Abstract
Shear-induced precursors obtained by fiber pulling with high shear rate in undercooled melts of isotactic polypropylene (iPP) at a wide temperature range were investigated by highly time and space resolved synchrotron radiation scanning X-ray microdiffraction (SR-μSXRD). X-ray diffraction patterns in spot area of 4.9 × 5.3 μm2 were obtained by scanning proximities around the fiber with step size of 7 μm immediately after shear and during the sequent crystallization process. The crystallinity derived from SR-μSXRD indicates that the structure of precursors was determined by shear temperature, providing a high shear rate was imposed. At shear temperatures around or below the melting temperature, the presence of crystal diffraction immediately after shear indicates the formation of crystalline precursors with reduced density by increasing shear temperature. At shear temperatures in between melting and equilibrium melting temperature, the formation of noncrystalline precursors is supported by three evidence: (i) formation of shear-induced ordered structure indicated by preference of crystallization on fiber surface; (ii) absence of crystal signal immediately after shear; (iii) induction time necessary for transforming noncrystalline precursor into crystal. The experimental results suggest that the structure of noncrystalline precursors probably is aggregates of partially ordered chain segments which orient along the shear direction.
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