Some new evidence for polymorphism in cold-crystallized and melt-crystallized poly(ether ether ketone)

Cold-crystallized or melt-crystallized poly(ether ether ketone) (PEEK) has been found to display multiple low-temperature minor peaks in addition to the major melting peak. Experimental evidences of thermal analysis and X-ray diffraction studies are provided in this study to support the proposal of multiple morphologies being responsible for the observed multiple melting behavior. The hypothesis of melting of original low-temperature crystals and reorganization into higher-melt crystals during DSC scanning has been found to lack plausible evidences. When step-wise crystallized or annealed in decending temperatures, the minor crystals could aggregate into several populations with each showing a distribution. The results of slow-cooled PEEK suggested that the minor crystals have a tendency to populate in the region where the minor melting peak position is located at 310 and 320°C. We have exemplified the co-existence of thinner lamellae of multiple thickness distributions simultaneously with the major lamellae after crystallization or annealing treatments. The polymorphism in PEEK exists on the lamellar scales.