11A. More on POLYMERS - synthetic macromolecules

Polymers are long chain molecule formed from lots of repeating units joined together by strong covalent bonds.

Modifying polymers, thermoplastics and thermosets

• First some reminders from section 7. about addition polymers which were discussed in some detail.
• As an example the formation of PVC is shown below.

  • the long chain PVC molecules

  • and what the molecules look like in the structure of PVC or any other thermoplastic.

  • Although the PVC molecules look straight, in reality, the long molecules will be all twisted-jumbled up as in the thermoplastic diagram above (a bit spaghetti like!).

• This is a typical addition polymer (formed by simple addition of monomer molecules), just like polythene and polystyrene etc. AND they are examples of thermoplastics, that is they can be heated and softened, reshaped and cooled to keep their new moulded shape.

• In addition polymerisation there is only one monomer molecule with a double bond and one product, the polymer, and the linking occurs via a reactive double bond. Detailed addition polymer notes

• In condensation polymerisation there are monomer molecules with a reactive functional groups.

  • There are two products, the condensation polymer itself, and, the small molecule that is eliminated between the two monomer molecules when the linking bond is formed from the two functional groups.

    • Notes further down the page.

    • Condensation polymers are usually thermoplastics too.

• Thermoplastic polymers (thermoplastics)

• A molecular model for a thermoplastic

• In thermoplastics the intermolecular forces between the polymer molecules are quite weak compared to the strong covalent bonds (C-C) holding the chain of atoms together.

• Because the 'intermolecular bonding' is weak, this explains that when heated, these 'plastic' materials will soften quite easily, which is why they are called 'thermoplastic' and have relatively low softening points and melting points.

• Even at room temperature the plastic is easily distorted because the polymer chains can slide over each other i.e. the external physical force applied on bending overcomes the intermolecular forces between the polymer molecules.

• Despite their relative weakness, on controlled heating until they are quite soft (but NOT molten), they are readily extrusion moulded or drawn out into useful shapes which retain their new formation on cooling.

• So, overall, thermoplastics are not that heat resistant or exceptionally rigid/strong - but their properties do vary quite widely e.g. poly(propene) and nylon can be drawn into strong fibres and both can be manufactured into quite strong and rigid forms.

• See nylon and Terylene

• COMPARISON OF THERMOPLASTICS and THERMOSETS and a mention of FIBRES

  • First a reminder that the use of a polymer mainly depends on its physical properties (which are derived from the polymers structure).

    • You can chemically modify polymers to change their physical properties to suit a particular use or application.

      • If you increase the chain length of the polymer molecules you increase the intermolecular forces between the chains so it is stronger and less flexible and has a higher softening/melting point.

      • If you shorten the average chain length the polymer has lower softening/melting and is easier to shape, and the plastic is more flexible.

      • Cross-linking, i.e. adding a cross-linking agent to the monomer/polymer mixture which forms strong chemical bonds between polymer chains is discussed below and this modification is most important when comparing thermosoftening plastics and thermosetting plastics. Cross

• In thermosoftening plastics like poly(ethene), poly(propene) or poly(chloroethene) PVC, because the inter-molecular attractive forces between the chains are weak, the plastic softens when heated and hardens again when cooled. See also addition polymers page.
  • It also means the polymer molecules can slide over each other especially when heated to their relatively low softening/melting points.
  • This means they can be easily stretched or moulded into any desired shape.
  • They are examples of thermoplastics (thermosoftening plastics), because they can be heated to make them softer - more plastic, reshape it e.g. in an injection mould system, and on cooling the plastic object retains its new shape - bottle, bowl, toy etc.
  • However it is possible to manufacture and process plastics in which the polymer chains are made to line up. This greatly increases the intermolecular forces between the 'aligned' polymer molecules and strong fibre strands of the plastic can be made.
  • Examples: The addition polymer poly(propene) and the condensation polymers nylon and Terylene.
• Thermosoftening polymers like poly(ethene) and poly(propene) consist of individual, tangled polymer chains and melt relatively easily when they are heated. This contrasts with thermosetting polymers consist of polymer chains with strong cross-links between them and so they do not melt when they are heated.
• When a thermosetting plastic is formed you not only get polymerisation to form long molecules, you also get chemical bonds formed between various points in one polymer chain molecule across to another polymer molecule.
  • These extra bonds are called cross-links and hold the linear polymer chains together in a much more rigid structure.
    • These cross links do not usually occur in the simpler addition polymerisations when thermoplastics like poly(ethene) and PVC are made.
  • Commercially, many thermosets consist of a partially polymerised (but not cross-linked) resin, which contains a cross-linking agent and a catalyst, so that when the mixture is exposed to air or a the mixture warmed, cross-linking polymerisation occurs and the hard thermoset is formed. This type of mixture is used to make fibre-glass reinforced structures e.g. light car bodies or the hulls of sailing boats and canoes.
• These extra cross-linking covalent bonds formed between adjacent chains of the polymers change the physical properties considerably and thermoset polymers have much higher high melting points (giving greater heat resistance and thermal stability) as well as greatly increased strength and rigidity.
  • Compared to thermosoftening plastics, thermoset polymers do not soften or melt and only break down and degrade at much higher temperatures compared to the softening/melting points of thermoplastics described above.
    • Thermosets are harder, more rigid/stiffer and not as easily bent or stretched, in fact they can be quite brittle and almost impossible to stretch (not very elastic!).
  • Note that thermosets type polymers can be formed at room temperature, heating may not be required.
    • Many super glues form this kind of structure.
  • However, you have to get it right first time because thermosetting polymers cannot be softened with heat and therefore cannot be stretched or re-shaped, but the advantage is that thermosets are much more heat resistant than thermoplastics.
  • But these cross-linked thermoset polymers are much more rigid (e.g. can't be stretched) and stronger material (though they can be brittle) and not as flammable as most thermoplastics.
  • On heating them strongly they do NOT melt, but tend to char, gradually giving off gases.

  • 

  • A simple diagram of the polymer molecules in the three different situation.

    • Thermoplastic: The polymer molecules tend to be randomly jumbled up, but no cross-linking bonds.

    • Fibres: Fibre molecules are thermoplastic molecules but manufactured in such a way to get the 'molecules more lined up' to increase intermolecular forces between the long molecules, and this increases the strength of the fibre, but no cross-linking bonds are formed.

    • Thermosets: Their great strength and very rigid structure derives from the strong cross-links between the polymer strands. These cross-links are full chemical covalent bonds, NOT the much weaker intermolecular forces/bonding in thermoplastics.

      • In thermoplastics you have intermolecular bonding (weak attractive forces) between polymer molecules.

      • In thermosets you have intramolecular chemical bonding (very strong attractive forces) between the adjacent polymer molecule chains.

  • Heat resistant polymers are usually thermosets e.g. like melamine resin (plastic plates), but even thermoplastics like poly(propene) can be used in hot situations e.g. plastic electric kettles.

• Examples of Thermosets:
  • Melamine (used in furniture), Bakelite (was used for electrical fittings, a horrible brown colour but a good insulator, not used now?), Formica (table tops) and some super glues are examples of thermosetting polymers.
• See also ....
  • Problems with using plastics are in section 7. Polymers-plastics page.
  • Some basic notes on polymers and plastics in the Section 7 Oil Products Polymer Notes e.g. the formation of poly(ethene), polypropene and poly(chloroethene) PVC.
  • Some important structure, strength and 1D, 2D 3D dimension structural concepts) in the Chemical Bonding notes.
  • See also A survey of the properties and uses of a wide range of materials

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11B. More on Other Synthetic Polymers - macromolecules

SYNTHETIC FIBRES like NYLON and TERYLENE - condensation polymers

Condensation polymerisation involves linking lots of small monomer molecules together by eliminating a small molecule. This is often water from two different monomers, a H from one monomer, and an OH from the other, the 'spare bonds' then link up to form the polymer chain plus H₂O.

In addition polymerisation there is only one monomer molecule with a double bond and one product, the polymer, and the linking occurs via a reactive double bond.

Condensation polymerisation involves monomers with two functional groups (one at each end of the molecule). When these types of monomers react, they join together (polymerise), small molecules such as water are eliminated in the process, and so the reactions are called condensation reactions, hence the process is called condensation polymerisation.

The simplest polymers are made from two different monomers with two of the same functional groups on each monomer.

• Terylene (a polyester) and nylon are good for making 'artificial' or 'man-made' fibres used in the clothing and rope industries.
  • In the manufacturing process the polymer chains are made to line up.
  • This greatly increases the intermolecular forces between the 'aligned' polymer molecules and strong fibre strands of the plastic can be made.
• A polyester can be made from ethane diol (an alcohol with two hydroxy groups two -OH's) and hexanedioic acid (a carboxylic acid with two -COOH groups, a dicarboxylic acid).
  • These are the two starting monomers prior to polymerisation and both must have a reactive group at each end - and a different functional group that can react with the other.
    • The ethanediol monomer molecule   HO-CH₂-CH₂-OH
      • This type of molecule is called a diol, because it has two alcohol groups -OH.
    • The hexanedioic acid molecule monomer molecule   HOOCCH₂CH₂CH₂CH₂COOH
      • This type of molecule is called a dicarboxylic acid, because it has two carboxylic acid groups -COOH.
  • Alcohols react with carboxylic acid to form esters with the elimination of water.
  • In this case the ester linkage is formed at both ends of each molecule with the elimination of water molecules.
  • Polyesters are condensation polymers because of how they are formed - by this condensation reaction that eliminates a small molecule to form the ester bond between the monomers.

Diagram to explain how a diol (alcohol) and a dicarboxylic acid condense together to give an ester linkage.

Here just one of each monomer have condensed together to make a bigger molecule - the water is eliminated as the new linking covalent bond is formed - an ester bond.

BUT, at each end of this molecule, the functional group (alcohol -OH, on left) can link with the other functional group (carboxylic acid, -COOH, on right) to create an even bigger molecule - eventually, a long chain polyester polymer will form.

• Where n is the very large number of monomer molecules, the condensation polymerisation of ethane diol and hexanedioic acid can be represented as ...

n HO-CH₂-CH₂-OH + n HOOCCH₂CH₂CH₂CH₂COOH

===> -(-CH₂-CH₂-OOC-CH₂CH₂CH₂CH₂-COO-)_n-  +  2n H₂O

or more simply: n HO-[][]-OH + n HOOC-[][][][]-COOH ==> -(-[][]-OOC-[][][][]-COO-)_n-+  2n H₂O

[][] and [][][][] represent the rest of the molecules, and n is a very large number !

• Terylene (a polyester) is formed by condensation polymerisation and the simplified structure of Terylene can be represented as
•   3 units as partially displayed formula

More advanced displayed formula representations of Terylene and its formation

• However, above is a more accurate representation of the polyester Terylene, where you can see the ester linkage (COOC) more clearly, but this level of molecular structure is probably not needed at GCSE/IGCSE chemistry level.
• This is the same kind of 'ester linkage' (-COOC-) found in fats which are combination of long chain fatty carboxylic acids and glycerol (alcohol with 3  -OH groups, a 'triol').

• 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" when used for fibres or fabrics, and "PET" or "PET Resin" when used for bottles, jars, containers and packaging applications.

  • 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.
     

• Nylon (a polyamide) is formed by condensation polymerisation, the structure of nylon represented below where the rectangles represent the rest of the carbon chains in each unit.
  • For advanced molecular representations see Organic Nitrogen Compounds (A level Notes)
• Nylon is made polymerising a dicarboxylic acid and a diamine with the elimination of water.
  • Both monomers have the same functional group at each end, hence di.... in their names.

Diagram showing the formation of the polyamide link as a water molecule is eliminated when the carboxylic acid group in one monomer, bonds with amine group of the other monomer.

In this case two amino acids have a formed the simplest possible polypeptide - a simple dipeptide.

Note * that at each end of the molecule, the amine group (-NH₂, on left) and the carboxylic acid group (-COOH, on right) can both form a bond with another diamine molecule by further elimination of water molecules.

Diagram to explain how a diol (alcohol) and a dicarboxylic acid condense together to give an ester linkage.

Here just one of each monomer have condensed together to make a bigger molecule - the water is eliminated as the new linking covalent bond is formed.

BUT, at each end of this molecule, the functional group (alcohol -OH, on left) can link with the other functional group (carboxylic acid, -COOH, on right) to create an even bigger molecule - eventually, a long chain polyester polymer will form.

• n HOOC-[][][][]-COOH + n H₂N-[][][][]-NH₂ ==> -(-OC[][][][]-CONH-[][][][]-NH-)_n-+  2n H₂O
• 3 units as partially displayed formula.
• is the displayed formula of a diamine.
• is the displayed formula of a dicarboxylic acid.

• This is the same linkage (-CO-NH-) that is found in linked amino acids in naturally occurring macromolecules called polypeptides and proteins, where it is called the 'peptide' linkage.
  • Nylon-6,6
  • Making Nylon-6,6 in the laboratory
    • In the 1st beaker, make a solution of 1,6-diaminohexane in water.
    • In a 2nd beaker, dissolve 1,6-hexanedioyl dichloride in a suitable organic solvent - it must be one that does not mix with water - an immiscible liquid like the organic solvent 1,1,2-trichloroethane.
    • Pour one solution on top of the other, and nylon is formed at the interface of the two solutions.
    • With a glass rod, you can extract a blob of nylon on the end, lift it up carefully and twist the glass rod around and wind up a steady 'mushy' thread of nylon..
    • At GCSE level you don't need to write an equation, but you might be expected to recognise the condensation polymerisation reaction AND the small molecule, hydrogen chloride HCl, is eliminated. The equation is:
  • n HOOC-(CH₂)₄COOH + n H₂N-(CH₂)₆-NH₂ ==> -(-OC(CH₂)₄-CO-NH-(CH₂)₆-NH-)_n- +  2n H₂O
    • Unless you are studying chemistry at an advanced level, you don't have to no why its called Nylon-6,6, but it's because both monomers have a chain of 6 carbon atoms!

See also Extra advanced Level notes on Nylon structure and synthesis

• Although these are actually thermoplastic polymers, nylon and Terylene can be drawn out into thin strong fibres for use in clothing.

• Some important structure, strength and 1D, 2D and 3D dimension concepts are in the Chemical Bonding notes.
• Nylon and polyester are typical synthetic fibres which have, in many cases, replaced cotton, silk and wool fabrics in the clothing industry.
  • They 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,
    • 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.
  • 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!
    • Fine polyester fibres can be made into a variety of articles of clothing which are lighter and cheaper than traditional materials like wool.
    • Plastic bottles made from polyester can recycled and turned into fibres again and reused in clothing, are you wearing a plastic bottle!

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11C. Examples of NATURAL POLYMERS, their structure, function and uses

• Note that silk and cotton fibres (strong fibres for fabrics), rubber (for tyres and elastic objects) are very useful natural materials that have been harvested for many years from the natural world.

• Silk has been used as a clothing and decorative fabric for thousands of years, and, like sheep's wool, is essentially a protein polymer material.

  • Synthetic man-made polymers like nylon and Terylene where developed and designed to try and mimic silk's useful clothing properties, and nylon's excellent properties eg strength have meant it has found many and diverse applications. You need to read the nylon and Terylene sections on this page.

• Rubber has been used for centuries as a natural elastic polymer, but it has been replaced by synthetic polymers like neoprene.

• Wood is an extremely useful construction material, and is mainly a polymer mixture of cellulose (a natural polymer of glucose) and lignin (with a rigid cross-linked polymer structure).
• The valuable crop of cotton (for fabrics) also has a molecular structure based on cellulose, in fact it is the purest form of cellulose that occurs naturally and forms strong useful fibres.
• Starch, cellulose and sugars are all carbohydrate molecules
  • Sugars are small molecules, but starch and cellulose are natural condensation polymers of sugars based on the condensation polymerisation of small sugar molecules like glucose.
  • Starch and sugars are used in the food industry, starch being a polymer based on sugar.
  • See Natural molecules - carbohydrates - sugars - starch, DNA for more details
• Amino acids have two functional groups from which peptides and proteins are made.
  • This involves a condensation polymerisation in the biochemistry of living systems.
  • The carboxylic acid group -COOH and the amino or amine group -NH₂
  • The simplest one is aminoethanoic acid (glycine) H₂N-CH₂-COOH
  • Proteins are naturally occurring polymers based on amino acids.
  • Amino acids can undergo condensation polymerisation via the two functional groups to form peptides, and all sorts of combinations of amino acids produce the huge variety of proteins found in living systems.
  • For glycine the condensation polymerisation to give a glycine peptide can be shown as ..
  • n H₂N-CH₂-COOH  ===>  -(NH-CH₂-COO-)_n-  +  2n H₂O
  • where n is a very large number, with the elimination of 2n water molecules, one from each link at either end of the glycine monomer molecule.
  • The equation illustrates the structure if only one amino acid is used, in reality, proteins are very complex using a variety of around 20 different amino acids, each with its own unique structure, leading to lots of different proteins with their own unique structure. Think of all the different tissues in your own body!
  • For more details see Amino acids, proteins, polypeptides, enzymes & chromatography for more details.
• DNA (deoxyribonucleic acid)
  • DNA is a very large molecule essential for life and the basis of genetic chemistry in living systems.
  • In a cells biochemistry the DNA encodes genetic instructions for the development and functioning of all living organisms and viruses.
  • Most DNA molecules are two polymer chains made from four different monomers called nucleotides.
  • Two strands of the DNA molecule are coiled together in the form of a double helix.
  • For more details see DNA and RNA structure and Protein Synthesis gcse biology revision notes
• -

See also A survey of the properties and uses of a wide range of materials

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• Plastics are widely used in the manufacture of cars and other road vehicles because they are cheap to make of varied composition for a wide variety of uses, they are light, durable and can be dyed any colour, they can be flexible or rigid and so can be used for e.g. used for internal fittings e.g. dashboard cover, floor covers (can be rubber too), door coverings, transparent and coloured covers over headlights and brake lights, and the insulating sheathing for all the electrical wiring.

• See also A survey of the properties and uses of a wide range of materials

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LINKS

See also Extra advanced Level notes on Nylon structure and synthesis

and extra Advanced Level notes of poly(alkene) addition polymers

GCSE/IGCSE/O Level Oil Products & Organic Chemistry INDEX PAGE

See section 7. for full details on addition polymers

ALL my Advanced A Level Organic Chemistry revision notes

Multiple Choice Quizzes and Worksheets

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (easier-foundation-level)

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (harder-higher-level)

KS4 Science GCSE/IGCSE m/c QUIZ on other aspects of Organic Chemistry

and 3 linked easy Oil Products gap-fill quiz worksheets

ALSO gap-fill ('word-fill') exercises originally written for ...

... AQA GCSE Science Useful products from crude oil AND Oil, Hydrocarbons & Cracking etc.

... OCR 21st C GCSE Science Worksheet gap-fill C1.1c Air pollutants etc ...

... Edexcel GCSE Science Crude Oil and its Fractional distillation etc ...

... each set are interlinked, so clicking on one of the above leads to a sequence of several quizzes