Re-entanglement Kinetics of Shear-induced Isotropic and Nematic Poly-ether-ether-ketone

The relaxation—or re-entanglement—kinetics of semi-crystalline polymers after flow is critical in determining subsequent crystallization pathways and the resulting structure anisotropy. The delay in re-entanglement times has been observed for flexible polymers below the equilibrium melting temperatures due to the presence of flow-induced precursors. Poly(ether ether ketone) (PEEK) exhibits unique nematic interactions due to its chain rigidity and can form shear-induced nematic phases under strong flow. In this work, we explore the re-entanglement kinetics of PEEK with different molecular weights, across a wide range of shear strains and temperatures spanning below and above its equilibrium melting temperature. Small amplitude oscillatory shear experiments were used to follow the time-dependent recovery of the storage modulus after shear cessation. Distinct re-entanglement kinetics were observed depending on the flow-induced state and temperature: a single relaxation mode after shearing into the isotropic phase and nematic phase at high temperatures, and two relaxation modes in the nematic phase below equilibrium melting temperature, with the second mode exhibiting increasing timescales at higher strains. At temperatures above the equilibrium melting temperature, the Arrhenius temperature dependence of the re-entanglement time displayed similar activation energy as viscous flow and below equilibrium melting temperature, a much higher activation energy is observed for the re-entanglement process. The single relaxation mode of the isotropic phase has similar power law scaling to molecular weight as the reptation time and zero-shear viscosity.