Nonlinear Stress Relaxation of End-Associative Star Chain

For polymers having long-lived associative groups (stickers), slow rheological features are governed by the sticker dissociation. Under small strains, models treating the effect of thermal dissociation of the stickers as an enhancement of local friction have been successfully applied to various associative polymers. In contrast, large strains mechanically disrupt the stickers to activate fast chain motion, and the resulting conformation serves as the initial conformation for the slow sticky relaxation occurring after thermal reformation of the stickers.

We have examined this idea for tetra-PEG star chains having stickers at the arm ends [Zhang et al., J. Soc. Rheol. Japan. 2024, 52, 123, ibid. 143]. In unentangled aqueous solutions, the tetra-PEG chains formed an associative network through the stickers to exhibit almost single-Maxwellian relaxation (due to thermal dissociation of the stickers) under a small step strain. In contrast, under large step strains, the chains exhibited time-strain separable nonlinear damping. This nonlinear behavior was analyzed on the basis of the above idea, and the resulting damping function was in good agreement with the experiments. The analysis also demonstrated a breakdown of BKZ constitutive equation under double-step strain when the first and second strains were applied in the opposite directions. A summary of these results will be presented on site.