polarised light microscopy

Question 1

what are the two methods to polarise light

Answer 1

1. Reflection off 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 2

1. Reflection off a non-metallic surface, such as glass or paint

Answer 2

• Reflected beam is polarised with vibration directions parallel to the reflecting surface
• Refracted light is polarised in the plane of the paper

Question 3

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

Answer 3

• 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

Question 4

give 3 types of polarised light

Answer 4

linearly
circularly
elliptical

Question 5

circularly polarised light

Answer 5

electric vector rotates clockwise

Question 6

elliptical polarised light

Answer 6

composed of unequal contributions or right and left circular polarised light

Question 7

see pp for

Answer 7

types of polarised light diagrams

Question 8

what is polarised light microscopy important for

Answer 8

identification and comparison of Synthetic Fibers and Minerals

Question 9

are fibres anisotropic or isotropic

Answer 9

anisotropic

properties along length differ from there across width

Question 10

fibres are anisotropic

Answer 10

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)

Question 11

what is isotropic refractive index used for

Answer 11

to measure the density and hence crystallinity of a fibre

Question 12

isotropic refractive index equation

Answer 12

n-1= K_ρ

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

Question 13

see pp for

Answer 13

orientation and order of fibre polymer system

Question 14

stretching or drawing of a fibre

Answer 14

increases the orientation of the fibre, reduces its diameter and packs the polymer chains closer together –> higher density

Question 15

what can affect properties of fibres

Answer 15

• amount of crystallinity and the amount of orientation of the ordered regions affects properties e.g. dye-ability, tensile strength,

Question 16

name 3 methods of investigating extent of crystallinity

Answer 16

XRD, HTEM, DSC

Question 17

what is isotropic refractive index used for

Answer 17

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.

Question 18

birefringence

Answer 18

• Birefringence 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 19

see pp for

Answer 19

birefringence calculation

Question 20

calcite

Answer 20

• 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 21

see pp for

Answer 21

michel levy chart

Question 22

michel levy chart

Answer 22

• 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.

Question 23

birefringence of fibres

Answer 23

• 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 24

sign of elongation

Answer 24

• 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 25

see pp for

Answer 25

positive and negative elongation

Question 26

cellulose acetate fibres - sign of elongation

Answer 26

• 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 27

see pp for

Answer 27

dichroism

Question 28

see pp for

Answer 28

diagrams of pigmented fibres

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.