Daughter lamellae

The y-crystalline form of i-PP has been studied through best fitting of X-ray diffraction powder profile [9, 10]. A novel crystal architecture was found where layers of parallel helixes two chain wide (bilayers) present a chain axis orientation with a tilt of 80° with respect to the one of the adjacent bilayers. This packing can explain several experimental results and is consistent with the model proposed for the interface between parent and daughter lamellae in branching a-form of i-PP [11, 12]. 

Fig. 8. Pattern during recrystallisation of a stretched iPP sample at temperatures below 140 °C Isotropic pattern after crystallisation from the melt, a slight preferential orientation may be observed (creation of crystallites with a wide range of orientation) crystallisation of the common stretched lamellar fragments generation of a meridional double-reflex (daughter lamellae) creation of a new series of (h k l)-crystallites (from left to right). The stretching direction is vertical.

JS A polymer spherulite growing into the melt. In polyethylene the ciystalline fibrils are thin lamellae. The molecules crystallize most rapidly on to the (010) plane the b axis is therefore the direction of most rapid growth and is p lel to the spherulite radius R. The a and c axes are randomly distributed around R. If the solidification is isothermal, the lamellae are all of the same thickness. In order to fill space the raoiating lamellae must branch and give birth to daughter lamellae as they grow out into the melt. Amorphous polymer is left trapped between the crystals. 

Figure 3.18 shows a schematic representation of double-oriented-kebab strac-ture. Stacks of parent lamellae is responsible for meridional scattering the SAXS patterns, whereas stacks of daughter lamellae exhibited equatorial scattering. The equatorial scattering is attributed to the overlap of scattering from daughter lamellae and shish. 

This can then be partially melted (following curve 2) and the specimen quickly removed and quenched, so that the material has recrystallized in a much lower melting state (curve 3). The morphological effect can be seen in Fig. 2.5. The lower melting peak is seen to relate to daughter lamellae formed between the dominant ones, and especially to highly cross-hatched regions which melt out entirely. 

First set of values refers to crystallization under quiescent conditions. CNTs nucleated iPP crystals which further nucleated secondary crystallites (mother-daughter lamellae). Second set of values show Avrami exponents for crystalhzation during a step shear of 20 s for 5 s. 

Figure 3.17 compares the morphology of a commercial iPP in the a-form with the so-called cross-hatched arrangement consisting of primary lamellae radiating from a spherulite center and smaller secondary (or daughter ) lamellae perpendicular to the primary ones (at the bottom, compare also with Figs. 3.9 and 3.12). Several preparation techniques, necessary for the different microscopic techniques, are mentioned in Section 3.3. 

Recrystallization in the stretched state results in the formation of a crystalline network that prevents contraction of the oriented crystalline structure during strain recovery, and thereby freezes the orientation in the elastic film. The data suggests that recrystallization is possibly in the form of epitaxial crystallization of a -PP (daughter) lamellae on top of oriented a-PP (mother) fibrillar crystals. The origin and nature of the orientation of the y-PP needs explanation, and is currently under investigation. 

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