Shear-induced nematic phase in entangled rod-like PEEK melts

One of the most intriguing properties of rod-like polymers is the ability to form a nematic phase. Among a broad variety of external stimuli to promote the isotropic (I)-nematic (N) transition, a shear-induced nematic phase represents one of the most fascinating phenomena in polymer physics. Here, after reviewing some relevant findings on quiescent and shear-induced nematics, we present the novel shear-induced isotropic-nematic transition exhibited by poly(ether ether ketone) (PEEK) melts of various chain lengths. The key factor is the significant rigidity of the PEEK chain that makes it a rod-like polymer. The molecular weight (M_w) dependence of the zero-shear viscosity (η₀) of PEEK in the isotropic phase, follows the Doi-Edwards theoretical prediction for rod-like polymers in the entangled regime; η₀∼M_w⁶. The shear-induced I-N transition manifests in the apparent shear viscosity dependence on the shear rate (flow curves) with three regimes: I) an isotropic response with no measurable birefringence at low shear rates, II) an I-N transition with an isotropic-nematic biphase, two steady state values of apparent viscosity and mild birefringence at intermediate shear rates, and III) a continuous nematic phase with strong birefringence at high shear rates with η∼γ˙^−1/2. Additionally, the observed threshold shear rates for regimes II and III for the four PEEK samples were used to construct a dynamic phase diagram of PEEK at 370°C, revealing that such a transition is stress-controlled.