Concentration\nRegimes for Extensional Relaxation Times\nof Unentangled Polymer Solutions

Abstract

We study the extensional flow properties of polyacrylamide\n(PAM)\nsolutions with various molecular weights and dispersities using a\ndripping-onto-substrate (DoS) protocol. A recent study [Dinic and\nSharma, <i>Macromolecules</i> <b>2020</b>, <i>53</i>, 4821–4835] suggested that coil–stretch\nhysteresis, which occurs when the drag coefficient ratio of stretched\nand coiled polymer chains ς_<i>s</i>/ς_<i>c</i> > 4.5, controls the scaling exponent\nof\nthe extensional relaxation time λ_<i>E</i> with concentration. Here, we test this hypothesis by varying ς_<i>s</i>/ς_<i>c</i> through\nthe PAM molecular weight distribution. The scaling exponent of the\nconcentration dependence of λ_<i>E</i> is <i>m</i> = 0.34 for PAM solutions with ς_<i>s</i>/ς_<i>c</i> < 4.5 and <i>m</i> > 0.5 for PAM solutions with ς_<i>s</i>/ς_<i>c</i> > 4.5. The increase\nin\nthe scaling exponent is attributed to the presence of coil–stretch\nhysteresis, which screens the excluded volume interactions under extensional\nflow. For highly disperse solutions with <i>Đ</i> ≈\n21, the transition from an exponent of 0.67 to 1 occurs at overlap\nconcentration <i>c</i>* derived from the weight-averaged\nmolecular weight instead of viscosity-averaged molecular weight, highlighting\nthe role of long chains. These results provide insight into the role\nof the polymer size distribution in the concentration-dependent extensional\nmaterial response in dilute and unentangled semidilute solutions.