Structure development during shear flow induced crystallization of i-PP: In situ wide-angle X-ray diffraction study

In situ synchrotron wide-angle X-ray diffraction (WAY-D) was used to monitor crystallization of isotactic polypropylene (i-PP) in the subcooled melt at 140 degreesC after step shear. The melt was subjected to a shear strain of 1430% at three different shear rates (10, 57, and 102 s(-1)) using a parallel-plate shear apparatus. WAXD results were used to determine the type (alpha- and beta -crystals), orientation, and corresponding mass fractions of i-PP crystals. It was found that formation of oriented alpha -crystals occurred immediately after application of the shear field. Subsequently, growth of primarily unoriented beta -crystals was observed. WAXD patterns clearly showed that beta -crystals grew only after the formation of oriented alpha -crystals in the sheared i-PP melt. The contribution of beta -crystals to the total crystalline phase was as high as 65-70% at high shear rates (57 and 102 s(-1)) and low (20%) at low shear rates (10 s(-1)), which was attributed to the different amount of surface area of oriented alpha -crystal cylindrites generated at different shear rates. The growth of beta -crystals which is related to the surface area of the oriented alpha -form crystalline assembly has been proposed earlier. Also, the unoriented nature and fast growth of the beta -crystals determined from WAXD experiments provide an explanation for the 2 orders of magnitude increase in the kinetics of crystallization of the unoriented structures, which was previously observed (but not explained) in our crystallization study by small-angle X-ray scattering (SAXS).