Melt viscosities of the monofunctional samples were examined for evidence of end group clustering. The polymeric star model gave an excellent account of results for the sodium carboxylate series, indicating long-lived spherical clusters in the melt. Strong association was also evident in the zwitterion series, but the viscosities were much too large to explain by a simple star model, suggesting the possibility of extended aggregate structures. Judged by rheological evidence, the polymers capped by amine and carboxylic acid groups were only weakly associated. However, nearly complete dimerization of the carboxylic acid is expected for the experimental conditions used, suggesting a highly labile monomer-dimer equilibrium. Multifunctional versions of the zwitterion-capped polymers behaved like networks at room temperature but relaxed in the experimental time scales minutes) above 100 °C.