Shear-Induced Isotropic-Nematic Transition in Poly(ether ether ketone) Melts

In a previous work on a poly(ether ether ketone) (PEEK) melt, above its nominal melting temperature (T_m ≅ 335 °C), a severe Cox-Merz rule failure was observed. The abrupt decrease in the apparent shear viscosity was ascribed to the formation of flow-induced crystallization precursors. Here shear rheology and reflection polariscope experiments are utilized to unravel the structural changes occurring under shear on a similar PEEK melt above T_m. Three regimes of the flow curve were identified from low (0.01 s^-1) to high shear rates (1000 s^-1): (I) an isotropic structure with weak birefringence due to polymer chain orientation and mild shear thinning for γ̇ < 1 s^-1, (II) an isotropic-nematic transition accompanied by strong birefringence, two steady-state viscosities, and large nematic polydomain director fluctuations, and (III) shear-thinning behavior with an η ∼ γ̇^-0.5 dependence for γ̇ > 20 s^-1, typically found in nematic fluids. The findings reported in this experimental work suggest that the nematic phase may represent the early stage of the formation of shear-induced crystallization precursors.