Effects of entanglement and dispersity on shear strain hardening

Polystyrene solutions are the widely used model system to study polymer dynamics and rheology. In the strong nonlinear region, we shear polystyrene solutions of uniform or bimodal distribution up to unprecedented high rates. The unique shear strain hardening takes place on such solutions with high Tg solute, irrespective of dispersity. Non-Gaussian stretching of chains is suggested by full recovery even after a shear strain of 60. Shear strain hardening in uniform solutions happens at a universal Rouse-Weissenberg number, 150, regardless of the number of entanglements. The critical stress between traditional stress overshoot and hardening is around 104 Pa, for 10% solutions regardless of dispersity or number of entanglements. This systematic study charts the parameters for shear strain hardening.