Polyethylene electron diffraction

Jones JB, Barenberg, S, Geil PH (1977) Amorphous Linear Polyethylene Electron Diffraction, Morphology, and Thermal Analysis. J Macromol Sci, Phys B15 329-335. Chen W, Wunderlich B (1999) Nanophase Separation of Small And Large Molecules. Macromol Chem Phys 200 283-311. 

Fig. 4 Electron diffraction micrograph of polyethylene shish-kebab structure. (From Ref.Pf)...

The use of x-ray and electron diffraction has contributed considerably to our knowledge of the fine-structure of many polymers. The interpretation of diffraction diagrams of various polymers allows the drawing of certain conclusions as to the geometrical structure of the chain and the probable positions of substituting atoms in respect to the main carbon-carbon chain. Thus it has been established that polyethylene chains inside crystalline... 

Lamellae are thin, flat platelets on the order of 100-200 A (0.01-0.02 pm) thick and several microns in lateral dimensions, while polymer molecules are generally on the order of 1,000-10,000 A long. Since the polymer chain axis is perpendicular to the plane of the lamellae, as revealed by electron diffraction, the polymer molecules must therefore be folded back and forth within the crystal. This arrangement has been shown to be sterically possible. In polyethylene, for example, the molecules can fold in such a way that only about five chain carbon atoms are required for the fold, that is, for the chain to reverse its direction. Each molecule folds up and down in a regular fashion to establish a fold plane. As illustrated in Figure 1.14a, a single fold plane may contain many polymer chains. The height of the fold plane is known as the fold period. It corresponds to the thickness of the lamellae. 

LEED low-energy electron diffraction LPET hnear polyethylene terephthalate... 

When they are grown at sufficient dilution, the crystallites approximate to lamellae with a uniform thickness of about 12 nm, the precise value depending on the temperature of growth. Electron diffraction shows that the chain axes are approximately perpendicular to the planes of the lamellae. The crystals are not exactly flat, but have a hollow-pyramidal structure, with the chain axes parallel to the pyramid axis. This pyramidal structure is seen clearly in fig. 5.5, which shows a single crystal of polyethylene floating in solution. This should be compared with fig. 5.3(b), which shows similar crystals flattened on an electron-microscope grid. The dark lines on the crystals in fig. 5.3(b) show where the pyramid has broken when the crystal flattened. 

We analyzed the ICB-deposited polyethylene films using JEM-200CX transmission electron microscope. The results show that these thin films were uniform and a mixture of crystalline and noncrystalline phases. The crystalline phase area appears rhombic. Ihe size of the crystalline lamellae is 2-20 mm. The electron diffraction pattern of the crystalline phase appears as an ordered array of spots (Fig. 5a shows a TEM micrograph and Fig. 5b an electron diffraction pattern). The chemical elements of these samples are analyzed with LEED-AES (Parkin-Elmer... 

FIGURE 5 Polyethylene thin film using the ICB-TOFMS deposition system (a) TEM micrograph (b) electron diffraction pattern. 

FIGURE 7 TEM micrograph and electron diffraction pattern of polyethylene on crystalline mica. Moire fringes emerged in the TEM image, indicating interference between the crystalline polyethylene and the crystalline mica. 

Polyethylene was deposited on a single crystalline mica substrate. Larger area moire fringes can appear in the TEM image, caused by interference from the two crystalline thin films, i.e., the crystalline PE thin film and the crystalline mica. Figure 7 shows a TEM micrograph and an electron diffraction pattern. The crystal structure of the mica is monoclinic. The relation of the distances between crystal planes is as follows ... 

When depositing metal clusters in a polyethylene thin film, we obtain polycrystalline thin films with suspended metallic clusters. For the room temperature substrate, the Au cluster size of 2-5 nm is smaller and the clusters are distributed randomly in the polyethylene thin films, shown in Fig. 13a. For a substrate temperature of 90°C, the Au clusters pile up together but still maintain a small spherical form. TTie electron diffraction patterns of the samples with the substrate at room temperature and 90°C are similar (see Fig. 13c), which shows that the Au clusters are crystalline and the polyethylene thin film is polycrystalline. 

Single crystals of many polymers may be grown from dilute solution [8]. Linear polyethylene, for example, forms single-crystal lamellae with lateral dimensions of the order of 10-20 /Electron diffraction shows that the molecular chains are oriented approximately normal to the lamellar... 

Fig. 3.5 Electron diffraction patterns from (A) amorphous carbon, (B) oriented amorphous polystyrene (tensile direction indicated by arrows) and (C) a polycrystalline PE film. The sharpness of the rings in (C) indicates crystalline order. Highly oriented polyethylene is shown in the diffraction pattern (D) (tensile direction indicated by arrows). The off-axis spots prove the presence of three dimensional order.

Fig. 3.15 Sequence of electron diffraction patterns from a polyethylene crystal at 100 kV, showing how the sharp spots fade and spread so the final result is a ring pattern. The crystals have become completely amorphous because of radiation damage. The doses are (A) 35-27 C m (B) 53-55.5 C m" (C) 70.5-74 C m" ...

Fig. 5.20 Electron diffraction (top left) and defocus phase contrast micrographs of melt drawn polyethylene films. Optical diffraction patterns from the micrographs are at top right. As drawn film (A) is well oriented. Annealing (B) increases orientation and crystal size. Bright regions are interlamellar the crystalline regions are gray or dark if they diffract. (From Yang and Thomas [95] reproduced with permission.)...

The interpretation of diffraction data on amorphous polymers is currently a subject of debate. Ovchinnikov et al. (31,34) interpreted their electron diffraction data to show considerable order in the bulk amorphous state, even for polyethylene. Miller and co-workers (3536) found that spacings increase with the size of the side groups, supporting the idea of local order in amorphous polymers. Fischer et al. (32), on the other hand, found that little or no order fits their data best. Schubach et al. (37) take an intermediate position, finding that they were able to characterize first- and second-neighbor spacings for polystyrene and polycarbonate, but no further. 

The single-crystal electron diffraction pattern shown in Figure 6.5 was obtained by viewing the crystal along the c-axis. Also shown is the single-crystal structure of polyethylene, which is typically diamond-shaped (see below). The unit cell is viewed from the c-axis direction, perpendicular to the diamonds. 

Figure 6.5 A study of polyethylene single-crystal structure, (a) A single crystal of polyethylene, precipitated from xylene, as seen by electron microscopy, (b) Electron diffraction of the same crystal, with identical orientation, (c) Perspective view of the unit cell of polyethylene, after Bunn, (d) View along chain axis. This latter corresponds to the crystal and diffraction orientation in (a) and (b) (22). Courtesy of A. Keller and Sally Argon.

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