Time-Resolved Simultaneous SAXS/WAXS Studies of PEEK during Isothermal Crystallization, Melting, and Subsequent Cooling

Time-resolved simultaneous small- and wide-angle x-ray scattering (SAXS and WAXS, respectively) of poly(aryl ether ether ketone) (PEEK) during isothermal crystallization, melting, and subsequent cooling were conducted using synchrotron radiation. Unit cell parameters were determined using WAXS measurements, and the dimensions of lamellar-level moieties were extracted from SAXS profiles. It was found that the changes in unit cell parameters were reversible on heating and cooling, and there were remarkable differences in the thermal expansion coefficients in different lattice directions. The long period L, the lamellar thickness l_c, and the thickness of amorphous layers l_a were calculated from the correlation function, and the changes of these dimensions during the processes mentioned above were traced. Sharp decreases in L and l_c in the initial stage of isothermal crystallization and cooling from melt were observed, and a corresponding continuous distributed model is proposed. When cooling from the melt, the invariant Q exhibited a maximum. The maximum is attributed to a change in the difference of the densities of crystalline and amorphous phases (ρ_c-ρ_a) with decreasing temperature.