Rheology of Unentangled Polymer Solutions Depends on Three Macromolecular Properties: Flexibility, Extensibility, and Segmental Dissymmetry

Abstract

AbstractWe elucidate the influence of chemical structure on macromolecular hydrodynamics and rheological response using poly(ethylene oxide) or PEO and 2‐hydroxyethyl cellulose (HEC). We contrast the shear rheology response measured using torsional rheometry and find the wellknown universalities like comparable shear viscosity for dilute solutions at comparable degree of chain overlap. We show that dripping‐onto‐substrate (DoS) rheometry protocols that we developed facilitate characterization of pinching dynamics and extensional rheology response even for weakly elastic, low viscosity fluids. Even for unentangled solutions with comparable shear viscosity and molecular weights, the PEO solutions exhibit distinctively higher values of extensional relaxation time, extent of strain hardening, transient extensional viscosity, and an overall delay in pinch‐off. We find that the extensional relaxation times exhibit concentration dependence distinct from shear rheology response, or anticipated by blob models, developed for relaxation of weakly perturbed chains. Most significantly, we show that the influence of polymer choice can be evaluateda priori, using three macromolecular parameters: flexibility, extensibility, and segmental dissymmetry, defined as the ratio of Kuhn length to packing length. We identify the minimum concentration above which elastocapillary response appears and define a stretched overlap concentration below which the extensional relaxation time is concentration independent.