Characterization of i-PP Shear-Induced Crystallization Layers Developed in a Slit Die Marcelo Farah, Rosario E. S. Bretas Department of Materials Engineering, Universidade Federal de Sa ˜o Carlos, 13565-905 Sa ˜o Carlos, SP, Brazil Received 24 March 2003; accepted 24 June 2003 ABSTRACT: In 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, T c , allowed us to observe that the induction time for the beginning of the shear-induced crystallization, t i , decreased as the shear rate increased, whereas at a given shear rate, the higher the T c , the higher the t i . 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, T d , at a given flow rate, Q, and to increase with the increase in Q, at a given T d. Regarding the die length, it was found that only at T d = 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 thick- ness 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 -crys- tallites, while layer 2 was mainly composed of -crystallites; also it was found that the amount of the -phase decreased as the shear rate decreased. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3528 –3541, 2004 Key words: crystallization; polypropylene; morphology; rheology; extrusion INTRODUCTION The morphology along the thickness of semicrystalline polymers after extrusion or injection molding is usu- ally formed by crystalline layers of different sizes and forms. Near the wall, very small and highly extended crystallites or cylindrites are observed, while near the center, large and unoriented spherulites are seen. In the intermediate region, columnar- or transcrystalline- type crystals can be observed. This morphology is called skin-core type. The skin is highly oriented due to the large shear rates that are developed near the wall, while the core is spherulitic because of the zero or minimum shear rates. This skin is also known as the shear-induced crystalline layer, while the core is known as the quiescent crystalline layer. The effect of the shear rate gradient on the morphology can also be better visualized in the injection molding of immisci- ble polymer blends, 1 where the disperse phase de- forms according to this gradient. Although it is known that elongational flows enhance crystallization better than shear flows, 2,3 we will limit in this work to the study of only shear-induced crystallization. Eder et al. 4 were one of the first to formulate a kinetic theory for the development and understanding of this shear-induced crystalline layer. Their main as- sumptions were that the surfaces or precursors at which nucleation could start were created by the flow and they did not exist prior to the onset of the flow. These precursors also would disappear by relaxation, after the stop of the flow. However, if the sheared melt were quickly cooled, shear-induced crystallization would occur. Their model allowed them to estimate the size of the flow-induced layers. Recently, Doufas et al. 5 presented a continuum model for this flow-in- duced crystallization, which combined thermodynam- ics with the Avrami equation. Their model had two constitutive equations: one for the amorphous phase and the other for the crystalline one, which was cou- pled with the crystallization kinetics. The melt was modeled as a Giesekus material and the crystalline phase was modeled as a collection of multibead rigid rods that grew and were oriented in the flow field. Their basic assumption was that the melt was homo- geneous; that is, all polymer chains were considered to crystallize in the same manner and, therefore, to have the same degree of crystallinity at any time. The model did not account, however, for any morphological de- tails (spherulitic, folded/lamellar, or fibrillar micro- structures). One of the polymers that more easily forms this type of multilayer morphology after extrusion or in- jection molding is isotactic polypropylene (i-PP). Thus, it constitutes a very useful polymer to make studies of shear-induced crystallization. Correspondence to: R. E. S. Bretas (bretas@power.ufscar.br). Journal of Applied Polymer Science, Vol. 91, 3528 –3541 (2004) © 2004 Wiley Periodicals, Inc.