Polymers

Question 1

What are thermoplastics? (4)

Answer 1

● Considered linear, can be branched
– chains not cross-linked by covalent bonds 
– only by Van der Waals bonds.
● Soften when heated, harden when cooled
● Secondary bonds break easily with heat
● Have range of Tm , due to:
-- Range of molecular weights
-- Varying “packing” states (part crystalline/
part amorphous)

Question 2

What are 5 types of thermoplastics?

Answer 2

PE, PP, PS, PTFE, PVC

Question 3

What are thermosets? (2)

Answer 3

● Heavily cross-linked or networked polymers
- Lateral covalent bonds join long polymer chains.
- Consequently application of heat does not break lateral bonds - no “softening”.
● Produced by chemical reaction on mixing two components (resin + hardener).

Question 4

Why are thermoplastics linear? (2)

Answer 4

– chains not cross-linked by covalent bonds

– only by Van der Waals bonds.

Question 5

Why do thermoplastics have a range of temperatures? (2)

Answer 5

– Range of molecular weights
– Varying “packing” states (part crystalline/
part amorphous)

Question 6

Why do thermosets have heavily cross-links or networked polymers? (3)

Answer 6

• Lateral covalent bonds join long polymer chains.
• Consequently application of heat does not break lateral bonds
• no “softening”.

Question 7

What are elastomers?

Answer 7

Linear polymers with occasional cross‐links, hence elastic “memory”, and amorphous

Question 8

What are 8 factors affecting tensile properties?

Answer 8

1. Chemical compositions
2. Molecular weight/length
3. Structure
4. Tacticity
5. Polymer additives, such fillers, plasticisers…
6. Temperature
7. Strain rate
8. Environments

Question 9

How does molecular weight/length affect tensile properties? (2)

Answer 9

Increase DP (degree of polymerisation)
– Increases strength
– Increases viscosity

Question 10

How does structure affect the tensile properties? (crosslinks) (3)

Answer 10

Cross Linked (Thermosets)
– Primary lateral bonds between
chains.
– Greater degree of cross linking – leads to ‘stiffer’ more ‘stable’ structure.
– More ‘brittle’ at ambient temperature.

Question 11

How does structure affect the tensile properties? (interpenetrating polymer network (IPN)) (2)

Answer 11

– During manufacture catalytic additions can result
in intertwining network formation.
– Restricts polymer chain movements, therefore
strengthening.

Question 12

How does structure affect the tensile properties? (Linearity) (2)

Answer 12

– Linear polymers have large degree of flexibility due to independent movement of chains (van de waals & hydrogen bonding).

– Branched polymers are considered linear.
• Branches restricts polymer chain movement, therefore strengthening
• Low packing efficiency therefore low density

Question 13

How does structure affect the tensile properties? (branches) (2)

Answer 13

– Branched polymers are considered linear.
• Branches restricts polymer chain movement, therefore strengthening
• Low packing efficiency therefore low density

Question 14

What is tacticity?

Answer 14

Stereoregularity or Spatial Arrangement of R Units

Along Chain

Question 15

What are three types of tacticity?

Answer 15

Isotactic – all R groups on same side of chain Syndiotactic – R groups
alternate sides
Atactic – R groups randomly positioned

Question 16

What happens if you increase crystallinity in polymers?

Answer 16

Increasing crystallinity in polymers can lead to improved strength, modulus, stiffness/brittle, lowering toughness.

Question 17

What are liquid crystal polymers?

Answer 17

crystal formation promoted in liquid state (“self‐reinforced” plastics), high strength at high temperature.

Question 18

What are semicrystalline polymers? (2)

Answer 18

• Some semicrystalline polymers form spherulite structures

* Alternating chain‐ folded crystallites and amorphous regions

Question 19

What are two ways to increase polymer crystallinity? (2)

Answer 19

Heat treating

The molecular chemistry and chain configuration

Question 20

How does heat treating increase polymer crystallinity?

Answer 20

– Slow cooling rate during solidification causes crystalline

regions to grow and % crystallinity to increase.

Question 21

How does molecular chemistry and chain configuration increase polymer crystallinity? (4)

Answer 21

– an ordered structure is needed. The more irregular & random the repeated unit arrangement, the greater tendency for the development of non‐crystallinity
– Simple linear polymer form crystalline region even at high cooling rate
– Complex structure, and excessive branching, prevent crystallization.
– Networked and crosslinked polymers are almost totally amorphous.

Question 22

How does drawing affect a polymer?(3)

Answer 22

– increases the elastic modulus (E) in the
stretching direction
– increases the tensile strength (TS) in the
stretching direction
– decreases ductility (%EL)

Question 23

What is drawing a polymer? (2)

Answer 23

• • stretches the polymer prior to use

- - aligns chains in the stretching direction

Question 24

What happens if you do annealing after drawing? (2)

Answer 24

– decreases chain alignment
– reverses effects of drawing (reduces E and
TS, enhances %EL)

Question 25

What happens if you do annealing to undrawn materials? (2)

Answer 25

• • increased TS and E

- - Reduced %EL

Question 26

What is fracturing in thermosets like?

Answer 26

brittle fracture

Question 27

What is fracturing in thermoplastics like? (4)

Answer 27

transition between ductile and brittle, if

• • reducing temperature, or increasing strain rate, the presence of sharp notch, or anything reducing Tg.
• • Temperature falls below a specific level (0.75TG) polymers can behave like brittle solids (eg glasses and ceramics)
• • Low ductility, high viscosity, sensitive to flaws. – Can lead to “crazing” – sometimes encouraged.

Question 28

What does crazing do to a polmer?

Answer 28

Craze formation prior to cracking
– regions of localized plastic deformation
– formation of microvoids and fibrillar bridges
– molecular chains oriented (Spherulites), bridges elongated and break, causing growth and coalescence of micro‐voids

Question 29

What are some benefits to polymers? (8)

Answer 29

– Low friction coefficients.
– Low modulus allows high degree of deflection (elastic) in contact
– Light
– Design favourable for flow, less joints..
– Easy and low cost in manufacture/process – Large varieties
– Additives
– Anti corrosion