(317h) Role of Polymer Molecular Weight Distribution on Extensional Flow of Polymer Solutions and Colloid-Polymer Suspensions

Understanding of the role of complex fluid composition on the extensional rheology is of great relevance for processing of functional materials through electrospinning, 3-D printing, and electrospray, among other routes. Here, we examine the effects of the polymer size distribution on the extensional flow of polymer solutions and colloid-polymer mixtures using a dripping-onto-substrate (DoS) protocol. For polyacrylamide solutions, the scaling of the extensional relaxation time with polymer concentration depends on polymer molecular weight. The scaling exponents correlate with the ratio of the drag coefficient ratio of stretched and coiled polymer chains. The increase in scaling exponent for polymers of greater molecular weight and dispersity is attributed to the presence of coil-stretch hysteresis, which screens the excluded volume interactions under extensional flow. Second, we characterize extensional flow for depletion suspensions containing PAM and methacrylate copolymer colloidal particles in an index- and density-matched solvent. The addition of polymer delays and modifies the pinch-off dynamics of these dense suspensions, depending on the size and dispersity of the polymer. The extensional relaxation time of each colloid-polymer suspension collapses as a function of the normalized free volume polymer concentration c/c* with the corresponding polymer solution, indicating that the elastic properties of the polymer solutions control the extensional time scale. The filament lifespan of a colloid-polymer suspension and a polymer solution collapse onto a master curve as a function of c/c* when normalized by the filament lifespan of the corresponding fluid without polymer. Our work highlights the importance of the polymer size distribution in tailoring the extensional flow of complex fluid solutions and suspensions.