Thursday 19 May 2022

Ralph Colby
Department of Materials Science, Penn State University, USA
Shear-Induced Nematic Phase in Entangled Rod-Like PEEK Melts
Seminar room 1 (FORTH) and online. Please visit the link.

We present experimental evidence for a 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 that is in an isotropic phase at rest above the melting temperature. The molecular weight (Mw) 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 ~ Mw6. 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~γ̇-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.

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