Time-resolved X-ray study of poly(aryl ether ether ketone) crystallization and melting behaviour: 2. Melting

Heating measurements of amorphous and isothermally crystallized (260-320°C) samples were carried out using time-resolved small-angle X-ray scattering (SAXS) to study the melting behaviour of poly(aryl ether ether ketone) (PEEK). In the amorphous sample, the scattering maximum became detectable after the cold crystallization, with the peak position q_max decreasing with temperature. The latter was also seen in crystallized samples. Two types of long period (L_B and L^M_C) were determined, by using Bragg's law and the correlation function, respectively. These values are inversely proportional to q_max, and therefore they all increased with temperature. In the crystallized samples, two stages of increase in the long period were found: below the lower melting temperature of the double endotherms (T_m1, determined by d.s.c.), the increase was small, while above T_m1, both of the long periods increased rapidly. In addition, the invariant Q was found to increase with temperature but reached a maximum near the onset temperature of the last endotherm. These results were consistent with the hypothesis of sequential melting of lamellar bundles consisting of two populations of thickness. In the crystalline specimens, the increase in Q was mainly due to the thermal expansion, whereas in the amorphous sample, the larger increase in Q was additionally attributed to the broad cold crystallization.