A new energy dispersive x-ray diffraction method was used to observe isothermal phase transitions of a polyethylene oxide sample contained in a variable temperature cell. A series of scattering measurements were performed to follow the time evolution of the system at each temperature. The collected spectra were suitably processed and the transition coordinate x was obtained as a function of time. A detailed treatment of the errors was accomplished to evaluate the reliability of the method. The experimental points (dx/dt)(max) - T were plotted both for the amorphous-crystalline (below the melting temperature T-m) and for the crystalline-amorphous (over T-m) transitions. They reproduce the usual behavior for T < T-m and show a characteristic unexpected feature for T > T-m which is partially justified.
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