Time-resolved synchrotron small angle X-ray scattering (SAXS) was used to investigate the early stages of crystallization in melt crystallized polyethylene. Classic Gibbs nucleation or density fluctuation theory can be used to describe the primary nucleation mechanism. At 110 °C, no signal of crystallization can be detected by SAXS for 30 min. When it is lower than 110 °C, the low q scattering intensity (0.008 < q < 0.03 Å−1) begins to upturn, and the primary nucleation process starts. The measured fractal dimension of the critical nuclei is in the vicinity of 3 which is close to the prediction of classic Gibbs nucleation theory. The growth rate of density ?uctuations R(q) at different scattering vector q for different temperatures was obtained by analyzing the increase of scattering intensities. The results show that the growth rate of density ?uctuation gets much bigger with the decrease of the isothermal crystallization temperature, but there is no signal of spinodal decomposition mechanism, in which there should be a linear relationship between R(q)/q2 and q2.

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