Polyamide crystal lamella

The structure of crystalline polymers may be significantly modified by the introduction of fillers. All aspects of the structure change on filling, crystallite and spherulite size, as well as crystallinity, are altered as an effect of nucleation [9]. A typical example is the extremely strong nucleation effect of talc in polypropylene [10,11], which is demonstrated also in Fig. 2. Nucleating effect is characterized by the peak temperature of crystallization, which increases significantly on the addition of the filler. Elastomer modified PP blends are shown as a comparison crystallization temperature decreases in this case. Talc also nucleates polyamides. Increasing crystallization temperature leads to an increase in lamella thickness and crystallinity, while the size of the spherulites decreases on... 

Wu s explanations for this fundamental observation were couched in field theory, such as overlapping of concentrated stress fields around particles or a transition in local stress state from plane strain to plane stress, and could not furnish a specific material dimension, which, as we present below, depends on the type of the polymer and its crystalline state. The required fully consistent explanation for the discovery was provided by the studies of Muratoglu et al. (1995a), who proposed that the material-specific level of A is a consequence of a preferred form of crystallization of polyamide lamellae near particle interfaces, extending to a certain distance I away from the interface. This results in an anisotropic plastic resistance in this layer, which upon percolation through the matrix and in an... 

Figure 6.11 Single crystal of polyamide 6 precipitated from a glycerol solution. The lamellae are about 60 A thick. Black marks Indicate 1 um (34).

A typical shish-kebab crystalline structure has been foimd by Maiti and Okamoto (2003) and Kim et al. (2001) in polyamide/organoclay nanocomposite and by Choi and Kim (2004) in PP/EPR/talc nanocomposite where a preferential orientation of polymer lamellae perpendicular to the surface of organoclay layers was inspected by TEM measurements. The unique observation of lamellar orientation (Ml the clay layers was ascribed to nucleation and epitaxial crystallization at the interface between layered silicate and polymer matrix especially the surfaces of clay platelets acted as heterogeneous nucleation sites. Orientation of iPP crystals was also enhanced in rPP/PP-MA/o-MMT injection-moulded parts, especially manufactured by dynamic packing injection moulding (Wang et al. 2005). MMT... 

The staining of polypropylene was not as straightforward as that of the polyamides and polyesters. Hock [270] developed a method for staining melt crystallized PP and showed it to be eomposed of spherulites containing lamellae. The melt crystallized polymer was boiled for 4h in 70% HNO3 at 120°C, and then chips... 

Polymer chains in semicrystalline polymers such as polyolefines, aliphatic polyamides (nylons), and aliphatic polyesters crystallize in the form of lamellae of folded chains that are organized to form lamellar stacks (Fig. 2.14a). The spaces between the lamellae and between the stacks are occupied by... 

In the 1980s Keith and Padden proposed that chain tilt itself (not screw dislocations) results in axial twisting of growing lamellae that is manifested as banded spherulites. The reviews of Keith and Padden [44] and Lotz and Cheng [23] may be consulted for original references. Chain tilt is almost always found for chains crystallized in an extended (not helical) conformation. For orthorhombic polyethylene and a-poly(vinylidene fluoride), the chains are inclined to the basal lamellar surfaces to provide more volume for chain folding (Section 3.2.1). On the other hand, those aliphatic polyamides (nylons) and polyesters that crystallize with monoclinic... 

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