Part 5 High performance polymers like Kevlar - structure, properties and uses

 Doc Brown's Chemistry Revision Notes - Smart Materials and Materials Science

Part 5a. HIGH PERFORMANCE POLYMERS

• These are polymer materials that are extremely strong, chemically very inert and thermally very stable at higher temperatures where other polymers might melt or degrade-decompose.

• See also some extra Advanced Level notes on KEVLAR

Part 5b. KEVLAR

• KEVLAR is the trade name for an aramid (aromatic amide) polymer fibre (also known as NOMEX).

  • These aramid polymeric molecules contain the -NH-CO- linkage between benzene rings.

  • Its actually the same secondary amide linkage you get in protein and nylon molecules!

• Kevlar fibres are made by spinning liquid crystal aramid oligomers (oligomers are molecules consisting of a few monomers linked together - but much shorter than a polymer!).

• Kevlar is a very strong artificial fibre that can be woven into materials that are very light and strong.

• It is used to make flexible body armour, i.e. it is a polymer designed for 'heavy duty' use!

• It is strong and tough because its molecules are held together in sheets by strong chemical bonds and strong intermolecular forces.

  • The intermolecular forces between the kevlar molecules in the sheet are hydrogen bonds, formed by the strong directional interaction of the two permanently polar bonds ^δ-N-H^δ+ and ^δ+C=O^δ-, i.e. ^δ-N-H^δ+/^δ-O=C^δ+

  • The highly polar bonds and hence the hydrogen bonding, is due to the significant difference in electronegativity between nitrogen and hydrogen, AND, carbon and oxygen.

• The sheets of molecules are themselves held together by strong intermolecular forces and the 'molecular sheets' can be aligned to maximise theses forces and the combined strength of many such sheets gives Kevlar fibres their great strength.

• 

• Along the polymer 'backbone' is a chain of strong of -N-C-C-C-C-N- bonds continuously repeated for hundreds or thousands of units which produces a strong rigid linear molecule.

• Between adjacent aramid molecules there are regular points where a intermolecular force can operate (called hydrogen bonding - for A level students). This extra bonding force holds the aramid molecules together in strong 'molecular' sheets.

• 

• The sheets are held together in Kevlar fibres by the 'random' but 'continuous' intermolecular forces between adjacent layers of molecules.

• See also some extra Advanced Level notes on KEVLAR

WHERE NEXT?

INDEX of SMART MATERIALS PAGES

PART 1 CHROMOGENIC MATERIALS - Thermochromic, Photochromic & Electrochromic Materials

PART 2 SHAPE MEMORY ALLOYS e.g. Nitinol & Magnetic Shape Memory Alloys

PART 3 SHAPE MEMORY POLYMERS, pH and temperature sensitive-responsive polymers, self-healing materials

Part 4 LYCRA-SPANDEX

PART 5 High performance polymers like KEVLAR

PART 6 GORETEX, THINSULATE and TEFLON-PTFE

PART 7 PIEZOELECTRIC EFFECT (PIEZOELECTRIC MATERIALS) and PHOTOMECHANICAL MATERIALS

PART 8 CARBON FIBRES

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