Characterization of i‐PP shear‐induced crystallization layers developed in a slit die

Abstract

AbstractIn this work, the shear‐induced crystalline layers of isotactic polypropylene (i‐PP), developed in a slit die, were characterized by different techniques. Rheological studies made in a strain‐controlled rheometer, at different crystallization temperatures, Tc, allowed us to observe that the induction time for the beginning of the shear‐induced crystallization, ti, decreased as the shear rate increased, whereas at a given shear rate, the higher the Tc, the higher the ti. The thickness of the shear‐induced crystallized layer, after extrusion through the slit die, was found to decrease with the increase of the die temperature, Td, at a given flow rate, Q, and to increase with the increase in Q, at a given Td. Regarding the die length, it was found that only at Td = 169°C, the thickness of this layer increased with the length. By polarized light optical microscopy (PLOM), five different crystalline layers were observed along the thickness of the sample. By scanning and transmission electron microscopy (SEM and TEM), respectively, and wide‐angle X‐rays (WAXS), it was found that layer 1, the nearest to the wall, was formed mainly by very small and oriented α‐crystallites, while layer 2 was mainly composed of β‐crystallites; also it was found that the amount of the β‐phase decreased as the shear rate decreased. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3528–3541, 2004