Long time annealing of semicrystalline PEEK above the glass transition results in the observation of several time dependent phenomena - "physical aging", "secondary crystallization", "multiple melting" of lamellae with different thermal stability etc. Their interrelation - common origin and kinetics of development, is characterized extensively for the first time in this study.
The evolution of the crystallinity during the secondary crystallization process was monitored by DSC and density measurements. Crystallinity was characterized according to the standard two-phase model of semicrystalline polymers and analyzed with respect to the failure of the model to adequately describe the physical state of the polymer. A discrepancy was observed between DSC and density crystallinity values and their respective rates of development during the secondary crystallization stage.
WAXS reveals that the crystal density is not a physical constant, but depends on the crystallization and/or annealing temperature. Furthermore, the crystalline lamellae densify with time during crystallization and/or annealing. This observation leads to the conclusion that there is no one-to-one correspondence between density and crystallinity and necessitates the application of a revised equation for density crystallinity which accounts for the dynamics of crystal densification.
The characteristics of the low temperature endothermic peak in the DSC scan of PEEK (peak maximum, transition enthalpy etc.) were found to evolve with annealing time and temperature during the secondary crystallization process in a way similar to the kinetics of development of the enthalpy relaxation process in amorphous polymeric glasses.
This study reports for the first time in the literature the observation of "physical aging" above the glass transition in the case of PEEK (according to the definition of this term given by Struik). An extensive investigation of the "double melting"/"multiple melting" phenomenon, which is observed as a result of isothermal treatment of the polymer above Tg, was performed and several new observations reported.
After the end of the primary crystallization process, the semicrystalline polymer is a nonequilibrium system due to the fact that crystallinity is less than unity. The system's continuing approach to equilibrium and its response to mechanical perturbations follow kinetics similar to that of segmental relaxation below the glass transition.