Strain Hardening of Unentangled Polystyrene Solutions in Fast Shear Flows
Salvatore Costanzo, Vincenzo Ianniello, Rossana Pasquino, Nino Grizzuti, Giovanni Ianniruberto, G. Marrucci
Abstract
We investigate the linear and nonlinear shear flow properties of several polystyrene solutions in the unentangled state. The solutions are made up of long (linear) polystyrene chains dissolved in oligostyrene at sufficiently small volume fractions such that the critical entanglement molar mass remains larger than that of the polymer. The use of a cone-partitioned plate fixture for nonlinear rheological measurements, coupled with the relatively small moduli and long relaxation times of the solutions, allows one to explore unprecedented large values of the Rouse–Weissenberg number in shear flow. As a result, chains are stretched close to their maximum extensibility, thus displaying shear strain hardening. Data on normal stresses indicate that the second normal stress difference is virtually zero at high Weissenberg numbers. Brownian dynamics simulations of Rouse-like chains incorporating finite extensibility and friction reduction suitably describe the nonlinear behavior of the solutions provided flow-induced changes of hydrodynamic interactions are also accounted for.