Time-resolved SAXS, WAXS, and DSC study of the annealing of poly(aryl ether ether ketone) (PEEK) from the glassy state

The annealing of poly(aryl ether ether ketone) (PEEK) from the glassy state was investigated by time-resolved simultaneous small-angle (SAXS) and wide-angle (WAXS) X-ray scattering using synchrotron radiation and by differential scanning calorimetry (DSC). Each experiment consisted of a heating ramp from 100 degrees C to the annealing temperature (280, 300, 310, 315, 340, and 350 degrees C) at 10 degrees C/min, followed by a 30 min isotherm at this temperature and a cooling ramp down to 100 degrees C at -10 degrees C/min. The long spacing L and the invariant Q were estimated from the Lorentz-corrected SAXS intensity curves. The crystalline (L(c)) and amorphous (L(a)) thicknesses and the linear degree of crystallinity (upsilon(c)(lin)) within the stacks of lamellar crystals were obtained from the correlation function gamma(r). The annealing temperature range can be divided into two regions based on the final morphology of the PEEK films. Samples annealed at temperatures up to 315 degrees C contain lamellar crystals that have crystallized during the isotherm. For annealing temperatures above 315 degrees C, thick lamellar crystals grow during the isotherm. However, a large fraction of the material remains molten at the end of the isotherm. The magnitude of this molten fraction increases with the annealing temperature. This molten fraction nonisothermally crystallizes during cooling as revealed by calorimetry and X-ray diffraction. The linear degree of crystallinity of PEEK crystallized from the glassy state at temperatures between 280 and 340 degrees C is always below 0.5, implying that the crystalline thickness of the lamellae corresponds to the smallest value of the two partial thicknesses obtained from the correlation function. For the last experiment, the polymer is fully molten at 350 degrees C and nonisothermally crystallizes during the cooling as revealed by calorimetry. In contrast, the analysis of the corrected invariant and the integrated WAXS intensity for this last sample indicates that the latter consists of lamellar stacks with a linear degree of crystallinity of up to 0.75.