Lecture 8: Polymers

Question 1

What are the different types of polymers?

Answer 1

• Natural (biopolymer)
• Semi-synthetic
• Synthetic

Question 2

What is a semi-synthetic polymer?

Answer 2

A natural polymer that is chemically treated to give a new material e.g cellophane

Question 3

What are some examples of natural (biopolymers)?

Answer 3

• Proteins
• Some fibres
• Polysaccharides
• Resins/gums
• Chitin
• DNA

Question 4

What are some examples of synthetic polymers?

Answer 4

• Plastics
• Elastomers
• Rubber

Question 5

What is a polymer?

Answer 5

• Long chain molecule - molecular weights of several thousand to several million atomic mass units.
• Constructed from many ‘like-structured’ molecules called monomers covalently bonded together in any conceivable pattern

Question 6

What are the different structures of polymers?

Answer 6

• Linear
• Branches
• Cross-linked
• Colloidal

Question 7

What is a homopolymer?

Answer 7

A polymer that is made up of the same monomer.

Question 8

What is a co-polymer?

Answer 8

A polymer that is made up of different monomers

Question 9

What are the two different ways monomers can react?

Answer 9

Monomers may have complimentary reactivity (react with self to form dimer etc) or react with another different monomer to perpetuate chain growth

Question 10

What is required for a polymerisation reaction to occur?

Answer 10

In order for a polymerisation reaction to occur it usually requires an initiator to commence polymerisation but sometimes they can self-assemble.

Question 11

What is the advantage of complexity within a polymer?

Answer 11

If we’ve got some complexity it means it has a unique signature that allows us to identify a polymer and potentially allows us to identify where a particular unknown substance, which is polymeric, where its manufactured.

Question 12

What does the ratio of end-groups allow?

Answer 12

Ratio of end groups to in-chain groups allows measuring of polymer length

Question 13

What does the nature of end groups allow for?

Answer 13

Nature of end group allows method of synthesis to be identified

Question 14

What do reactive end groups allow for?

Answer 14

Reactive end groups allow further modification to control properties or add additional functionality

Question 15

In a polymer, when is there no end groups?

Answer 15

When you have a ring

Question 16

Why do end groups usually give us a unique signature?

Answer 16

• Most industrial processes usually have their own type of end groups they use so we can track back a polymer to a certain manufacturer.
• This isn’t always the case.
• End groups have good identifying features, ratio of end groups to core bulk of polymer is potential unique between batches although on an industrial scale they’re usually pretty good at getting them the same.
• The type of end group is also unique

Question 17

Why are end groups important?

Answer 17

• They provide a unique signature
• They are usually more reactive and will react with things in the outside world making them important in trace evidence.
• This allows us to identify they type of trace evidence.

Question 18

3 main types of polymers

Answer 18

• Thermoset
• Thermoplastic
• Elastomer

Question 19

Thermoset

Answer 19

• Burns when heated
• Irreversibly hardened once shaped
• Generally inflexible as the corss linking inhibits molecular motion.
• Chemical process called curing required to set them
• Curing agent allows for additioonal cross linking between the polymers
• Rigid once set
• Polyurethanes, epoxy resins (superglue) and silicones

Question 20

Thermoplastic

Answer 20

• When we apply heat it melts, it can be remelted and reshaped.
• They can be stretched but will usually break and be quite brittle afterwards.
• Generally more flexible than thermosets as disordered regions facilitate molecular motion.
• No chemical curing required.
• Eg polystyrene, nylon and polycarbonate

Question 21

Elastomer

Answer 21

• Can be thermoplastic or thermoset
• Is “viscoelastic” so has viscosity (can flow) and elasticity (returns to originals shape).
• They can flow and be stretched significantly and still return to original shape without the use of heat.
• Rubbers (natural, butyl, silicone, etc)

Question 22

High density polyethylene

HDPE

Answer 22

• Containers and lids
  – Food bottles, petrol tank
  – Motor oil bottles
  – Crates
  – Pipes
  Higher density than LDPE
  Very low and now branching
  High crystalline content (70-90%) as lots of aligened chains
  Less transparent than LDPE
  Stiffer and harder than LDPE
  Less gas permeable than LDPE
• More rigid
• Multiple chains of polymers packed together.
• Physical properties allow us to identify it.

Question 23

Low density polyethylene

LDPE

Answer 23

– Film & sheet packaging
– Toys
– Squeeze bottles
– Plastic bags
– Wire & cable coatings
- Low crystalline content (40-60%) as they’re far less aligned
- More transparent than HDPE
- Forms good films
- Lower density than HDPE
- More gas permeable than HDPE
- Highly branched, for every 1000 carbon atoms you’re liekly to get 60 branches
- Physical properties allow us to identify it.
- Flexible

Question 24

Physical properties of a polymer

Answer 24

- How the polymer behaves at a macroscopic scale is largely determined by how adjacent chains interact and are linked. - Supramolecular interactions - hydrogen bonds, van der waals interactions. - Covalent interactions - cross-linking of chains - Controlled by underlying chemistry - Measuring properties and understanding of chemistry allows us to identify and compare polymeric trace evidence types.

Question 25

Crystallinity

Answer 25

Defined as the regions of atomic ordering where intramolecular folding/stacking of adjacent chains occurs.

Question 26

What does crystallinity affect?

Answer 26

– Impact resistance – Young’s Modulus – Tensile strength – Stiffness – Crease – Thermal Behaviour – Transparency

Question 27

Degree of crystallinity

Answer 27

Degree of crystallinity: 0 = Completely amorphous (not aligned) 1 = Completely crystalline Only a few synthetic polymers are entirely crystalline.

Question 28

What does measuring crystallinity allow?

Answer 28

- Measuring crystallinity allows identification of polymer and comparison between samples. - Spectroscopic techniques allow us to identify or compare two different polymers. - Destructive techniques are potentially cheaper and easier. - Depending on the availability of your evidence type, a destructive technique may be preferable. - The amount of amorphous crystalline is unique to a polymer. - A polymer could be formed in a different synthetic way to have a different ratio of crystalline.

Question 29

Tensile strength

Answer 29

- How much something will stretch before it breaks - For polymers, tensile strength increased with polymerchain length and cross linking. - All solids are elastic to some extent. - Measuring is dne by taking a sample and stretching it. - But size matters as it is easier to stretch smaller things than larger things - Larger things have more matter to move so more force is required. - In order to standardise the sample size we need to normalise the length of the piece of the material and the cross sectional area of the sample we’re looking at in order to get a normalised value for the tensile strength.

Question 30

Measuring tensile strength

Answer 30

- Stretching anything is like stretching a spring – apply a force and get an increase in length.

Question 31

Equation for strain

Answer 31

- For a solid we use the term strain (ε) not “stretch” - -ε = l/L --> fractional change which is independant of sample length. - Minus sign is because they’re in the opposite direction to the applied force. - Stress and strain are sample size independent as they’re normalised.

Question 32

Stress (σ) equation

Answer 32

- For a solid we use the term stress (σ) not force. - -σ = F/A, independant of sample size. - Minus sign is because they’re in the opposite direction to the applied force. - Stress and strain are sample size independent as they’re normalised

Question 33

Force equation

Answer 33

- F = -kx - For a spring, Hookes Law applies - Minus sign is because they’re in the opposite direction to the applied force. - K is unique to any materisl snf is the force/spring constant - (gradient on a graph of Force (y) vs distance stretched (x)) - X is the distance stretched. - F = force

Question 34

Yound modulus equation

Answer 34

- E is the Young’s Modulus - Describes how stiff a solid is - E = -σ/ε - - sign because if you pull in one direction (x) then the spring pulls back the opposite way. - Large values so its usually reported in GPa

Question 35

Typical behaviour of a thermoplastic polymer

Answer 35

Two main deformation regions: - Elastic (Elastic region where if you stretch it it will return back to its original form) - Plastic (when it will not return back to its original form). - Start to apply some force to stretch it apart and at some point you get nicking where bits on the end are bulky but in the middle the start to align and stretch out. - Nicking is when it starts to form a crystalline region.

Question 36

Amorphous phase transitions

Answer 36

Something that's amorphous generally as you increase temp to go from glassy material at a particular temp then would go rubbery and eventually liquid.

Question 37

Crystal material phases

Answer 37

It will start of crystalline and at a certain temperature will eventually melt due to being thermoplastic.

Question 38

What is T_g?

Answer 38

* T_g = Glass transition material which is unique to a material, this is an identify region of material. * T_g is not the melting point but is a temperature at which tension in polymer backbone lessens sufficiently to impart flexibility but not flow.