METHOD FOR MEASURING OR INVESTIGATION OF FIBER STRUCTURE

Presentation on

METHODS

There are many methods for measuring of fiber structure. Such as: The absorption of infrared radiation

Raman scattering of light

Optical and X-ray diffraction

Optical microscopy

Electron microscopy and electron diffraction

Nuclear magnetic resonance

Optical properties

Thermal analysis

Density

General physical properties

Here I only describe about optical and X-ray diffraction & electron microscopy and electron diffraction method.

OPTICAL AND X-RAY DIFFRACTION

When a beam of light is passed through a photographic slide, the light is scattered in many directions.

By using a lens in the right place, we can recombine this scattered information about the picture into an image on a screen.

But the information is there before it is recombined, and diffraction is the science of understanding and using this information in all sorts of ways.

Diffraction is the study of the particular patterns that may be found when waves pass through or round objects of particular shape.

OPTICAL AND X-RAY DIFFRACTION

For example, there is a characteristic diffraction pattern from a single slit. The difference between the image that must be focused at a particular place and the angular diffraction pattern that can be intercepted anywhere is shown in Fig. 1.6.

OPTICAL AND X-RAY DIFFRACTION

The use of polarized light in either of the above two techniques changes the pattern and thus, in principle, increases the available information about structure if it can be interpreted.

The diffraction patterns from objects with some regular repetitive structure are more simple and immediately useful. Thus a diffraction grating of regularly spaced lines, illuminated normally by parallel light, will give a set of fringes, with the maxima of the bright bands at angles φ defined by the relation: nλ = a sinφ

Where n is an integer, λ the wavelength of light and a the spacing of the lines in the grating.

OPTICAL AND X-RAY DIFFRACTION

X-ray diffraction is a most important tool for the study of fiber structure, Firstly, because it gives information at the most important level

of fine structure; &

Secondly, because focusing of X-rays is not possible, so that diffraction methods have to be used.

Three advances have made the technique more powerful than was available to the pioneers of X-ray diffraction: Arrays of detectors give enhanced quantitative information on

the diffraction pattern;

Computer software then enables the data to be analyzed and interpreted; &

The increased power of synchrotron radiation reduces exposure times and allows small spot sizes to be used.

OPTICAL AND X-RAY DIFFRACTION

A crystal can be regarded as made up of layers of atoms, themselves regular in their two-dimensional plan, stacked regularly on top of one another. Although analysis of the diffraction from such a three-dimensional lattice is more complicated than for a simple grating, it does result in a very similar equation; for it can be shown that, if a beam of X-rays is directed at a crystal, it is strongly reflected whenever it strikes layers of atoms at an angle θ, shown in Fig. 1.8, such that: nλ = 2d sinθ

OPTICAL AND X-RAY DIFFRACTION

The condition that a particular reflection should occur is that the layer of atoms should make the required angle with the X-ray beam. This will happen for a series of orientations of the crystals distributed around a cone. The X-rays will be reflected around a cone of twice this angle, as shown in Fig. 1.10.

OPTICAL AND X-RAY DIFFRACTION

Layers of atoms giving rise to a particular reflection will make a constant angle, φ, with this crystal axis, but, if there is no preferred orientation perpendicular to the fiber axis, the layers can occur at a series of positions distributed around the fiber axis on a cone, as shown in Fig. 1.11. If an X-ray beam is directed at right angles to the fiber axis, the reflections will now occur, not round a whole cone, but only at those four angles at which the cone of Fig. 1.10 (defining the characteristic angles of reflection) intersects with the cone of Fig. 1.11 (defining the angles at which the particular layers of atoms occur). This is illustrated in Fig. 1.12

ELECTRON MICROSCOPY AND ELECTRON DIFFRACTION

Electrons, although usually regarded as particles, can act as if they were waves with a wavelength of the order of 0.005 nm.

They can be focused by bending their paths in electric and magnetic fields in the same way that light rays are bent by lenses.

Electron microscopes can form an image with a limit of resolution that is far smaller than is possible with an optical microscope.

A limitation is that the specimens must be in a vacuum.

The early applications of electron microscopy to fibers are discussed by Chapman, Hearle and Greer, Hearle and Simmensand Hearle have used tomography to make a quantitative determination of the twist angles in the helical assembly of the intermediate filaments (micro fibrils) in the macro fibrils of the ortho-cortex of wool.

ELECTRON MICROSCOPY AND ELECTRON DIFFRACTION

Much better method for examining surface detail is scanning electron microscopy (SEM).

The principle of this method is that a fine spot of electrons is traversed across the specimen and some response is used to form an image on what is, essentially, a television screen scanned synchronously with the spot.

In the usual mode of operation, where the scattered electrons picked up by a collector are used to generate the image, the picture looks like an ordinary enlarged image of the specimen as viewed along the column followed by the electrons forming the spot.

The main use of scanning electron microscopy in fiber science has been in the range of medium to high magnification, which is near or beyond the limit of the optical microscope.

The scanning electron microscope has the great advantage of a much larger depth of focus.