Crystal lamella stack dominant

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). ... 

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. 

Figure 22 Schematic illustration of the lamellar stack (top) and Its possible evolution during the crystallization process (bottom). The structure in the left bottom panel shows formation of separate stacks of thicker and thinner crystals whereas the structure in the right bottom panel corresponds to the so-called insertion growth mode where the thinner crystal grow In-between the dominant thicker lamellae.

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