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

Abstract

We 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 evaluated a 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.

Solvent Properties Influence the Rheology and Pinching Dynamics of Polyelectrolyte Solutions: Thickening the Pot with Glycerol and Cellulose Gum

“…The exponent takes the value of n e = 2/3 for inertiocapillary (IC), n e = 1 for the viscocapillary (VC) pinching, and possibly n e = n for the power law exponent for the shear thinning fluids. Here, the characteristic time scales for IC and VC behavior is, respectively, called the Rayleigh time, , and viscocapillary time, t vc = η R 0 /σ. ,, The extrinsic length scale, R 0 , is chosen to be the outer nozzle radius and the parameters viscosity, η, density, ρ, and surface tension, σ, depending on the fluid choice. The IC regime is observed if the dimensionless measure of viscosity called the Ohnesorge number, , is relatively small or Oh < 1, whereas the VC regime arises for Oh > 1.…”

“…Additional details about the experimental setup and design considerations, specific advantages of DoS rheometry contrasted to other extensional rheology methods, and the various approaches to analyzing radius evolution datasets are included in our previous contributions. ,,,− ,− We investigate and describe the extensional rheological response of complex fluids such as neutral and charged polymer solutions, inks, nail lacquers, surfactant-based cosmetics (shampoos and conditioners), proteins such as egg albumin, spinnable polymer solutions, , and foods (honey, molasses, mayo, and ketchup). ,,,− ,− Since we introduced DoS rheometry in 2015, it has emerged as the method of choice for innumerable studies benefitting from the virtues of a simplified, cost-efficient setup and the ability to analyze the pinching dynamics and extensional rheological response of complex fluids, including those with viscosity comparable to water or weak elasticity, even with sub-millisecond relaxation times. − …”

“…, and viscocapillary time, t vc = ηR 0 /σ. 34,64,139 The extrinsic length scale, R 0 , is chosen to be the outer nozzle radius and the parameters viscosity, η, density, ρ, and surface tension, σ, depending on the fluid choice. The IC regime is observed if the dimensionless measure of viscosity called the Ohnesorge number,…”

“…35,56,62,63 However, due to finite time (∼50 ms) needed for initial step stretch, the characterization of low-viscosity unentangled polymer solutions was beyond the reach of these methods, especially for polysaccharides and polyelectrolytes. 33,35,41,64 In a series of papers, we developed drippingonto-substrate (DoS) rheology protocols to address many of the characterization challenges, including characterization of extensional rheology of low-viscosity systems such as unentangled solutions of neutral and charged polymers. 18,19,64−70 The DoS rheometry protocols rely on the visualization and analysis of capillarity-driven pinching dynamics of the stretched liquid bridge formed between a nozzle and a sessile drop on a substrate.…”

“…The introduction of methods that analyze capillarity-based pinching, for example, capillary breakup extensional rheometer (CaBER), presented the opportunity to emulate dispensing flows and achieve higher extensional rates (ε̇ > 10 s –1 ). ,,, However, due to finite time (∼50 ms) needed for initial step stretch, the characterization of low-viscosity unentangled polymer solutions was beyond the reach of these methods, especially for polysaccharides and polyelectrolytes. ,,, In a series of papers, we developed dripping-onto-substrate (DoS) rheology protocols to address many of the characterization challenges, including characterization of extensional rheology of low-viscosity systems such as unentangled solutions of neutral and charged polymers. ,,− The DoS rheometry protocols rely on the visualization and analysis of capillarity-driven pinching dynamics of the stretched liquid bridge formed between a nozzle and a sessile drop on a substrate. The extensional relaxation time, λ E , and the strain and strain-rate dependent extensional viscosity (also referred to as tensile growth coefficient), , are typically extracted from the analysis of radius evolution data. − The rate-independent steady, terminal extensional viscosity, η E ∞ , is also measured occasionally. − Our previous studies showed that the concentration-dependent variation in the shear and extensional rheological response of aqueous solutions of sodium poly­(styrene sulfonate) (NaPSS), poly­(acrylic acid) (PAA), and sodium carboxymethyl cellulose (NaCMC) is distinct from the corresponding solutions prepared in a glycerol/water (70/30) mixture. , To decipher the physicochemical factors that lead to the observed differences on changing the solvent, we contrast the pinching dynamics and shear and extensional rheology of NaCMC solutions prepared in a range of glycerol–water mixtures.…”