Lecture handout including QS - Department of Materials Science ...

BH7 Course B: Materials for Devices BH7
Depending upon the structure of a polymer molecule, the slow direction may be along the length of
the molecule, or perpendicular to the length. e.g. (for light propagating perpendicular to the plane of
the paper):
→ n 1
↓ n 2
polyethylene polystyrene
€
n 1
> n 2 fast
n 1
< n 2
slow
slow
fast
The birefringence is defined as the difference between the refractive indices:
Δn = n 1
− n 2
Whether or not a polymer sample exhibits non-zero birefringence will depend on the degree of chain
orientation:
random, isotropic = non-birefringent
aligned (or crystalline), anisotropic = birefringent. €
Polarizers or Polaroid materials are used as polarizing filters to produce linear polarized light.
These are polymer films (e.g. polyvinyl alcohol, PVOH, doped with iodine), in which the long-chain
polymer molecules are uniaxially aligned. The molecules absorb light vibrating parallel to their long
axes, and transmit that vibrating perpendicular:
[ Note: the lines
drawn to illustrate a
polarizer generally
indicate the resultant
direction of polarization
of the light,
rather than the
alignment of the
absorbing structure.]
Direction of alignment
of polymer molecules ↓
Crossed polars:
(all light will be blocked)
polarizer
analyzer