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 crystal C11.3 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 direction 4-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