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Doc Brown's GCSE/IGCSE/O Level KS4 science-CHEMISTRY Revision Notes

Oil, useful products, environmental problems, introduction to organic chemistry

11. Comparison of thermoplastic addition polymers, synthetic fibres from condensation polymers and thermosets

This further page on macromolecules-plastics describes the difference in formation of addition polymers like poly(chloroethene)/PVC and condensation polymers like Terylene and Nylon, all of which are thermosoftening polymers. The properties of thermosoftening polymers (thermoplastics) are compared with thermosetting polymers (thermosets).

Index of KS4 Science GCSE/IGCSE Chemistry Oil & Organic Chemistry Pages: 1. Fossil Fuels : 2. Fractional distillation of crude oil & uses of fractions : 3. ALKANES - saturated hydrocarbons and combustion : 4. Pollution, carbon monoxide, nitrogen oxides, what makes a good fuel?, climate change-global warming : 5. Alkenes - unsaturated hydrocarbons : 6. Cracking - a problem of supply and demand, other products : 7. Polymers, plastics, uses and problems : 8. Introduction to Organic Chemistry - Why so many series of organic compounds? : 9. Alcohols - Ethanol - properties, reactions, biofuels : 10. Carboxylic acids and esters : 11. Condensation polymers, Nylon & Terylene, comparing thermoplastics, fibres and thermosets : 12. Natural Molecules - carbohydrates - sugars - starch : 13. Amino acids, proteins, enzymes & chromatography : 14. Oils, fats, margarine and soaps : 15. Vitamins, drugs-analgesic medicines & food additives and aspects of cooking chemistry! : 16. Ozone, CFC's and free radicals : 17. Extra notes, ideas and links on Global Warming and Climate Change : Multiple Choice and Gap-Fill Quizzes: m/c QUIZ on Oil Products (GCSE/IGCSE easier-foundation-level) : m/c QUIZ on Oil Products (GCSE/IGCSE harder-higher-level) : IGCSE/GCSE m/c QUIZ on other Aspects of Organic Chemistry : and 3 Easy linked GCSE/IGCSE Oil Products word-fill worksheets

ALL my Advanced Level Organic Chemistry revision notes

 

11a. More on POLYMERS - synthetic macromolecules

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!).

  • Advanced Chemistry Page Index and LinksThis 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.

  • 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.
    • 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
  • 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 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 and the strength of the fibre, but no cross-linking bonds.

      • 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.

  • 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 ....

Advanced Chemistry Page Index and Links


11b. More on Other Synthetic Polymers - macromolecules

SYNTHETIC FIBRES like NYLON and TERYLENE

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.

  • 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.
  • nylon (3 units etc.)
  • This is the same linkage (-CO-NH-) that is found in linked amino acids in naturally occurring macromolecules called proteins, where it is called the 'peptide' linkage.
    • Nylon-6,6 (c) doc b
  • Terylene (a polyester) is formed by condensation polymerisation and the structure of Terylene represented as
  •  terylene  (3 units etc.)
  • 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').
  • Terylene (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.
  • 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 to 3D dimension concepts are in the Chemical Bonding notes.
  • Advanced Chemistry Page Index and LinksNylon 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.
  • A don't forget that silk fibres (for fabrics), rubber (for tyres and elastic objects) are very useful natural polymers.
  • Wood, an extremely useful construction material, and is mainly a polymer mixture of cellulose (a polymer of glucose) and lignin (with a rigid cross-linked structure).
  • The valuable crop of cotton (for fabrics) also has a molecular structure based on cellulose, in fact its the purest form of cellulose that occurs naturally.

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 (c) doc b 3 linked easy Oil Products gap-fill quiz worksheets

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

... AQA GCSE Science (c) doc b Useful products from crude oil AND (c) doc b Oil, Hydrocarbons & Cracking etc.

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

... Edexcel 360 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

ALL my Advanced Level Organic Chemistry revision notes

 

Notes information to help revise KS4 Science Additional Science Triple Award Separate Sciences Chemistry revision notes for GCSE/IGCSE/O level Chemistry Revision-Information Study Notes for revising AQA GCSE Science AQA GCSE Chemistry, Edexcel GCSE Science, Edexcel GCSE Chemistry, OCR 21st Century Science Chemistry, OCR Gateway Science chemistry, WJEC/CBAC GCSE science-chemistry CCEA/CEA GCSE science-chemistry (and courses equal to US grades 8, 9, 10)

Advanced Chemistry Page Index and Links
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