Part 7. The chemistry of AROMATIC COMPOUNDS

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

Part 7.12 The structure, properties and uses of polyesters and polyamides involving aromatic monomers - including high performance polymers

Sub-index for this page

7.12.1 PET and Terylene

7.12.2 The high performance polymer Kevlar

7.12.3 The high performance polymer Nomex

INDEX of AROMATIC CHEMISTRY NOTES

All Advanced A Level Organic Chemistry Notes

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7.12.1 The formation, molecular structure and uses of the polyester PET/Terylene

PET is the acronym for Poly Ethylene Terephthalate, and it has the same molecular structure as Terylene!

Terylene (a polyester) is good for making 'artificial' or 'man-made' fibres used in the clothing industry.

benzene-1,4-dicarboxylic acid, (terephthalic acid) a carboxylic acid functional group at each end of the molecule

Ethane-1,2-diol (ethylene glycol), two primary alcohol groups, one at each end of the molecule;

benzene-1,4-dicarboxylic acid forms a condensation polymer with ethane-1,2-diol, the polyester Terylene and the eliminated molecule is water H₂O, 1st diagram below, and the 4th diagram shows several of the linked ester units.

You can carry out the reaction with a different aromatic acid derivative to make PET/Terylene e.g. using ...

 the diacid chloride of benzene-1,4-dicarboxylic acid and the eliminated molecule is hydrogen chloride (HCl).

 the dimethyl ester of benzene-1,4-dicarboxylic acid and the eliminated molecule is methanol (CH₃OH)l.

(see the two middle diagrams below)

More advanced displayed formula representations of Terylene/PET and its formation

The plastic PET is the same as Terylene!

The acronym PET stands for Polyethylene terephthalate.

When used in clothing fibres, it is called Terylene, but when used in bottles, it is called PET   !!

PET is a clear, strong and lightweight plastic belonging to the polyester family.

It is typically called "polyester" e.g. the brand Terylene when used for fibres or fabrics, and "PET" or "PET Resin" when used for bottles, jars, containers and packaging applications - but they have the same molecular structure - its just how its processed into a particular product.

PET is a thermoplastic, softens on heating, that can be extruded into different shapes, particularly for water and fizzy drinks. The bottle keeps the dissolved carbon dioxide from escaping and it is shatterproof if dropped on the floor - so its quite a tough plastic.

PET is the world's packaging choice for many foods and beverages because it is hygienic, strong, lightweight, shatterproof, and retains freshness.

PET is most commonly used to package carbonated soft drinks and water.

The most common use of polyester today is called PET (for short!) and is used to make the plastic bottles for storing liquids in like soft drinks. PET is very useful because it is transparent, shatterproof and cheap!
 

Terylene polyester is a typical synthetic fibres which have, in many cases, replaced cotton, silk and wool fabrics in the clothing industry.

Fine polyester fibres can be made into a variety of articles of clothing which are lighter and cheaper than traditional materials like wool.

Shirts, sheets, socks and trousers are manufactured from a mix of a polyester and a natural fibre such as cotton and wool.

The polyester gives fabric strength and creases resistance and the natural fibre gives the material a softer more natural feel and also allows the material to absorb some perspiration from the wearer - remember plastic materials are usually hydrophobic - water repellent.

   

The manufacturing process of drawing out fibres makes them stronger

In the manufacturing process the polymer chains are made to line up by drawing out the polymer into a fine fibre.

Increasing the polymer chain alignment greatly increases the intermolecular forces between the 'aligned' polymer molecules and strong fibre strands of the plastic can be made - the process increases the surface - surface contact, so increasing the instantaneous dipole - induced dipole forces and the permanent dipole - permanent dipole forces.

Terylene fibres are cheap to make on an industrial scale compared to cotton from fields, silk from silkworms and wool from sheep.

As well as being cheaper, the physical properties of synthetic fibres have several advantages compared to their natural predecessors like cotton, silk and wool.

Compared to natural fibres, synthetic fibres tend to be ....

lighter - outdoor or indoor clothing,

more durable - harder tougher wearing fibres,

water-resistant - better water-proofed fabrics,

Polyester fibres are good thermal insulators - trap air e.g. the stuffing in duvets.

However, there are some disadvantages e.g.

they are not very breathable and sweat builds up making you feel uncomfortable.

The use of 'breathable' fabrics like GORE-TEX are described and discussed on the smart materials page 6. Gore-Tex and thinsulate etc.

 

Stability to hydrolysis

Since the monomer linkage is an ester, its not unreasonable to consider the possibility that PET/Terylene might degrade in the environment by hydrolysis.

Fortunately, the ester linkage in polyesters is quite stable and no such degradation takes place with water and very slowly in contact with acids.

(i) However, if you reflux fragmented bits of PET/Terylene (to increase the surface area) for long enough in aqueous sodium hydroxide solution, they will eventually dissolve to form a solution of ethane-1,2-diol (one of the monomers) and the disodium salt of benzene-1,4-dicarboxylic acid, disodium benzene-1,4-dicarboxylate (the salt of the other monomer material).

(ii) If you add excess dilute hydrochloric acid to the mixture, a white precipitate of benzene-1,4-dicarboxylic acid forms.

C₆H₄ represents the benzene ring structure in these simple structural formula equations.

State symbols are omitted for simplification: PET and the acid (s) and everything else is (aq).

(i) -(-CH₂-CH₂-OOC-C₆H₄-COO-)_n-  +  2n NaOH ===> n HO-CH₂-CH₂-OH + n Na^+-OOCC₄H₄COO^-Na⁺

-(-CH₂-CH₂-OOC-C₆H₄-COO-)_n-  +  2n OH^- ===> n HO-CH₂-CH₂-OH + n ^-OOCC₄H₄COO^-

(ii) Na^+-OOCC₄H₄COO^-Na⁺  +  2HCl  ===>  HOOCC₄H₄COOH  +  2NaCl

^-OOCC₄H₄COO^-  +  2H⁺  ===>  HOOCC₄H₄COOH

Therefore it is unwise to store acidic, and especially alkaline, chemicals in PET containers and Terylene clothing should not come into contact with acids or alkalis.

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7.12.2 High performance polymers - KEVLAR

Kevlar is an aramid, a polyamide in which the amide links (HN-CO-) are directly attached to benzene rings.

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.

Both Kevlar and Nomex are stable to hydrolysis and unlike Nylons, they are not attacked by aqueous acids or alkalis.

KEVLAR is the trade name for an aramid (aromatic amide) polymer fibre.

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

The diagram above shows the condensation polymerisation equation for the manufacture of KEVLAR.

Both monomer molecules have their substituent groups in the 1,4 orientation.

Its actually the same secondary amide linkage you get in protein (peptide linkage) and nylon molecules!

The polymer molecule chain in Kevlar is a little more 'linear' than Nomex (7.12.3) which means the molecules can pack more closely together increasing the intermolecular force between them.

The equations for the formation of Kevlar are repeated in structural formula style and with the repeating unit shown.

 You need to be able to work out the structural formula of the original monomers i.e. the original aromatic dicarboxylic acid or its dichloride and the aromatic diamine.

You also need to be able to point out the link formed on condensation i.e. the HN-CO- linkage.

 

The properties and uses of Kevlar fibres

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 including bulletproof vests!

Kevlar is used to make aircraft parts (low density - light and strong), reinforce tyres.

It is used in the manufacture of brake linings and cables.

It is strong and tough because its molecules are held together in sheets by strong chemical bonds and strong intermolecular forces - see diagrams below.

 

A more detailed look at the polymer structure of Kevlar and why it is so strong

The longer polymer chains are held together by strong C-C and C-N covalent bonds, but there are no cross-linking covalent bonds.

However, there intermolecular forces between the long kevlar molecules within each sheet include 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+llll^δ-O=C^δ+  (see diagram further down).

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 hydrogen bonding supplements the instantaneous dipole - induced dipole intermolecular attractive forces, so the long polyamide molecules are strongly held together within each sheet (several diagrams below show this).

The sheets of molecules are themselves held together by strong intermolecular forces (instantaneous dipole - induced dipole attractive 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 an intermolecular hydrogen bonding force can operate.

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, the instantaneous dipole - induced dipole forces attractive forces.

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7.12.3 The high performance polymer Nomex

Nomex is an aramid, a polyamide in which the amide links (HN-CO-) are directly attached to benzene rings.

Nomex is another polymer material that is extremely strong, chemically very inert and thermally very stable at higher temperatures where other polymers might melt or degrade-decompose.

Both Kevlar and Nomex are stable to hydrolysis and unlike Nylons, they are not attacked by aqueous acids or alkalis.

Nomex has very good flame resistant properties.

The diagram above shows the condensation polymerisation equation for the manufacture of NOMEX.

Both monomer molecules have their substituent groups in the 1,3 orientation.

The polymer molecule chain in Nomex is less 'linear' than Kevlar (7.12.3) which means the molecules can't pack as closely together decreasing the intermolecular force between them compared to Kevlar.

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INDEX of AROMATIC CHEMISTRY NOTES

 All Advanced Organic Chemistry Notes

All My synthetic polymer-plastics revision notes pages Introduction to addition polymers: poly(ethene), poly(propene), polystyrene, PVC, PTFE - structure, uses More on the uses of plastics, issues with using plastics, solutions and recycling methods Introducing condensation polymers: Nylon, Terylene/PET, comparing thermoplastics, fibres, thermosets Extra notes for more advanced level organic chemistry students Polymerisation of alkenes to addition polymers - structure, properties, uses of poly(alkene) polymers The manufacture, molecular structure, properties and uses of polyesters Amides chemistry - a mention of polyamides The structure, properties and uses of polyesters and polyamides involving aromatic monomers The chemistry of amides including Nylon formation, structure, properties and uses Stereoregular polymers -  isotactic/atactic/syndiotactic poly(propene) - use of Ziegler-Natta catalysts and note that polypeptides are also polyamides

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