Describe and explain, in molecular terms, the workings of a twisted nematic liquid crystal.

(nematic) LC’s placed between (two glass) plates/sheets with (two) polarizing filters at 90° to each other / each pixel contains LC (film/molecules) sandwiched between (two glass) plates/sheets;

plates/sheets have (fine) scratches/grooves and LC molecules (in contact with glass) align with these;

LC molecules adjacent to top layer are orientated at 90° to those adjacent to bottom layer / OWTTE;

LC molecules align in twisted (nematic) arrangement due to intermolecular forces;

Accept “intermolecular bonds” for “intermolecular forces”.

LC interacts with (plane-)polarized light which is rotated 90°;

when molecules are aligned (with the scratches) light will pass through (and pixel appears bright);

when a potential difference/voltage is applied molecules align with electric field (losing their twisted structure);

they no longer allow (polarized) light to pass (and pixel appears dark);

A suitably labelled diagram may be used to explain some marking points.

One teacher commented that part (a) seemed too easy for two marks. Candidates generally scored well on the first part of this question though the weaker candidates often did not draw molecules with the irregular arrangement in space and hence only scored half marks. Some did not include eight molecules. In (b) (i), reference to the CN bond was often omitted. (b) (ii) proved a real challenge whereby candidates had to explain why a liquid-crystal device may be unreliable at low temperatures. Several different answers were allowed here on the markscheme ranging from molecules becoming more ordered to molecules unable to change orientation as they approach the fixed arrangement of the solid state to the increase in viscosity of the medium (leading to an increase in the LCD response time). In (c), very few scored all four marks though many did score half marks. The markscheme was quite expansive for this question and a suitably labeled diagram also could have been used to explain many of individual marking points. Diagrams were used to good effect by some of the better candidates in supporting their written answers.