Shear-Induced Isotropic-Nematic Transition in Poly(ether ether ketone) Melts

Area of Science:

• Polymer Science
• Rheology
• Materials Science

Background:

• Poly(ether ether ketone) (PEEK) melts show unusual viscosity behavior above melting temperature (Tm ≈ 335 °C).
• Previous work suggested flow-induced crystallization precursors cause Cox-Merz rule failure in PEEK melts.

Purpose of the Study:

• Investigate structural changes in PEEK melts under shear above Tm.
• Identify flow regimes and their associated rheological properties.
• Determine the role of the nematic phase in shear-induced crystallization.

Main Methods:

• Utilized shear rheology experiments.
• Employed reflection polariscope experiments.
• Analyzed PEEK melt behavior across a range of shear rates (0.01–1000 s⁻¹).

Main Results:

• Identified three flow regimes: isotropic, isotropic-nematic transition, and nematic.
• Observed weak birefringence in the isotropic regime (γ̇ < 1 s⁻¹).
• Detected strong birefringence and dual viscosities during the isotropic-nematic transition.
• Characterized shear-thinning behavior (η ∼ γ̇⁻⁰.⁵) in the nematic regime (γ̇ > 20 s⁻¹).

Conclusions:

• The nematic phase in PEEK melts exhibits distinct rheological properties.
• The observed nematic phase may be an early precursor to shear-induced crystallization.
• Understanding these flow regimes is crucial for processing PEEK.