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 (Tm ≅ 335 °C), a severe Cox-Merz rule failure was observed. The abrupt decrease of the apparent shear viscosity was ascribed to the formation of Flow-Induced Crystallization (FIC) precursors. Here, shear rheology and reflection polariscope experiments are utilized to unravel the structural changes occurring under shear on a similar PEEK melt above Tm. Three regions of the flow curve were identified from low (0.01 s-1) to high shear rates (1000 s-1): I) isotropic structure with weak birefringence due to polymer chain orientation and mild shear thinning below1 s-1, II) isotropic-nematic transition accompanied by strong birefringence, two steady-state viscosities and large nematic polydomain director fluctuations and III) shear thinning behavior with viscosity scaling as the reciprocal square root of shear rate above 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 for the formation of shear-induced crystallization precursors.