A novel flow cell for the investigation of elongational flow induced nucleation

Crystallization of semi-crystalline polymers can be strongly enhanced by chain orientation during flow. Elongational flow may cause extraordinarily large molecular orientations, specially for the high end tail molecular weight part, which then can act as nuclei during crystallization, since relaxation time of these molecules is very large. In this study, a novel cross slot flow cell for polymer melts is described in which a stagnation flow can be created and for which the thermal history is controlled. Such flow is characterized by the presence of a free stagnation point, around which the extensional strain is high. With this flow cell, the influence of elongational flow on crystallization kinetics can be studied, using in situ Wide Angle X-ray Diffraction (WAXD) and point-wise birefringence measurements. First results are presented for an isotactic polypropylenes (iPP). The melt was annealed at a temperature of 220oC for 90 minutes to erase crystal and flow memory. At the same temperature the flow is applied. Next, the cell is cooled to a temperature below the (quiescent) melting temperature (about 170oC) to investigate isothermal crystallization. A strong flow induced enhancement of turbidity in the birefringence experiments was found. This effect was larger at higher temperatures, but more difficult to detect at lower temperatures (<130°C), because the onset time of crystallization decreases exponentially with decreasing temperature. WAXD measurements showed in most cases an oriented phase, despite the long cooling times. The orientation of the structure could be explained by the structure proposed by Fujiyama, with the c-axis oriented in inflow and outflow direction. In one experiment also the phase was detected. The reproducibility of the thermal history in the flow cell was excellent. Unfortunately, the stress controlled deformation history was not reproducible. Therefore, constant velocity experiments, which are much better reproducable, should be performed in future.