(527a) Flow-Induced Nematic Alignment and Nucleation Acceleration in Polymer Melts

Using rheo-optical methods we show that poly(ether ether ketone) melts of various chain lengths begin to exhibit a sudden nematic alignment at the critical shear rate beyond which shear flow accelerates nucleation. The shear-induced isotropic-to-nematic (I-N) transition manifests in the shear rate dependence of viscosity with three regimes: I) An isotropic response with no measurable birefringence at low shear rates, with zero-shear-rate viscosity proportional to the 6th power of weight-average molecular weight, predicted for isotropic melts of rodlike polymers by Doi. II) An I-N transition with an isotropic-nematic biphase, two steady state values of apparent viscosity and measurable birefringence at intermediate shear rates. III) A fully nematic alignment with strong birefringence at high shear rates with shear thinning viscosity scaling as the reciprocal square root of shear rate, predicted by Marrucci and observed for all nematic polymers. We speculate that this nematic alignment of polymer chains just prior to crystallization results in many tie chains between crystals, and is responsible for the outstanding mechanical properties of PEEK We find that other aromatic backbone polymers exhibit similar features, even ones that never crystallize. We also study the effects of shear flow on nucleation acceleration of more flexible polymers to determine which aspects of these flow effects are universal to all semicrystalline polymers.