(602f) Macromolecular Dynamics and Extensional Rheology of Aqueous Solutions of Flexible and Semi-Flexible Polymers

We elucidate the influence of chain flexibility on macromolecular hydrodynamics, rheological response, and drop formation/liquid transfer by utilizing solutions of two polymers with matched overlap concentration, and distinct chemical structure and flexibility. We contrast the shear and extensional rheology response of aqueous solutions of semi-flexible 2-hydroxyethyl cellulose (HEC) with solutions of flexible, polyethylene oxide (PEO). Liquid transfer and drop formation/ deposition processes associated with printing, spraying and coating flows involve complex free-surface flows including the formation of columnar necks that undergo spontaneous capillary-driven instability, thinning and pinch-off. For simple Newtonian fluids, the interplay of capillary, inertial and viscous stresses determines the self-similar thinning and pinch-off dynamics. In rheologically complex fluids, extra elastic stresses alter the pinch-off dynamics. Here we show that dripping-onto-substrate (DoS) rheometry protocols that involve visualization and analysis of capillary-driven thinning of a columnar neck can be used for measuring extensional viscosity and extensional relaxation time of polymeric fluids and for investigating the role of chemical structure. We show that the HEC solutions display a higher shear viscosity and a stronger concentration-dependent increase in specific viscosity above overlap concentration. In contrast, the PEO solutions exhibit distinctively higher values of extensional relaxation time, extent of strain hardening and transient extensional viscosity as well as overall delay in pinch-off at matched concentration.