TIME-RESOLVED SYNCHROTRON X-RAY STUDY OF CRYSTALLINE PHASE-TRANSITIONIN POLY(ARYL ETHER KETONE KETONE) CONTAINING ALTERNATED TEREPHTHALIC ISOPHTHALIC MOIETIES

Unique crystallization and melting behavior in poly(aryl ether ketone ketone) containing alternated terephthalic and isophthalic moieties we re studied by time-resolved synchrotron x-ray methods. Recently, this material has been shown to exhibit three polymorphs (forms I, II, and III). In this work, we further investigated their distinctive thermal properties and found that form I is the dominating and the most therma lly stable phase while form II is favored by fast nucleation condition s and is the least stable phase. On the other hand, form III represent s a minor intermediate phase that usually coexists with form I and can be transferred from form II and to form I. Structural and morphologic al changes in form I have been followed by simultaneous wide-angle x-r ay diffraction (WAXD)/small-angle x-ray scattering (SAXS) measurements during cold- or melt-crystallization and subsequent melting. In all c ases, a larger dimensional change was found in the crystallographic a- axis than the b-axis during heating and cooling. This may be due to th e greater lateral stress variation with respect to temperature along t he a direction of the primary lamellae which is induced by either the formation of secondary lamellae or the preferential chain-folding dire ction in poly(aryl ether ketone ketone)s. During the phase transitions of form II --> III in the cold-crystallized specimen and form III --> I in the melt-crystallized samples, lamellar variables (long period, lamellar thickness, and invariant) obtained from SAXS remain almost co nstant. This indicates that the density distribution in the long spaci ng is independent of the melting in form II or III. For melt-crystalli zation, the corresponding changes in unit-cell dimensions and lamellar morphology during the annealing-induced low endotherm are most consis tent with the argument that these changes are due to the melting of th in lamellar population. (C) 1995 John Wiley & Sons, Inc.