The nucleation rate was measured by directly counting the number of nuclei, which were developed while an isotactic polypropylene melt was flowing under shear in a thin film. The nucleation rate was enhanced with an increased rate of shear, e.g., by a factor of 10 larger at the rate of shear of 14 s 21 compared with the quiescent state, at 1348C. The ratio of the shear-enhanced nucleation rate to the nucleation rate in the quiescent state was larger at a higher temperature of crystallization, i.e., about 10 times at 1348C to 590 times at 1408C. The increase of the nucleation rate under shear flow was explained by a reduction of the lateral and end (fold) surface free energies; the product s s 2 s e decreased to 3.2 Â 10 27 for the sheared melt, from 6.0 Â 10 27 (J m 22 ) 3 for the isotropic state. The free energy reduction was caused by transition of the nucleus formation mode from three-dimensional folded chain nuclei to two-dimensional bundle nuclei, in which chains lie down on the glass substrate, aligning parallel to the flow direction.
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