Lamellae crystalline/amorphous

Many polymers solidify into a semi-crystalline morphology. Their crystallization process, driven by thermodynamic forces, is hindered due to entanglements of the macromolecules, and the crystallization kinetics is restricted by the polymer s molecular diffusion. Therefore, crystalline lamellae and amorphous regions coexist in semi-crystalline polymers. The formation of crystals during the crystallization process results in a decrease of molecular mobility, since the crystalline regions act as crosslinks which connect the molecules into a sample spanning network. 

Figure 1 Semi-crystalline lamellae. Crystalline, interfacial and liquid-like (amorphous) regions are indicated.

The semi-crystalline HDPE being modeled is initially of a sphernlitic morphology described in Chapter 2. It is made up of a 3D packing of crystalline lamellae and their attached amorphous layers as idealized in Fig. 9.25(a). The basic elements of the spherulite are two-phase composite inclusions that consist of integrally coupled crystalline lamellae and their associated amorphous layers between lamellae. Owing to their large aspect ratio, the composite inclusions are modeled as infinitely extended sandwiches with a planar crystalline/amorphous interface as shown in Fig. 9.25(b). Each composite inclusion I is characterized by its interface normal rt and the relative fractional thicknesses and / = 1 / of... 

Nojima et al. [145] determined the long period in the crystalline regions from scattering data and separated it into the thicknesses of lamellae and amorphous layers in the crystalline regions. The lamellar thickness was independent of blend composition but the thickness of the amorphous layer increased at low ol-igo-PS contents and then became constant (Fig. 63). Nojima et al. [145] discussed the composition of the amorphous layers under various conditions. 

Linear PE (HDPE) without such defect layers and with large crystalline blocks forms longer crystalline lamellae (white), covered by the crystalline-amorphous interphase (dark) see Fig. 2.8. 

The optical observations cannot resolve the crystalline-amorphous structure. Observation of single crystallites requires methods which provide an analysis in the 10-nm range. Electron microscopy is particularly suited for this purpose. Figure 4.4 shows as an example the surface of a partially crystalline polyethylene, as it becomes reproduced in the electron microscope when using a carbon film replica technique. The picture of the surface resembles a landscape with many terrasses. These obviously result from cuts through stacks of laterally extended, slighty curved lamellae whiqh have thicknesses in the order of 10 nm. 

Figure 2.24 Hierarchy of structures observed when a polymer crystallizes. Molecules adopt an extended conformation, or fold. The crystallized molecules are organized into lamellae. Crystalline layers and amorphous layers form a lamellar stack. Radial growth of lamellae leads to spherulitic supermolecular structures. The spokes of the spherulite result from defects in lamellar organization, such as giant screw dislocations...

Zhu L, Calhoun BH, Ge Q, Quirk RP, Cheng SZD, Thomas EL, Hsiao BS, Yeh F, Liu L, Lotz B. Initial-stage growth controlled crystal orientations in nanoconfined lamellae of a self-assembled crystalline-amorphous diblock copolymer. Macromolecules 2001 34 1244-1251. 

---