Concentration Regimes for Extensional Relaxation Times of Unentangled Polymer Solutions

Abstract

We study the extensional flow properties of polyacrylamide (PAM) solutions with various molecular weights and dispersities using a dripping-onto-substrate (DoS) protocol. A recent study [Dinic and Sharma, Macromolecules 2020, 53, 4821−4835] suggested that coil−stretch hysteresis, which occurs when the drag coefficient ratio of stretched and coiled polymer chains ς s /ς c > 4.5, controls the scaling exponent of the extensional relaxation time λ E with concentration. Here, we test this hypothesis by varying ς s /ς c through the PAM molecular weight distribution. The scaling exponent of the concentration dependence of λ E is m = 0.34 for PAM solutions with ς s /ς c < 4.5 and m > 0.5 for PAM solutions with ς s /ς c > 4.5. The increase in the scaling exponent is attributed to the presence of coil− stretch hysteresis, which screens the excluded volume interactions under extensional flow. For highly disperse solutions with Đ ≈ 21, the transition from an exponent of 0.67 to 1 occurs at overlap concentration c* derived from the weight-averaged molecular weight instead of viscosity-averaged molecular weight, highlighting the role of long chains. These results provide insight into the role of the polymer size distribution in the concentration-dependent extensional material response in dilute and unentangled semidilute solutions.

Pinching dynamics, extensional rheology, and stringiness of saliva substitutes

“…Unlike CaBER or Trimaster or FISER, the DoS protocols involve no moving parts and therefore, allow analysis of low viscosity and low elasticity fluids as the same interplay of stresses that is relevant for jetting, CaBER or dripping drives capillarity-driven pinching dynamics in DoS rheometry. The robustness and utility of this frugal technique has resulted in its increasing use for studying the extensional rheology response of polymer solutions and macromolecular biological fluids, 88,89,153,154,171–177 but a thorough examination of the influence of interfacial rheology, complexation between polymers and other ingredients, and adsorption kinetics are also warranted.…”