Questions on Liquid crystals

1. One compound in use in liquid crystal displays is 4-pentyl-4’-cyanobiphenyl.

Explain the importance of the following features, which help to give the compound good liquid crystal properties:

(a) the cyanide group, −CN.

(b) the pentyl group, −C₅H₁₁.

(c) the two disubstituted phenyl groups, -C₆H₄.

2. Explain why:

(a) Soap solutions only tend to show liquid crystal behaviour at quite high concentrations.

(b) Cholesteryl benzoate (the first liquid crystal discovered) turns from a solid into a cloudy liquid when heated to 145.5 ^oC and then into a clear liquid at 178.5 ^oC when heated further.

(c) The screen on laptops and digital watches may give strange images when used at low temperatures.

(d) Old television sets, which contained a cathode ray tube, got much hotter than modern sets, which use liquid crystal display.

(e) Liquid crystals need to be in the nematic phase in order to be able to function in an LCD device.

Liquid crystal questions