Shear-induced crystallization of PB-1 up to processing-relevant shear rates

Abstract

Two experimental methods to study shear-induced crystallization of poly(butene-1) (PB-1) in the high shear rate region are presented: one using a concentric cylinder rheometer and the other a capillary rheometer equipped with a cylindrical die. The crystallization onset time (t on) is used as the parameter to monitor crystallization progress through the output signal from each device. By combining the new data with the results from a previous paper (2005) in which a plate–plate rheometer was used, onset time data covering a shear rate range from 10-4 to 500 s-1 at temperatures 99–107°C are obtained. In this range, a decrease in onset time spanning five decades is observed. The onset times obtained from the capillary rheometer are larger compared to those from the other two methods, which can be explained from the different type of flow. The data also confirm the procedure to construct a temperature-invariant curve, which can be extended to high shear rates for three PB-1 samples having different molecular weight distributions. The slope of the fitted curve for all three cases is −1, which suggests that a critical value is required for shear-induced crystallization. The morphology of the formed crystals (spherulitic or rod-like) depends on the molecular weight, but this does not affect the validity of the T-invariant curve. Above the melting point, it is shown that the amount of long chains influences the temperature limit where shear-induced crystallization can still take place.