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

More documents

Recommendations

Info

$Writing in LaTeX under Windows or Linux$

BH12 Course B: Materials for Devices BH12 A sample of nematic LC will not necessarily have a single, uniform director across its whole volume, but instead may be broken down into smaller, differently oriented regions, or domains. Where differently oriented domains meet there is a ‘glitch’, called a disclination. Observation of a LC sample between crossed polars typically shows a schlieren texture: bright regions of nematic LC with uniform director, separated by dark boundary regions where the director is aligned with the polarizer or analyzer (or is parallel to the light path), and hence the sample is in extinction. Further types of Liquid Crystals There are several different classes of LC, based upon their overall tendency for molecular alignment. Smectic: molecules organise into layers ⇒ orientational order, some positional order Chiral nematic: helical twist Smectic A D parallel to layer normal Smectic C ↑ D D D The molecules in a Chiral Nematic (also called cholesteric) have their long axes (and hence director) in a plane (in the above, this is shown as the horizontal); the director rotates along the axis perpendicular to this plane, tracing out a helix. For polarized light propagating along this axis, the plane of polarization rotates with D. The pitch is the distance along the axis taken for a complete 360° rotation of the director. It can vary over a very wide range, e.g. ~100 nm - 100 µm, depending upon the LC molecules involved, the degree of polymerization, and the concentration in solution.
BH13 Course B: Materials for Devices BH13 This twisted structure occurs because the molecules are chiral (lack inversion symmetry) [see AH68], and show an asymmetric packing preference. e.g. Poly(γ-benzyl-L-glutamate) (PBLG): with R: Monomer: Polymer: The polymer molecule is an α-helix, i.e. is asymmetric (there’s a difference between a right-hand and a left-hand helix). This leads to the observed twist in alignment, i.e. twist of the director. dark → (light propagation along symmetry axis of molecules ⇒ nonbirefringent) Observation of a chiral nematic LC between crossed polars, with light travelling perpendicular to the axis of the helix, shows a stripy pattern, often appearing like a fingerprint: dark → strongly birefringent ⇒ bright
Page 1 and 2: Natural Sciences Tripos Part IA MAT
Page 3 and 4: BH1 Course B: Materials for Devices
Page 13: BH11 Course B: Materials for Device
Page 17 and 18: BH15 Course B: Materials for Device
Page 61 and 62: Qu. Sheet 5 MATERIALS SCIENCE BQ1 C
Page 63 and 64: Qu. Sheet 6 MATERIALS SCIENCE BQ2 C
Page 65 and 66:
Qu. Sheet 7 MATERIALS SCIENCE BQ3 C
Page 67 and 68:
Qu.Sheet 8 MATERIALS SCIENCE BQ4 Co
show all

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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?