Layer-line streaking

Figure 1 Example of an x-ray fiber diffraction pattern suitable for analysis using AXIS. The spots are discrete, there is no layer line streaking, and the background scatter may be seen clearly. The sample (potassium hyaluronate) was slightly tilted out of the plane perpendicular to the x-ray beam.

Full circles denote observed reflections, open circles absent reflections that would be observed if all orientations about the fiber axis were present (fiber type orientation). While the 100 reflection is observed in poly(i.-methionine), it is weaker than would be expected if the orientation were fiber type, and it appears that the (100) planes develop preferentially parallel to the surface and (110) planes perpendicular (5). The diagram for poly(y-methyl-ia-glutamate) is constructed from earlier data (12) with a very similar unit cell a = 11.8 A, c = 27.0 A, compared with a = 11.46 A, c = 26.8 A for poly(i.-methionine). The cell size and the positions of the reflections are consistent with the presence of an a-helix containing 18 residues in five turns. The hatched areas denote the position of layer line streaks. The meridional 00,18 reflections at about 1.5 A are not shown. 

Figure 8.8. Electron dilTraction pattern of wollastonite with the electron beam parallel to [001]. Note the streaking parallel to a on odd layer lines. (From Hutchison and McLaren 1976.)...

A further very marked feature of the diflFraction photographs is a streak along the fifth layer line (as opposed to a crystalline arc), similar to that observed in poly(L-alanine) (25) and shown there to be a consequence of random parallel-antiparallel packing of the molecules in the... 

The third type of disorder, related to the variability of the unit cell, is relatively less common. Usually, it is caused by the fact that polymer chains, although packed in good order laterally, are able to slip along their axes relative to one another. This occurrence is manifested by streaks on layer lines other than the equator if there are streaks on the equator also, some disorder in the lateral packing, as well, is indicated. The most severe type of unit-cell disorder, that of the simultaneous presence of more than one type of unit cell in the same specimen, is rather rare. It would be indicated by spots that appear where they do not belong, as, for example, between layer lines, or by layer lines of variable separation. In its milder cases, some reflections on a diagram otherwise proper-looking would not be indexed satisfactorily. 

The observability of the (001) reflection has been a source of major controversy over the years. Even when the intensity at the (001) position is non-zero, it can be argued that it is due to streaking on the layer line, attributable to regions of more nematic character. The question is also whether the data can be accurate enough and the methods reliable enough to be used for a determination of the magnitude and the phase aspects of the bond-length alternation. 

Qualitative SAED consists of observation of the pattern of diffraction spots obtained on the TEM viewing screen from a randomly oriented fiber or particle. Such a pattern indicates that the material is crystalline. Chrysotile fibrils, with their cylindrical form, will usually give the same characteristic pattern, corresponding to a 0.73 mn spacing for (0 0 2) planes, and a layer line repeat of 0.53 nm, as well as streaking of the (110) and (130) reflections. These observations and measurements can be made directly from the screen if the appropriate calibrated screen... 

If all the lamellar stacks are p fectly oriented parallel to the fiber-axis, then the lamellar peak would be streak or a layer line of constant axial widdL But in a typical SAS pattern, the width of the layer line increases with the distance from the meridian, as shown In Figure 6. Whole body rotation of the lamellar stack (misorientation) causes the width Azj of the layer line to increase with the distance fit)m the meridian. The rate of such increase in the width is determined by the average angle that the lamellar stack makes with the fiber-axis. This orientation angle O) of the lamellar stacks is calculated using the expression... 

Metallic cobalt exhibits this phenomenon, and so do layered silicates and layered halides like Cdl2 or Bil3. In X-ray diffraction, stacking faults cause the appearance of diffuse streaks (continuous lines in the diffraction pattern). 

Instead of a microscope slide, you usually use a 12 X 12-in. glass plate and coat it with a thick layer of adsorbant (0.5-2.0 mm). Years ago, I used a small paintbrush to put a line (a streak rather than a spot) across the plate near the bottom. Now you can get special plate streakers that give a finer line and less spreading. You put the plate in a large developing chamber and develop and visualize the plate as usual. 

The basic structural features of MCM-36 inferred from its X-ray powder pattern are confirmed directly by TEM. The image of a particle oriented with sheets perpendicular to the plane of viewing [see reference 6] shows MCM-22 layers (25 A thick, with a streak of pores possibly forming a continuous line in the middle) spaced at -25 A intervals. The presence of props ( pillars ) keeping the layers apart is not observed directly probably due to insufficient contrast and/or ordering. 

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