Isotopic RI and dichroism

Question 1

Polarization of Light
Two methods to polarise light:
step 1

Answer 1

1. reflection off of a non-metallic surface, such as glass or paint.
    reflected beam is polarized with vibration directions parallel to the reflecting surface (perpendicular to the page)
    Refracted light is polarised in the plane of the paper.

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Question 2

Polarization of Light
Two methods to polarise light:
step 2

Answer 2

2. Light is polarised on passing through a substance that absorbs light vibrating in all directions except one.

Anisotropic crystals have this property and were used in microscopes built before ~1950
 Now polarisers are made of plastic film made by the Polaroid Corporation.
 long-chain organic molecules are aligned closely together in one direction forming a grid in a plastic sheet.
 allows the passage of light vibrating only in the same direction as the grid. Light vibrating in all other directions is absorbed

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Question 3

Polarization of Light

Two methods to polarise light:

Answer 3

1. reflection off of a non-metallic surface, such as glass or paint.
2. Light is polarised on passing through a substance that absorbs light vibrating in all directions except one.

Question 4

polarised light

Answer 4

linearly polarised
circularly polarized
elliptical polarized light

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Question 5

Polarised Light Microscopy :

Answer 5

Most important method for identification and comparison of Synthetic Fibers and Minerals:
 Anisotropic and isotropic refractive index
 Birefringence
 Sign of elongation

Question 6

fibres are anisotropic

Answer 6

– properties along length differ from there across width
n_iso= [n_∥+2 (n_⊥)]/3
niso = isotropic RI (similar to RI of bulk polymer)
n_∥ = RI parallel to fiber length
n_⊥ = RI perpendicular to fiber length
niso, n_∥, n_⊥ all ~ 1.5 (all polymers are made from C, H, O and RI depends on refraction of the atom)

look on ppt for correct symbols

Question 7

Isotropic Refractive Index can be used to measure:

Answer 7

the density and hence crystallinity of a fibre
n-1= K_ρ

n = isotropic refractive index,
K = atomic refractivity of material
ρ= density (used for quality control of fibres in industry)

Question 8

Orientation and Order of Fiber Polymer System.

Answer 8

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(a) Non Orientated Non-crystalline
(b) Non-Orientated but Sections Ordered
(c) Orientated and Crystalline.

Question 9

stretch or drawing of a fibre

Answer 9

 stretching or drawing of a fiber increases the orientation of the fiber, reduces its diameter and packs the polymer chains closer together -> higher density
 amount of crystallinity and the amount of orientation of the ordered regions affects properties e.g. dye-ability, tensile strength,
 Other methods of investigating extent of crystallinity
+ XRD, HTEM, DSC

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Question 10

Isotropic refractive index

Answer 10

used to compare identical fibers which differ only in crystallinity – useful for colourless nylon fibers in carpets
 usually heat set if twisted
 on heating above Tg (glass transition temperature) then cooled the isotropic RI increases
 Above Tg molecules can move and be more ordered, on cooling the ordering is set in.
 look at ppt
 Note Γ remains constant as longitudinal order unchanged therefore can use niso to discriminate non-descript fibers

Question 11

Birefringence:

Answer 11

• is when the refractive index depends on the polarization and propagation direction of light.
• Birefringence Γ occurs when an optical material (crystal or orientated crystalline polymer) in the path of a beam of light causes the beam to be split into two polarization components which travel at different velocities.

Question 12

birefringence equation

Answer 12

Γ= n_∥- n_⊥  
 Γ=  R/1000 T  
Where R is retardation (nm) – amount by which || ray is slowed down in c.f. to ⊥ ray
T is thickness of fiber (µm)
n is refractive index

Question 13

Calcite .

Answer 13

Its birefringence is extremely large, with 𝑛⊥ and 𝑛∥ of 1.6584 and 1.4864 respectively.

This is because it has an anisotropic crystalline lattice.

Anisotropic crystals, have crystallographically distinct axes and light is refracted into two rays, each polarized so that they travel at different velocities and that their vibration directions are oriented at right angles to one another.

This phenomenon is termed “double” refraction.

Question 14

birefringent crystals between crossed polarizers

Answer 14

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After the light rays pass through the analyzer they vibrate in the same plane,
Constructive or destructive interference occurs and a spectrum of colour is observed

Question 15

michel- levy chart

Answer 15

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• sample is rotated to a position of maximum brightness in the microscope
• the column of colour produced gives the path difference in nm e.g. magenta (1st order) has a path difference of 570nm
• the nearest vertical line to the colour is followed to the nearest horizontal line to the known thickness.
• Birefringence is determined by following the diagonal line at the intersection of the colour and thickness value.
• An interactive Michel Levy Chart can be found at
• This chart can also be used to find thickness if birefringence and colour are known

Question 16

Birefringence of fibers:

Answer 16

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-  Acetate and acrylic fibres have a very low birefringence <0.05
 Viscose rayon and polypropylene fibers have a medium birefringence 0.015-0.04
 Polyamide fibres has a high birefringence of 0.04-0.07
 Polyester fibres have a very high birefringence >0.07

Question 17

sign of elongation

Answer 17

n∥ > n⊥ - Fiber is positive (+) – Most fibres

n∥ < n⊥ - Fiber is negative (-) – Acrylic, cellulose triacetate

-occurs when groups hanging off main chain have high molar
refraction

Question 18

Sign of Elongation (cont.)

Cellulose acetate fibres:

Answer 18

 90% of hydroxyl groups are esterified
 Birefringence decreases as n⊥ increases due to high molar refraction of acetate groups which are aligned ⊥ to polysaccharide molar chain
 sign of elongation ranges from –ve to +ve.
 most acetates appear isotropic
e.g. Cellulose triacetate –ve BUT cellulose diacetate +ve
 Acetate group slows down light
+ good way to discriminate between extent of acetylation

Question 19

dichroism

Answer 19

look on ppt for equation

• • when R > 1 Hypochromic effect
• when R < 1 Hyperchromic effect
• Change in hue is due to a shift in the band of highest intensity:
• hypsochromic (blue shift) -> λ gets shorter
• bathochromic (red shift) -> λ gets longer
• Linear dichroism spectra (LD) = AII - A┴ ( at each λ)

Question 20

congo red

Answer 20

is the sodium salt of 3,3′-([1,1′-biphenyl]-4,4′-diyl)bis(4-aminonaphthalene-1-sulfonic acid. ( not correct IUPAC name) It is an azo dye.
To exhibit dichroism the dye molecule must be elongated and the chromophore linear.
Dye molecules must be aligned with polymer molecules
Absorption of plane polarised light usually greater along fibre than across fibre.

Question 21

polyester dyed with Disperse Yellow 56

Answer 21

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• LD spectrum same shape as that with no polarisation but greater intensity
  Hypochromic effect

Question 22

polyester dyed with Disperse Orange 5 (azo dye)

Answer 22

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• LD spectrum different to that with no polarisation and intensity reduced

Question 23

polyamide dyed with Vialon Fast Violet RR

Answer 23

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• LD spectrum has -ve values

Question 24

Acid black 222

Answer 24

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inversion of the dichroic effect is found on polyamide only.
Largest dichroic effects for polyamide and silk, wool shows no dichroic effect
In rare cases, silk behaves differently than polyamide and wool (depends on the dye)

Question 25

Diazol Light Bordeaux 2BL - strong dichroic effect

Answer 25

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Question 26

Acrylic fibre dyed with Basic Blue 5

Answer 26

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-LD spectrum has evry low values over the whole range for acrylics and also acetates

Question 27

DICHROISM IN PIGMENTED FIBRES

Answer 27

depends strongly on the crystal structure (shape of the pigment grains) and the draw ratio (orientation of the polymer chains).

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• 50% of pigmented fibres in general show a strong dichroic effect
  20% have weak dichroism.
  Both regular (80%) and inversed dichroic effects (20%) occur

Question 28

Carbon Black pigmented polypropylene

Answer 28

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-absorption spectrum is very broad with a plateau between 425 and 675 nm. The LD-spectrum is mostly positive with negative values at both ends of the visible range.

Question 29

pigmented fibres

Answer 29

 formed by adding finely ground pigments to the spin dope.
 After spinning the man-made filaments are drawn
 alignment of the polymer chains and reorientation of the pigment grains.

If the crystal structure of the pigment has long elongated grains, an orientation parallel to the polymer chains is favoured.
If the crystal structure forms disc-like particles, these will be oriented perpendicular to the polymer chains.

 dichroic behaviour affected by pigment’s crystal structure (influences pigment shape) and the fibre draw ratio (influences the orientation of the pigment grains).
 bulk effect of pigments is different from the molecular effect in dyeing.