Flow induced crystallization in isotactic polypropylene during and after flow

- We studied the evolution of flow induced crystallization in isotactic polypropylene.
- Crystallization can already be detected during the short flow period of 0.25 s.
- All crystallization aspects, kinetics, orientation and polymorphism, are revealed.
- Parent and daughter evolution is quantified as a function of the flow strength.
- The oriented polypropylene α-phase can induce oriented polypropylene β-phase.

Using a unique slit flow device and in situ synchrotron X-ray methods, the entire evolution of flow induced crystallization in isotactic polypropylene (iPP) was studied from the start-up of flow up to completion of crystallization. Two WAXD detectors, including the ultrafast Pilatus and the two-dimensional Frelon, were combined to achieve a sufficient time resolution for a fairly short period (during and just after flow) and a sufficient spatial resolution for measuring the kinetics of different crystallites oriented in different directions. The complete evolution of these structures is obtained and this reveals several important issues on crystallization. Firstly, the appearance of crystallites can occur already within a short flow duration of maximum 0.25 s. The specific formation time strongly depends on the flow strength. The formation of crystallites just after flow can be distinguished from that occurring during flow, although both happen on the sub-second time scale. Next, we quantified the subsequent appearance of iPP daughter lamellae and determine the time dependent ratio between parent and daughter lamellae as a function of the flow strength. The average orientation of the initial shish is relatively high while the orientation of parent lamellae decreases with their lateral growth. Finally, at the experimental temperature of 145 °C and depending on the shear strength, iPP β-phase can be induced. The quantitative information provided by this data set is well suited for validation and extension of our models for flow induced crystallization of polymers. Such complete data sets, including the fully specified initial and boundary conditions are not available yet for the (nearly) processing conditions as we applied in this study.

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