Reinforcement efficiency of carbon nanotubes and their effect on crystal-crystal slip in poly(ether ketone)/carbon nanotube composite fibers

Poly(ether ketone) (PEK)/carbon nanotube (CNT) composite fibers have been produced using dry-jet wet spinning. Carbon nanofibers (CNF), few-walled carbon nanotubes (FWNT), and multi-walled nanotubes (MWNT) have been utilized as nanofillers, with few-walled carbon nanotube loadings as high as 28 wt%. The interfacial strength of the PEK/FWNT and PEK/MWNT fibers were evaluated through the monitoring of the G-mode Raman peak shift as a function of fiber strain. Interfacial shear strengths as high as 14.2 MPa were measured for the PEK/FWNT fiber with 28 wt% CNT loading, a 1320% increase as compared to the PEK/MWNT fiber with 5 wt% loading (1.0 MPa). Tg of the PEK/FWNT fibers increased by 19 °C as the FWNT loading was increased from 5 wt% to 28 wt%. A second peak in the tan δ behavior of all PEK/CNT fibers was also observed. This second tan δ peak (T ∼ 240 °C-250 °C) is attributed to the α*-transition (crystal-crystal slip) in the PEK crystalline regions, and its presence is more pronounced (higher magnitude of tan δ) in all PEK/FWNT fibers as compared to PEK/MWNT and PEK/CNF fibers. FWNTs are restricting the mobility of the amorphous PEK as evidenced by the increasing Tg. Enhanced PEK crystal-crystal slip corresponds to increasing large scale chain mobility in the crystalline regions of the PEK at Tα*. We propose a model which correlates the increase in PEK-FWNT interfacial shear strength to an increase in large scale chain mobility where crystal-crystal slip precedes failure of PEK/CNT interface at Tα* for the PEK/FWNT fibers.