FINE STRUCTURE OF FIBRE A fiber is defined as a unit matter characterized by flexiblity, fineness, and a high

ratio of length sufficient high temperature stability and a certain minimum

strength and extension are also required.

Fine structure relates to the physical arrangements of molecules

constituting the fiber from general consideration and also chemical analysis it is

known that the molecules constituting the fiber are considerably long as

compared to their width. Further these molecules are made a large number of

smaller units called monomers repeating them several times. For this reason

these fiber forming molecule are called as polymers (derived from Latin word

poly-many and mer-unit).the unit of the polymer is the monomer, it is also

derived from the Latin word mono-one. At the molecular level the polymer is

extremely long and linear, whereas the monomer is very small. Monomers are

usually chemically reactive where as the polymers are unreactive. “The chemical

reaction which causes the monomers to join end to end to form a polymer is

called polymerization.”

Polymer are chemically unreactive this does not prevent its being subsequently

attacked by chemicals and other degrading agents. The length of the polymer is

important .all fibbers both of man-made or natural have to extremely long

polymers. For measuring the length of the polymer is a complicated task and can

be determined by its DEGREE OF POLYMERISATION.

Degree of polymerization = average molecular weight of the polymer

Molecular weight in repeating unit in the polymer

METHODS OF INVESTIGATION OF FINE STRUCTURE The optical methods are not suitable for this purpose, since the smallest object

which cannot be seen with the help of visible light is equal to its wavelength

which is in the range of 0.5µ where as the molecules have dimensions of the

order of few Ao . So we have to use the waves or rays which have a very fine wave

length ( prefferably high frequency). Infrared rays,X-rays,and electron beam are

used for investigation of fine structure of fiber.

Infrared red rays The infrared radiation gives the idea about the atoms constituting the moleculrs

and the type of bonds between them. Where as the X-ray gives the idea about the

physical arrangement of the molecules,and the electron beam gives the idea very

much enlarged images of fine stucture as agains the indiret information given by

X-rays and infrared rays.

The infrared radiation are composed of electromagnetic waves whose

wave length varies between 1 µ -15 µ. When these rays are passed through the

material , it is mostly absorbed at certain characterstic frequency. With the help

of the sprectometer the variation in the absorption can found and plotted against

wave length of more commonly called wave number.

The wave number on which the absorption takes place depend upon the

nature of two atoms and the bond between them. The absorption frequency of

such group are as ≥C-H,>C=O, C-O-,-O-H-,>N-H, ,>C=C and so on. To a

smaller extent it also depends upon the group of its neighborhoods. For example

the absorption frequency for a carbon hydrogen bond in a terminant group is

different from that for the same bond in(-CH2 -) therefore infrared spectroscopy

can help in deducting the chemical structure of fiber. This method can also be

used for estimating quantity of substance like water and other chemical present

in the fiber.

Further if infrared rays are polarized than they give maximum absorption for

orientation and minimum absorption at an orientation at right angles. This is also

known as infrared diachroism. This can be used in investigate the degree of

orientation of molecule in the fiber.

An advantage of the infrared absorption meted is that it is influenced by all the

molecule in the fiber in both the crystalline and non-crystalline regions. Whereas

X-ray diffraction method gives detailed information only about crystalline region.

One of the major techniques involves exposure of a material to the vapour of the

heavy water (D2O) there by replacement of hydrogen atoms by deuterium atoms.

In case of the fibers only non crystalline region are assessable to heavy water.

Hydrogen atom of this entire region will be replaced by deuterium. Whereas as

those in crystalline region will remain unaffected. From this difference in infrared

absorption the percentage of two regions can be estimated.

X-RAY DIFFRACTION METHOD If a beam of X-ray is directed at a crystalline region it is strongly reflected

whenever it strikes the layer of atoms at an angle θ such that

nλ=2dsinθ

Where, n=integer, λ=wavelength of X-ray, d= distance between atomic layer.

Under this condition the reflection from individual layer rain force each other

since they are in phase with each other whereas at other angle they interface

with one another.

There may be many layers of atoms of varying density in different direction. There

will be a series of characteristics angle for each case. From these angles and from

the variation in the intensity of reflection, the general crystal structure can be

work out.

In fibers we are not dealing with single crystal there will be a lot of small crystal

and they are oriented parallel to the fiber axis. But it is simpler to consider first

the diffraction pattern which is found where there is no preferred orientation.

This is what we get, the process of passing an X-ray beam through powdered

crystals is called powder photograph.

For a strong reflection to occur the layer of atoms should make the required angle

with X-ray beam. This will happen for a series of orientation of crystals distributed

around a cone. The X-rays will be reflected around a cone of twice this angle. Due

to this all the orientation present and all the other reflection will occur with the

appropriate layers of the atoms distributed around the cones given in a

characteristic angle of incidence.

Thus, the powdered photograph, therefore series of circles subtending angle can

be determined by the distance between the layers of the atoms.

When there is a preferred orientation such that the layers are distributed round a

cone making an angle of Ф as shown in fig below….

Now if an X-ray beam is passed at an right angle to the fiber axis, the reflection

will occur at the four point of intersection of two cone as shown in fig. above and

below…….each

Each of the layers of atoms will contribute different sets of four spots distributed

symmetrically in the four quadrants and these will be repeated at different value

of ‘n’. There are two special cases:-

1. If Ф =∏/2 then the cone of fig-2 become a plane cutting the other one in

only two places& the reflection occur at two spots on the equator of the

photograph.

2. If Ф = (∏/2—θ) than the two cones just touch and again we obtain two

spots but this time at the poles.

If Ф < (∏/2—θ) than no reflection occurs.

If the orientation is not perfect we get reflection over a wide range of angle

& the spots can be converted into arcs. Due to imperfect or varying crystal

structure, the arcs become thicker in the radial direction.

Apart from the strong reflection from the crystalline region , there is a

scattering of X-rays from the non-crystalline region which gives the diffuse

back ground from the relative intensities of the two, the proportion of

crystalline and amorphous region can be obtained.

Electron microscope The observation of fine structure of fibers by optical microscope is restricted by

the limit of resolution imposed by the wavelength of the light i.e., 0.5µ. The

development of electron microscope enables picture to be obtained which shows

much finer structure. The wavelength of electron used in electron microscope is

of 0.05 A0 . In practice the best resolution so far obtained in the most favorable

condition is about 5 A0 which almost enables to identify individual atoms of fiber

or fine structure.

There are mainly 3 types of electron microscope. They are as follows:-

1. Transmission electron microscope

2. Reflection electron microscope

3. scanning electron microscope

Transmission electron microscope

The ray from the electron source are condensed on the specimen and then

focused by electric or magnetic fields acting as lenses to give a magnified image

on a fluorescent screen or photographic plate. For the passage of electrons, it is

necessary for the instrument to be evacuated. Only dry specimen can be

examined. Contrast in the image depends on the variation on the scatterings of

the electrons by the parts of the specimen of different density. The unscattered

electrons are focused on the screen. The scattered electrons, which have lost

their energy and slowed down, would come to focus at a different point. They

form a diffuse background to a picture so reducing the contrast.

In principle, the electron microscope is similar to the optical microscope. The

electron beam is analogous to the ray of light where magnetic or electric fields

are similar to optical lenses. On major advantage of electric lenses is that they can

be continuously varied by varying the current but magnetic lenses are more

preferable because there is no danger of high voltage as in case of electric lenses.

One condition is provided in case of using the specimen in transmission electron

microscope that the specimen should be very thin i.e. less than 0.1µ which allows

the passage of electron.

Reflection electron microscope

This is useful for the direct examination of surface of materials against indirect

method of replica technique used in the transmission electron microscope. The

elements of this microscope are the same as that of the transmission type but the

illuminating system (electron beam) is inclined at small angle ‘α’ (which generally

lies between 80-150) to the axis of the instrument. The specimen which is not

much thin is arranged with its surface at an angle ‘β’ such that α>β>α/2. The

image is formed by objective and projector lenses system from electrons

scattered or reflected from the specimen surface into the objective aperture.

A thin layer or sliver is evaporated on to the specimen so as to make it conductive

and prevent accumulate of charge. The magnification in the direction

perpendicular to the plane of a incidence is much greater than that right angles to

parallel, but because of great depth of focus of image a considerable length of

fiber is in focus and it helps in evaluating the shape and surface characteristics. A

serious drawback of this method is that the possibility of fiber damage by electron

bombardment.

Scanning electron microscope

Scanning electron microscope uses a different approach to the direct

examination of surface structure or fine structure. Electron lenses are used in

illuminating source to form a generally reduced image of electron source (a very

narrow and strong electron beam). The electron probe scans the specimen across

by the electrostatic deflecting electrode as in a T.V. system. Secondary electrons

leaving the specimen are collected directly into an electron amplifier and after

amplification the signal is used to modulate the beam intensity of cathode ray

tube. The beam starts scans the screen of the tube in synchronization with the

scanning spot in the microscope. Thus an image is built on the screen of cathode

ray tube.

Although the intensity of the probe is high the average of electron intensity

incident on the specimen is negligibly small so that chances of the fiber damage

are less. The specimen is coated with a thin layer of film of evaporated metal to

prevent the accumulation of charge on surface.

Specimen preparation For getting proper contrast of image of the specimen or fine structure by electron

microscope, we have to use the test specimen very thin. Since the fibers are

mainly composed of lighter atoms they scattered the electrons to a lesser extent

so that many of the scattered electrons reach the screen there by reducing the

contrast. Improving contrast is one of the main problems in electron microscope.

Supporting films As in the case of t5he optical microscope the specimen is held on a glass plate

which is transparent to light. We need supporting films in electron microscope

which do not scatter electrons but are strong enough to support the specimen.

Generally colloidal or framework film are used. The films are prepared by

dissolving this substance in a volatile solvents and spreading the solution either

on a water surface or glass plate. The thickness is in the range of 100Ao. for the

high resolution work carbon films are used which are prepared by evaporating a

thin film of carbon on a substance which is subsequently dissolved away.

Shadow casting The contrast may be increased by evaporating on to the surface of the specimen

or by thin coating of a suitable metal at a small angle. The metals used are

paradium alloy of gold and palladium, chromium or platinum, since these metals

have high atomic weight their scattering power is much more and hence they

improve the contrast. The metal is evaporated in vacuum in such an angle that

the shadow to object ratio is about 4:1. Knowledge of shadow angle and the

length of shadow enable the height of the specimen above the supporting

membranes to be determined.

Thinner section Section for the examination in the electron microscope should be thinner than

0.1µ and some time they should be only a few hundred A0 thick. Special section

machine called “microtome” are available for this purpose. Since fibers are

flexible a hard embedding medium is necessary during section cutting. The

material is embedded in a mixture of butyl& methyl Meta crystal monomers

which is than polymerized, epoxy resins are also used for this purpose. The

forward movement of specimen on the microtome which is equal to the thickness

read is obtained by the thermal expansions of a metal rod to ensure smooth and

regulate movement. Steel glass and diamond knives are used for this section

cutting.

One of the limitations of this section is that some of the features observed may be

a result of the section cutting.

Disintegration method This method overcomes the difficulties involved in the section cutting but they

provide a different type of information. Fibers may be disintegrated into small

fragments by mechanical action (grinding), irradiation with ultrasonic’s,

dissolution by chemicals or enzymatic attack or by a combination of these

techniques. The suspension of a disintegration product in a suitable liquid

(generally water) is placed on a membrane coated grid and the liquid is allowed to

evaporate. The particles from the suspension dry down on to the supporting film

and are usually shadow cast to increase contrast. With this technique the fibril

structure of cellulosic and fibers can be clearly visible and identified.