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 adripping-onto-substrate (DoS) protocol. A recent study [Dinic andSharma, Macromolecules 2020, 53, 4821-4835] suggested that coil-stretchhysteresis, which occurs when the drag coefficient ratio of stretchedand coiled polymer chains & sigma;( s )/& sigma;( c ) > 4.5, controls the scaling exponentofthe extensional relaxation time & lambda;( E ) with concentration. Here, we test this hypothesis by varying & sigma;( s )/& sigma;( c ) throughthe PAM molecular weight distribution. The scaling exponent of theconcentration dependence of & lambda;( E ) is m = 0.34 for PAM solutions with & sigma;( s )/& sigma;( c ) < 4.5 and m > 0.5 for PAM solutions with & sigma;( s )/& sigma;( c ) > 4.5. The increaseinthe scaling exponent is attributed to the presence of coil-stretchhysteresis, which screens the excluded volume interactions under extensionalflow. For highly disperse solutions with D & AP;21, the transition from an exponent of 0.67 to 1 occurs at overlapconcentration c* derived from the weight-averagedmolecular weight instead of viscosity-averaged molecular weight, highlightingthe role of long chains. These results provide insight into the roleof the polymer size distribution in the concentration-dependent extensionalmaterial response in dilute and unentangled semidilute solutions.