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Rheology of associating polymers : effects of ionic and hydrogen bonding interactions Tomkovic, Tanja

Abstract

Associating polymers, consisting of relatively small functional groups capable of forming reversible physical associations, represent an important class of materials since they can be used in stimuli-responsive systems. They have found numerous applications in self-healing systems, and they can be used as shape memory polymers and biomaterials. Their complex behavior depends on the delicate interplay between the chemistry of functional groups and polymer dynamics. Therefore, it is essential to understand the physics of these complicated networks to exploit their full potential for important/innovative applications. Two classes of associating polymers, namely ion-containing polymers (ionomers), and hydrogen bonding polymers, were studied. Using a rotational rheometer and the Sentmanat extensional fixture, a full rheological characterization of several commercial ionomers and a series of novel amine-containing polynorbornenes and polycyclooctenes was conducted. Emphasis has been placed on the distribution of the relaxation times to identify the characteristics times such as reptation, Rouse times and the characteristic lifetime of the ionic and hydrogen bonding associations. These characteristic relaxation times were related to the structure of these polymers using developed scaling theories. Complete removal of ions was performed to produce copolymers in order to study the relative effects of ionic associations. It was demonstrated that ionic interactions increase the linear viscoelastic moduli and the viscosity by up to an order of magnitude and cause significant strain hardening effects in uniaxial extension. Similarly, for the novel hydrogen bonding polymers, the reversible associations significantly increase the rheological properties and thus delay their relaxation, causing chains to strain-harden before failure. Additionally, it was discovered that the amine-containing polymers exhibit outstanding self-healing properties. Optimal, fast self-healing response was achieved by varying the molecular weight and structure of these materials. Finally, the capillary flow of several commercial ionomers was studied both experimentally and numerically. The excess pressure drop due to entry, the effect of pressure on viscosity, and the possible slip effects on the capillary data analysis were examined. Rheological data were successfully fitted to a viscoelastic model developed by Kaye, Bernstein, Kearsley, and Zapas, (known as the K-BKZ model) to perform relevant capillary flow simulations.

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