Influence of Shear Homogeneity on Flow-induced Crystallization of Isotactic Polypropylene

Influence of shear homogeneity on rheology and flow-induced crystallization (FIC) behavior of polymer is investigated by contrasting the sudden startup shear with the slow shear rate ramp-up procedure. The shear experiments are performed via changing only the rate ramp-up time at a fixed shear rate of 17.7 s(-1) and a constant strain of 20. And the corresponding motor acceleration time is set to 0, 0.5, 1.0 and 1.5 s, respectively. Combining wide angle X-ray scattering and polarized microscopy characterization, this set of experiments is designed to study the rate ramp-up time dependence of the rheological properties and FIC at the same shear rate and strain. When the rate ramp-up time is 0.5 s, it is found that a slow acceleration cannot completely eliminate the flow inhomogeneity. The stress overshoot still occurs in the stress-strain curve and the sample orientation is only slightly improved. As the rate ramp-up time is further increased to 1.0 s, the stress overshoot disappears, indicating that the inhomogeneity induced by destruction of melt structures vanishes. In this case, the degree of sample orientation is further improved, and the amount of oriented crystals is increased. However, when the rate ramp-up time is 1.5 s, the effect of flow induced crystallization is weakened, which is indicated by a decrease in orientation degree and the amount of oriented structures. Results obtained sufficiently illustrate that the FIC is dependent on the rate ramp-up time. However, a slower rate ramp-up procedure is not always associated with a better the flow-induced crystallization. Actually, there exists an optimal rate ramp-up time for a fixed shear rate. In addition, the optimal rate ramp-up time is not the same for different shear rates. These results suggest that rate ramp-up time should be considered as a critical parameter for FIC. And the underling mechanism still needs further investigation.