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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BH13 Course B: <strong>Materials</strong> for Devices BH13<br />
This twisted structure occurs because the molecules are chiral (lack inversion symmetry) [see AH68],<br />
and show an asymmetric packing preference.<br />
e.g. Poly(γ-benzyl-L-glutamate) (PBLG):<br />
with R:<br />
Monomer:<br />
Polymer:<br />
The polymer molecule is an α-helix, i.e. is asymmetric (there’s a difference between a right-hand and<br />
a left-hand helix). This leads to the observed twist in alignment, i.e. twist <strong>of</strong> the director.<br />
dark →<br />
(light propagation<br />
along<br />
symmetry axis<br />
<strong>of</strong> molecules<br />
⇒ nonbirefringent)<br />
Observation <strong>of</strong> a chiral nematic LC between crossed<br />
polars, with light travelling perpendicular to the axis <strong>of</strong><br />
the helix, shows a stripy pattern, <strong>of</strong>ten appearing like a<br />
fingerprint:<br />
dark →<br />
strongly<br />
birefringent<br />
⇒ bright