Concentration Regimes for Extensional Relaxation Times of Unentangled Polymer Solutions

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 ςs/ςc > 4.5, controls the scaling exponent of the extensional relaxation time λE with concentration. Here, we test this hypothesis by varying ςs/ςc through the PAM molecular weight distribution. The scaling exponent of the concentration dependence of λE is m = 0.34 for PAM solutions with ςs/ςc < 4.5 and m > 0.5 for PAM solutions with ςs/ςc > 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.