MORPHOLOGICAL-STUDY AND MELTING BEHAVIOR OF NARROW MOLECULAR-WEIGHT FRACTIONS OF POLY(ARYL ETHER ETHER KETONE) (PEEK) ANNEALED FROM THE GLASSY STATE

The morphology of several semicrystalline poly(aryl ether ether ketone ) (PEEK) samples with narrow molecular weight distributions and of a c ommercial sample of PEEK has been investigated by means of differentia l scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS). The use of different molecular w eights allows us to enlarge the macroscopic crystallinity range usuall y attainable for the commercial PEEK grades. WAXD and DSC data are in good agreement to estimate the weight degree of crystallinity values r anging from similar to 15% for the highest molecular weight sample to similar to 55% for the lowest molecular weight sample. Most of the inv estigated samples exhibit the well-known double melting behavior with a low and a high melting endotherm. This behavior is interpreted in th e light of a melting-recrystallization mechanism which is strongly aff ected by the presence of entanglements in the amorphous zones. The ana lysis of the SAXS data shows that, under constant annealing conditions , the crystal thickness (given by the smallest length obtained from th e correlation function) remains unaffected by a change of the molecula r weight while the amorphous layer is found to strongly increase with increasing chain length due to the presence of entanglements. This lea ds to a decrease in the linear degree of crystallinity within the lame llar stacks which is consistent with the behavior of the macroscopic d egree of crystallinity obtained from WAXD, DSC, and density measuremen ts.