Solvent Quality Exponent Inferred from Extensional Relaxation Times of Unentangled Polymer Solutions

Polymer-solvent interactions influence static and dynamic properties of polymer solutions, including molecular weight and concentration-dependent variation in coil size, chain diffusivity, and rheological properties like zero shear viscosity and relaxation time. Therefore, quantitative measurement of solvent quality exponent that provides the power law dependence of equilibrium coil size on molecular weight or the number of Kuhn segments is a critical step in the characterization of macromolecular properties, and making inferences about their phase behavior, rheology, and processability. Traditionally, the solvent parameter is determined by making measurements of how intrinsic viscosity varies with molecular weight, presenting profound challenges in terms of material, time, patience, and equipment needs. Here we propose an alternative procedure based on analyzing extensional relaxation time as a function of concentration for a given polymer. We collate and revisit the recent measurements of extensional relaxation time using analysis of capillary-driven pinching dynamics in experiments carried with diverse techniques like dripping-onto-substrate (DoS) rheometry, capillary breakup extensional rheometer (CaBER), Cambridge Trimaster, and dripping. We include measurements over a wide range of solvents and polymers with varied molecular weights to illustrate how the response is influenced by the solvent quality and the macromolecular properties set by the number, length, and diameter of Kuhn segments.