Lamellae structure, stacked

Much effort has been devoted to investigating the detailed architectures and the construction of spherulites. Early investigations of the crystallization of polymers through optical microscopy (OM) [7,8] posited that polymer spherulites consisted of radiating fibrous crystals with dense branches to fill space. Later, when electron microscopy (EM) became available, spherulites were shown to be comprised of layer-like crystallites [9,10], which were named lamellae. The lamellae are separated by disordered materials. In the center of the spherulites, the lamellae are stacked almost in parallel [5,6,11-15]. Away from the center, the stacked lamellae splay apart and branch, forming a sheaf-like structure [11,13-15]. It was also found that the thicknesses of lamellae are different [5,6,11,12]. The thicker ones are believed to be dominant lamellae while the thinner ones are subsidiary lamellae. 

Depending on the nature of the components used (layered silicate, organic cation, and polymer matrix) and the preparation method, three types of hybrid PCNs can be obtained [17]. Phase-separated microcomposites (conventional composites) are obtained when the polymer chains are unable to intercalate within the inorganic sheets clay lamellae remain stacked in structures marked as tactoids as in the pristine mineral. Otherwise, when the polymer chains penetrate in between the clay galleries, an intercalative system is obtained. In this case, the nanocomposite shows, at least in principle, a well-ordered multilayer morphology built up with alternating polymeric and clay layers. When clay platelets are randomly dispersed in the polymer matrix and the lamellae are far apart from each other, so that the periodicity of this platelet arrangement is totally lost, an exfoliated structure is achieved. 

Keller has shown that a terrace-like structure is observed reflecting the stacking of the lamellae structure in a melt-grown crystal (Figure 5.6). 

Blends of linear and branched polyethylene normally crystallize in two stages. The components crystallize separately provided that they are of similar molar mass. Linear polyethylene will crystallizes at the highest temperatures, forming regular shaped crystal lamellae. Branched polymers crystallize at lower temperatures in finer, S-shaped lamellae located between the stacks of the dominant lamellae. Although linear and branched polyethylenes are chemically very similar they can phase separate in the molten state. A characteristic of phase separated behaviour is the observation of a dominant lamella structure (Figure 6.14). ... 

Figure 2 Transmission electron mictt aph of the different deformation behavior of the highly oriented layer (top) with stacked lamellae and the typical cross-hatched lamellae structure of PP (bottom). The lamellae appear bright, the amorphous phase dark and the damaged structures black. The melt flow direction inside of this micrograph is horizontal.

Structure (stacks of lamellae). The peaks position remains unchanged with the addition of graphene nanoflakes. 

To some polymer scientists the word morphology means the assembly and relative arrangement of crystals or of second phase particles, while others use supermolecular structure to describe the same thing. Whatever the name, optical and electron microscopy and the complementary techniques of light and x-ray scattering are used to determine such structure. Common arrangements in crystalline polymers solidified frpm the melt are spherulites, row structures, stacks or bundles of lamellae, rods and fibrils. Poorly ordered materials may have... 

The beta crystalline form of isotactic polypropylene differs from the alpha form by having a lower crystalline density and lower melting point. The beta form is metastable to the alpha form and will rearrange to the alpha structure when heated to approximately 100 °C or placed under strain. Beta lamellae form parallel stacks as shown in Fig. 19.4. 

A third effect that has been encountered [185] is a structure that is made from a mixture of different kinds of clusters which are all finite stacks and vary by the number of their members. At the first glance such a material looks as if the zero-sum rule were violated - but for each individual cluster it is not. Figure 8.45 shows a sketch of the probability of different clusters in such a structure. In the study [ 185] the parallel fit using three finite stacks (solos, duos, trios of lamellae) yielded a peculiar coupling of cluster fractions according to the relation... 

It should be noted that there is no difference in structure between isolated stacked lamellae of PE without NA and those of PE with NA in the early stage of the overgrowth process. The difference between them is limited to the difference of the number of isolated stacked lamellae. A significant difference between them should occur when stacked lamellae start colliding with each other in the latter stage of overgrowth. In this work we will not focus on the latter stage. 

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