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Notes by Category University Engineering

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Polymers

In this notes sheet...

  1. Polymeric Structure

  2. Thermoplastics

  3. Thermosets

  4. Elastomers

  5. Tensile Properties

  6. Temperature-Dependent Properties

  7. Creep in Polymers

  8. Failure in Polymers

 

Broadly, polymers (also known as plastics) are split into three types:

  1. Thermoplastics

  2. Thermosets

  3. Elastomers

Each type has a specific set of properties that are dictated by its structure and the bonding within the polymer.


Polymeric Structure

All polymers consist of long macromolecules which contain chains of covalently bonded atoms. The process of turning single monomer molecules into long chains of polymers is known as polymerisation:

Polymerisation is a chemical reaction that generally happens at high temperatures with the help of a catalyst (this is to break the double bond between the carbon atoms).


The schematic above shows the polymerisation of ethylene, C₂H₄ into polyethylene (PE).

  • Chains are typically between 10³ and 10⁵ monomers long.

  • The molecular weight of the polymer is the product of the chain length and monomer weight.

  • Chains vary hugely in weight, and so commercial polymers are classified in terms of mean molecular weight:

The chains can be linear, branched, or cross-linked networks:

This is what dictates their properties.


Bonding

  • Monomers are bonded as chains with covalent bonds

  • Chains are connected at branches and cross-links with Van der Waals bonds

  • Often, hydrogen bonds form here too


Thermoplastics

As seen above, these can be regular semi-crystalline structures or amorphous (no ordered structure). The more branches there are to a chain, the less regular the structure.

We say semi-crystalline, as no polymers are truly ordered. All thermoplastics contain crystalline and amorphous regions.

The Van der Waals bonds between chains break when heated at lower temperatures than the covalent bonds holding the monomers as chains. This means they easily melt and can be formed into different shapes over and over again.


Semi-Crystalline Thermoplastics

Some of the most common examples include:


Polythene (PE)

  • Very cheap

  • Easy to mould

  • Tough

  • Generally used in bottles, packaging, pipes

Polypropylene (PP)

  • Same properties as PE, but stiffer

  • Good UV resistance

  • Fatigue resistance

  • Used as fibres, outdoor furniture, rope

Polytetrafluoroethylene (PTFE)

Excellent temperature and chemical resistance

  • Good non-stick properties

  • Used in lubricants, chemical containers & bottles, non-stick surfaces


Amorphous Thermoplastics

Common examples are:


Polystyrene (PS)

  • Transparent

  • Very cheap

  • Very mouldable

  • Brittle

  • Can be expanded into foam

  • Used for stationary, packaging, food containers, electrical insulation

Polyvinylchloride (PVC)<