Fracture of polymer chains in extensional flow: Experiments with DNA, and a molecular-dynamics simulation

Abstract

The behaviors of polymer chains in simple shear flow and in flows with a large component of extension are now considered to be qualitatively different [R. B. Bird, C. F. Curtiss, R. C. Armstrong, and O. Hassager, Dynamics of Polymeric Liquids, 2nd ed. (Wiley, New York, 1987), Vol. 2, Chap. 13]. To examine the extensional case, we have fractured DNA molecules in solution in steady sink (primarily extensional) flow. DNA was chosen for this study because it can be obtained as monodisperse, unique-sequence material, and because the size distribution of the fracture fragments can be obtained by gel electrophoresis. Dilute monodisperse T7 DNA (Mw=26×106) solutions were recirculated through a 0.13 mm orifice in a flat plate. The flow field upstream of the orifice closely approximated an ideal sink flow, being free of vortices under the conditions used, DNA concentrations less than 12.5 μg/mℓ and flow rates less than 0.045 cm3/s. The kinetics of the fracture at low flow rates showed an initial lag period followed by a period of first-order rate; the lag period disappeared at higher flow rates while the first-order period persisted. The fracture rate increased exponentially with the flow rate. Contrary to the classic theory by Frenkel, our experimental results showed fracture products broadly distributed in size; this anomaly was explained by a bead-spring molecular-dynamics computer simulation. The simulation showed that just prior to chain fracture the chain was aligned parallel to the flow but contained many folds, so that points of maximum stress were not usually at the molecular center. These results suggest that the residence time in the converging flow was too short for the chain to reach complete extension.