polarised light microscopy Flashcards
what are the two methods to polarise light
- Reflection off a non-metallic surface, such as glass or paint
- Light is polarised on passing through a substance that absorbs light vibrating in all directions except one
- Reflection off a non-metallic surface, such as glass or paint
- Reflected beam is polarised with vibration directions parallel to the reflecting surface
- Refracted light is polarised in the plane of the paper
- 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
give 3 types of polarised light
linearly
circularly
elliptical
circularly polarised light
electric vector rotates clockwise
elliptical polarised light
composed of unequal contributions or right and left circular polarised light
see pp for
types of polarised light diagrams
what is polarised light microscopy important for
identification and comparison of Synthetic Fibers and Minerals
are fibres anisotropic or isotropic
anisotropic
properties along length differ from there across width
fibres are anisotropic
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)
what is isotropic refractive index used for
to measure the density and hence crystallinity of a fibre
isotropic refractive index equation
n-1= K_ρ
n = isotropic refractive index, K = atomic refractivity of material ρ= density (used for quality control of fibres in industry)
see pp for
orientation and order of fibre polymer system
stretching or drawing of a fibre
increases the orientation of the fibre, reduces its diameter and packs the polymer chains closer together –> higher density
what can affect properties of fibres
• amount of crystallinity and the amount of orientation of the ordered regions affects properties e.g. dye-ability, tensile strength,
name 3 methods of investigating extent of crystallinity
XRD, HTEM, DSC
what is isotropic refractive index used for
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.
birefringence
- 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.
see pp for
birefringence calculation
calcite
- 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.
see pp for
michel levy chart
michel levy chart
- 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.
birefringence of fibres
- 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
sign of elongation
• 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
see pp for
positive and negative elongation
cellulose acetate fibres - sign of elongation
• 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.
see pp for
dichroism
see pp for
diagrams of pigmented fibres
pigmented fibres
- 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.
