Flow-induced crystalline precursors in entangled Poly(vinyl alcohol) aqueous solutions

Flow-induced formation of crystalline precursors was systematically investigated for entangled poly (vinyl alcohol) (PVA) aqueous solutions. The preshear flow was applied at 25 degrees C, a temperature well above the crystallization temperature. The crystalline precursors formed when the Weissenberg number (Wi(R) = gamma Tau(R), defined with respect to the Rouse time) became larger than one such that polymer chains are stretched. The precursors lead to an additional relaxation process that was much slower than the chain relaxation process in the linear viscoelasticity. The precursors were very stable at 25 degrees C, which could be effectively smeared by application of a secondary shear flow that was much slower than the preshear, but faster than the inverse of a characteristic time scale of the slow process (arising from the crystalline precursors). During the secondary shear flow, the stress growth coefficient shows a strong stress overshoot, indicating that the flow-induced crystalline precursors had a structure akin to the associative network, probably owing to the disruption of the protective hydration shell around PVA molecules by the preshear flow, allowing the interchain hydrogen bonds to form.

Automated summary

The flow-induced formation of crystalline precursors in entangled PVA aqueous solutions was investigated systematically. The precursors were stable at 25 degrees C but could be effectively removed by a secondary shear flow. The stress growth coefficient during the secondary shear flow indicated a structure similar to an associative network.