Spherulitic shear zone

Figure 8. Skin/core morphology of injection-molded PP—polarizing optics, (a, top) Skin and shear zone (b, bottom) spherulitic core.

Fig. 29a and b. Structure of the shear zone at various temperatures in PP a T = —196 °C, two sets of discrete shear band A and B, b T = —40 °C, diffuse shear zone with quasi-homogeneous deformation of spherulites... 

Finally, when the testing temperature is near the glass transition temperature, a broad shear zone instead of discrete coarse shear bands develops at an angle of 45° to the direction of the external stress (Fig. 29 b). Inside this zone the spherulites are homogeneously deformed in direction to the main shear stress. It is expected that... 

Typically, in a semicrystalline polymer there are three zones within the molded part an oriented, nonspherulitic skin a subsurface region with high shear orientation, or a transcrystalline region and a randomly oriented spherulitic core. The thickness of the skin and shear zone is known to be an inverse function of the melt and mold temperature with decreased... 

An example of the multilayered structures common in polyacetals is shown in the polarized light micrographs (Fig. 5.35). They depict a uniformly nucleated crystalline structure formed due to mold filling and variations in the rate of cooling of the melt. The skin surface in the microtomed section (top in Fig. 5.35A) is birefringent, non-spherulitic and highly oriented. The molecular chains are oriented parallel to the injection direction. The central portion of the bar consists of a core (bottom Fig. 5.35A) with randomly oriented spherulites (Fig. 5.35C). It has no preferred molecular or lamellae orientation. There are usually one or more layers between the skin and core that are transitional shear zones with intermediate... 

Figure 7 shows that the micro-injection molded polyethylene parts exhibit typical skin-core morphology similar to that observed for conventional injection molding parts. While the interface between the skin layer and the transitional shear zone is apparent, the interface between the transitional shear zone and the spherulitic core is hard to locate. The skin layer probably has shish-kebab structural characteristics. The Kebabs , which are crystalline lamellae, fill the crystalhzed space. Fibrous crystals, or the Shishs , are ahgned parallel to the injection direction. They penetrate those lamellae. The fibrillar structure follows the direction of the flow, as shown in Figure 7. The transitional shear zone may be thought of as crystalline ribbons that branch and fill crystallized space with some loss of orientation. Crystallization occurring at the sites of both the skin layer and the transitional shear zone is significantly influenced by shear or elongational stress history. On the other hand, the influence of shear on the crystalhzation occurring in the spherulitic core is negligible. The crystalline structure...

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