PEEK oligomers as physical model compounds for the polymer. 4. Lamellar microstructure and chain dynamics.

Crystals of monodisperse linear poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene) (PEEK) oligomers have been reinvestigated by wide-angle and small-angle X-ray scattering and atomic force microscopy (AFM). For the shorter oligomer investigated (containing six phenyl groups), a correlated packing of the chains is observed in the direction of the chain axis, with the formation of crystals being much thicker than the oligomer extended chain length. This results from the alignment of chain ends in planes, as revealed by AFM. A crystal structure is proposed for this oligomer, consistent with both AFM and X-ray diffractometry results. By contrast, the longer oligomer (containing 12 phenyl groups) presents a crystalline morphology consisting of stacks of atomically uncorrelated thin lamellar crystals, whose thicknesses are about the extended chain length computed for the oligomer. This is due to random staggering of the chains along their axes in the crystals, as suggested previously. CP/MAS C-13 solid-state NMR proton spin-lattice relaxation experiments reveal that these morphological differences induce variations in the chain dynamics of the oligomers and confirm that only the longer oligomer can be taken as a good model for a isolated crystal phase. From this study, reference values at room temperature are proposed for the specific mass of a perfect PEEK crystal [1.412(1) g.cm(-3)] and for the proton spin-lattice relaxation time of such a crystal [3.6(2) s].