Elsevier

Polymer

Volume 45, Issue 1, January 2004, Pages 207-215
Polymer

Shear induced shish–kebab structure in PP and its blends with LLDPE

https://doi.org/10.1016/j.polymer.2003.10.020Get rights and content

Abstract

In order to better understand the effect of shear stress on the crystal morphology and orientation of polyolefins, dynamic packing injection molding was used to prepare oriented pure polypropylene (PP) and its blends with linear low density polyethylene (LLDPE). The obtained samples were characterized via 2d-SAXS, 2d-WAXD and AFM. Macroscopically, shear induced morphology with surface skin, central core and oriented layer between the skin and the core was observed in the cross-section areas of the samples. For pure PP, a highly oriented structure was seen in the sheared layer but much less oriented structure exists in the core. The orientation in the skin lies in between. The shish–kebab structure, composed of stretched chains (shish) and layered crystalline lamellae (kebabs), was found in the sheared layer. Shish structure exists mainly in the skin layer and oriented spherulits dominates in the core. For PP/LLDPE (50/50) blends, a change of phase morphology from less-phase-separated structure (homogeneous) in the skin, to co-continuous structure in the sheared layer and sea-island structure in the core was observed. PP formed a shish–kebab structure in all the three layers. And on the other hand, a very unique crystal morphology and lamellar orientation of LLDPE were obtained, with the lamellar stack oriented either perpendicularly or 45–50° away from the shear flow direction.

Introduction

Shear-induced orientation and crystallization in semicrystalline materials, which is very common in the industrial processing (e.g. extrusion, injection molding), has attracted more and more attentions due to the significant theoretical values and practical applications [1], [2], [3], [4], [5], [6], [7]. Many studies have been carried out to investigate the molecular orientation in the deformed melt and the resultant morphological changes during crystallization process. When polymers are crystallized from an oriented or strained melt, shish–kebab textures are usually a predominant feature of their morphology [8], [9]. The shish–kebab structure in polyethylene has been intensively studied since it was first reported by Pennings in 1965, where he found a new kind of polymer crystallite in stirred polyethylene solutions [10]. The main effect of shear is to assist the formation of nuclei by the alignment of polymer chains in the supercooled melt along shear direction. This alignment may act as a precursor for the formation of stable primary nuclei. Then, lamellae will be constrained to grow from the primary nuclei laterally due to their high aspect ratio, resulting in formation of a highly oriented, row-nucleated morphology. It has been concluded that the factors that is responsible for the alignment and relaxation of molecular chains can affect the formation of primary nuclei as well as the shish–kebab texture [11], [12]. Observation of shish crystal growth of isotactic polystyrene (iPS) extending into non-strained polymer melt was also reported and explained by a model of an autocatalytically reproduced extensional flow field in the front of the grow tip [13]. Hsiao has recently carried out elegant experiment to study the shear-induced precursor (shish) in iPP melt by in situ Rheo-SAXS and Rheo-WAXD and concluded that the shish–kebab morphology was developed after applying a shear stress at 165C, near the nominal melting point of iPP [14], [15], [16], [17]. The shish-induced layered crystalline lamellae (kebabs) were found to be oriented perpendicularly to the flow direction. Even more, for the first time, Hsiao has qualitatively described the shish–kebab morphology in iPP.

Recently, we have carried out intensive investigation on polyolefin blends achieved by dynamic packing injection molding [18], [19], [20], [21], which relies on the application of shear stress fields to melt/solid interfaces during the packing stage by means of hydraulically actuated pistons. The main feature is that after the melt is injected into the mold the specimen is forced to move repeatedly in a chamber by two pistons that move reversibly with the same frequency as the solidification progressively occurs from the mold wall to the molding core part. Shear-induced morphology with core in the center and oriented zone surrounding the core was observed in the cross-section areas of the samples [20], [21]. It was found that a shish–kebab structure is formed in oriented zone and the amount of shish crystal of HDPE can be promoted by adding second component, such as EVA [22]. In this work, the attention is focused on the shear-induced shish–kebab structure in iPP and its blends with linear low density polyethylene (LLDPE) to gain better understanding of formation of shish–kebab structure of iPP and LLDPE.

Section snippets

Materials

The polypropylene (PP) and LLDPE used in the experiment are commercial products, PP (2401) was purchased from the Yan Shan Petroleum Chemical, China (melt flow index is 2.5 g/10 min); and LLDPE (7042) was Purchased from the Ji Lin Petroleum Chemical, China (melt flow index is 2.0 g/10 min).

Samples preparation

Various binary blends were prepared by varying the LLDPE content in iPP matrix. Melt blending of a pair of polymers was conducted using twin-screw extruder (TSSJ-25 co-rotating twin-screw extruder). After

2d-WAXD result

As mentioned before, for a dynamic sample, shear induced morphology with core in the center and oriented zone surrounding the core was observed in the cross-section areas of the sample. One expects a highly oriented structure in the oriented layer and less oriented structure in the core. Showing as an example, Fig. 1 are the 2d-WAXD patterns of PP at different zone obtained perpendicular to the shear flow direction. For pure PP, in the skin, an intensity distribution of PP is observed, which

Conclusion

In summary, the oriented iPP and its blends with LLDPE were prepared via the dynamic packing injection molding technology, and the oriented samples were characterized through 2d-WAXD, 2d-SAXD and AFM. For the pure iPP, a strongly oriented structure could be obtained in the oriented layer and in the skin, and much less oriented structure in the core. Furthermore, the 2d-SAXD results suggested that the shish structure existed mainly in the skin, and shish–kebab structure mainly in the oriented

Acknowledgements

We would like to express our great thanks to the China National Distinguished Young Investigator Fund (29925413) and National Natural Science Foundation of China (20274028) for Financial Support. This work is also partly supported by Ministry of Education of China for Doctoral Degree (20020610004).

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