New Insight of Isothermal Melt Crystallization in Poly(aryl ether ether ketone) via Time-Resolved Simultaneous Small-Angle X-ray Scattering/Wide-Angle X-ray Diffraction Measurements

Isothermal melt crystallization (stage I, at 290 °C) and subsequent annealing (stage II, at about 287 °C) of poly(aryl ether ether ketone) was studied by time-resolved simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) methods using synchrotron radiation. At stage I, both crystalline WAXD and SAXS peaks occurred almost simultaneously within the resolution time of our setup (30 s). WAXD profiles revealed a typical increase in the apparent crystallinity index and a slight rise in the crystal density with time, whereas SAXS profiles revealed decreases in both long period L and lamellar thickness l_C (estimated from the correlation function assuming a two-phase model) and a slight increase in the interlamellar noncrystalline layer thickness. At stage II, the annealing at a lower temperature caused anisotropic changes in the unit cell dimensions: b (along the spherulitic growth direction) was found to remain about constant, while a (perpendicular to the growth direction) decreases noticeably. Other induced variations include a subsequent increase in the crystallinity index and decreases in L, l_C, and invariant Q. These results are consistent with a proposed model comprising primary crystallization from the unrestrained melt which produces thicker lamellar stacks, and subsequent infilling secondary crystallization from the restrained melt which produces thinner lamellar stacks with smaller L and l_C.