C11 Liquid Crystals (HL)

C11.1 Identify molecules that are likely to show liquid-crystal properties, and explain their liquid-crystal behavior on a molecular level.C11.2 Describe and explain in molecular terms the workings of a twisted nematic liquid crystalC11.3 Describe the liquid-crystal properties of Kevlar, and explain its strength and its solubility in concentrated sulfuric acid.

C11.1 Identify molecules that are likely to show liquid-crystal properties, and explain their liquid-

crystal behavior on a molecular level.

• Biphenyl Nitriles are molecules which display liquid crystal behavior– The Nitrile group in this molecule is

polar– This helps to ensure that the molecules

can align in a common direction4-pentyl-4’-cyanobiphenyl

C 11.1 Continued• Long Alkane chain

groups on end of molecules– Makes it hard for molecules

to pack too closely together– Ensure molecules can stay

in liquid crystal state

• Two benzene rings in biphenyl molecules– Gives more strength and

rigidity– Gives molecules a more

rod-like shape to help align in liquid crystal state

4-azoxyanisole 4- methoxybenzylidene 4-butylanilien

C11.2 Describe and explain in molecular terms the workings of a twisted nematic liquid crystal

• A pixel in an LCD screen is composed of 3 colors red, green and blue

• Liquid crystals in LCD screens have a twisted shape or geometry– Liquid crystals touching the plates,

align with tiny grooves in the plates– Intermolecular forces cause the

molecules to align together and form the helical structure

C 11.2 Continued• Twisted Nematic Liquid

crystals are the basics of LCD Displays

• Composition of LCD Screen– Polarizing filter– Glass layer– electrodes– Liquid crystal layer– electrodes– Glass layer– Polarizing filter

C 11.2 Continued• When a pixel appears

white, RGB are all on• If a pixel needs to be

dimmed or turned off, electricity is run through the liquid crystals

• When electricity is run through the liquid crystals, they all align

• Light then cannot pass through the polarizing filter

No electricity running through LC, light passes

Electricity run through LC, light cannot pass through

C11.3 Describe the liquid-crystal properties of Kevlar, and explain its strength and its solubility in

concentrated sulfuric acid.

• Kevlar is lyotropic liquid crystal when in a solution of concentrated sulfuric acid

• Molecular alignment depends on concentration

• Kevlar has a rod-like shape• Kevlar molecules form extra hydrogen

bonds between molecules• In very strong acid solutions, oxygen and

nitrogen becomes protonated and hydrogen bonds break