The transient nonlinear rheology of concentrated polymeric materials is of interest for a range of applications including inks for additive manufacturing and advanced coatings. Precise control of printed objects, for instance, relies on the rheological behavior of polymer inks far from equilibrium, and often involves the process of yielding where the ink changes from solid to liquid under an applied load. Typical rheological observations of yielding include that of the Payne effect, where an overshoot is observed in the dynamic loss modulus during oscillatory shear straining of increasing magnitude. We show this overshoot to be due to an increase in energy dissipated through unrecoverable processes by way of an iterative recovery rheological protocol.  Studying di-block bottlebrush block co-polymers, we show that the dynamic loss modulus can be experimentally decomposed into two components relating to recoverable and unrecoverable processes that may be associated with conformational changes and center of mass motions, respectively. We use our experimental observations to construct a model in which elastic effects influence the center of mass motion. The model accurately predicts a range of physical phenomena including the Payne effect and overshoots in steady-shear start-up tests, as well as some behaviors not predicted by other models.

Transient yielding of bottlebrush block copolymers

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ACS Fall 2022

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