Cross-hatched lamellar structure

This results in the orientation of PE on the iPP substrate with the (100)p and (010),pp planes in contact and both molecular chains 50°apart from each other, producing a cross-hatched lamellar structure of PE (Fig. 12.9a). On the other hand, matching between the interchain distances of the PE/PTFE system causes a parallel chain alignment of the PE with PTFE, inducing a parallel aligned lamellar structure of PE (Fig. 12.9b). 

In order to avoid misunderstanding, we emphasize here that the parallel aligned lamellar structure is not the sole morphology of parallel chain polymer epitaxy. As an example, when crystallizing the poly(ethylene adipate) (PEA) on uniaxially oriented PE substrate [97], a cross-hatched edge-on lamellar structure of PEA with its lamellae ca. 66.5° apart from the chain direction of the PE substrate film is observed (see Fig. 12.10), indicating the occurrence of epitaxial crystallization of PEA on oriented PE substrate. Known from the experience of PE/iPP epitaxy, where the 50° intersect of both polymer chain directions leads to the formation of a cross-hatched lamellar structure of PE (see Fig. 12.9a), one may conclude that the PEA and PE molecular chains... 

Figure 12.29 Schematic representation of epitaxially oriented cross-hatched lamellar structure, in which the mechanically soft amorphous inter-lamellar regions of one phase are bridged by the crystalline lamellae of the other phase. Lotz and Wittmann [94]. Reproduced with permission of American Chemical Society.

SAXS is a widely used method for the investigation of lamellar structure in the two phase systems. To obtain structural parameters, such as the average crystal separation and crystal thickness, the one dimensional elecbon density correlation function is often used. In syndiotactic poly(propylene) the one dimensional model calculation [24] can be applied since the amorphous phase and crystalline phase form one-dimensional stacks of crystalline lamellae. But in the case of monoclinic iPP, electron microscopy reveals the existence of unique cross-hatched lamellar stracture [25-30], and the applicability of the one dimensional model has been questioned by Albrecht and Strobl [31]. These researchers used SAXS and dilatometry to study structure development in PP, and presented a scheme to check for the feilure of the... 

Morphological explanations for the improved ductility of the fl nucleated materials have focussed on the lamellar texture. That crazes in a iPP are more localised and better defined than in /3 iPP may reflect both the influence of the cross-hatched structure on lamellar slip described in the previous section, and the strong correlation between deformation in ft iPP and the local orientation of the lamellae with respect to the tensile axis. Indeed, the trend towards more localised deformation in a spherulites may simply reflect the relatively homogeneous lamellar textures of these latter [24]. 

The ideas underlying elemental structures models are to establish microstructures experimentally, to compute free energies and chemical potentials from models based on these structures, and to use the chemical potentials to construct phase diagrams. Jonsson and Wennerstrom have used this approach to predict the phase diagrams of water, hydrocarbon, and ionic surfactant mixtures [18]. In their model, they assume the surfactant resides in sheetlike structures with heads on one side and tails on the other side of the sheet. They consider five structures spheres, inverted (reversed) spheres, cylinders, inverted cylinders, and layers (lamellar). These structures are indicated in Fig. 12. Nonpolar regions (tails and oil) are cross-hatched. For these elemental structures, Jonsson and Wennerstrom include in the free energy contributions from the electrical double layer on the water... 

Under defined conditions, the toughness is also driven by the content and spatial distribution of the -nucleating agent. The increase in fracture resistance is more pronounced in PP homopolymers than in random or rubber-modified copolymers. In the case of sequential copolymers, the molecular architecture inhibits a maximization of the amount of the /1-phase in heterophasic systems, the rubber phase mainly controls the fracture behavior. The performance of -nucleated grades has been explained in terms of smaller spherulitic size, lower packing density and favorable lamellar arrangement of the /3-modification (towards the cross-hatched structure of the non-nucleated resin) which induce a higher mobility of both crystalline and amorphous phases. 

There are many modifications of lamellar and superlamellar structures in different polymers, for instance, the cross-hatched lamellae in a-iPP (see Fig. 2.2) or the so-called shish kebab structures in oriented PE and PP see Fig. 2.13. Such structures appear when highly oriented melts crystallize (in the form of a bundle of oriented, longer macromolecules, the shish, and smaller perpendicular lamellae of folded macromolecules, the kebabs ). A peculiar shape is formed by spiral lamellae in TPU (also see Fig. 4.95 in Section 4.5.3). 

This difference is reflected in the morphologies of samples. Figure 8.17 shows TEM micrographs of all samples. A typical thick lamellar structure is observed for iPP-IC. For iPP-PMQ and iPP-MQ, thinner lamellae than iPP-IC and a cross-hatched structure are observed. On the other hand, iPP-POW has no lamellar structure, which means the crystalline component is... 

Structure shown in the AFM image indicates that the PEA lamellae are ca. 66.5° apart from the chain direction of the PE substrate crystals. It causes a 23.5° inclination of the PEA chains with respect to its lamellar normal direction. This is reasonable when the lamellae with 00/ fold surfaces are considered. As illustrated in the right part of Figure 12.12, a chain folding in the 001 surface leads exactly to the formation of PEA lamellae 66.5° apart from the chain direction of the PE substrate crystals. This cross-hatched structure should take the response from a remarkable property improvement of the PE/PEA system reported in Ref. [100], as also found for the PE/iPP system [101,102] which is discussed later. 

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