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