Phase behavior and rhythmically grown ring-banded spherulites in blends of liquid crystalline poly(aryl ether ketone) and poly(aryl ether ether ketone)

Phase behavior and rhythmically grown ring-banded spherulites in thin films of liquid crystalline poly(aryl ether ketone) (LC-PAEK) and poly(aryl ether ether ketone) (PEEK) blends have been investigated by differential scanning calorimetry (DSC), polarized light microscopy (PLM), atomic force microscopy (AFM), transmission electron microscopy (TEM), and electron diffraction (ED) techniques. The results show that the composition of the blends has great effect on the phase behavior and structure. Thin films of pure LC-PAEK and PEEK crystallized from the melts exhibit typical mosaic and spherulitic structures, respectively. For the blends with higher LC-PAEK contents (> 50%), the two components are miscible at the molecular level in the melting state, and a phase separation takes place during the cooling process, as a result of the competition between the phase separation and the LC-PAEK phase transition, resulting in an unusual ring-banded spherulite. The bright core and rings of the ring-banded spherulites under PLM are composed of the LC-PAEK phase, while the dark rings consist of a coexisting phase of PEEK with part of LC-PAEK. The formation of the ring-banded spherulites is attributable to structural discontinuity caused by a rhythmic radial growth. For the 50:50 LC-PAEK/PEEK blend, the ring-banded spherulites and individual PEEK spherulites coexist, which implies that a partial phase separation between the two components takes place in the melting state. In PEEK-rich blends, LC-PAEK is rejected into the boundary of PEEK spherulites. In addition, the effect of cooling rate and crystallization temperature on the phase behavior, especially the ring-banded spherulite formation in the blends, is discussed.