Polymers

Question 1

Benefits of polymers

Answer 1

• Abundant resources, low cost
• Ease of processing, versatile properties
• Low density relative to metals and ceramics
• Good strength-to-weight ratios for certain polymers
• High corrosion resistance

Question 2

Limitations of polymers

Answer 2

• low strength relative to metals and ceramics
• low modulus of elasticity (stiffness)
• service temps. limited to a few hundred degrees
• viscoelastic properties (limitation in load bearing)
• some polymers degrade when subjected to sunlight and other forms of radiation

Question 3

Plastics:

Answer 3

polymers containing a variety of additives

Question 4

Polymer composites:

Answer 4

polymers containing different types of material phases, such as inorganic fillers, metal particles etc.

Question 5

Linear polymers

Answer 5

A chain in which all of the
monomers exist in a single
line

Question 6

Branched polymers

Answer 6

Branched polymers occur when groups of units branch off from the long
polymer chain. These branches are known as side chains and can also be very
long groups of repeating structures

Question 7

Cross-linked polymers

Answer 7

Covalent bonds between the polymer molecules

Question 8

Thermoplastics:

Answer 8

• Composed of long chains formed by joining monomers (linear or branched)
• Behave in ductile, plastic manner
• Can be amorphous or semi-crystalline
• Chains have weak van der Waals bonds
• Mould above Glass transition temperature and return to
  normal state upon cooling
  – Easily recycled

Question 9

Thermosetting Polymers:

Answer 9

• Long chains of molecules strongly cross-linked to form a 3-D
  network structure
• Stronger, but more brittle than thermoplastics
• Decompose on heating instead of melting
• Not easily recyclable due to cross-linking

Question 10

Elastomers:

Answer 10

• Rubber like polymers that are thermoset or thermoplastic

- Sustain elastic deformations greater than 200%

Question 11

Thermoplastic elastomers:

Answer 11

• Special group of polymers having the processing ease of
  thermoplastics and the elastic behavior of elastomers
• e.g. styrene –butadiene-styrene block copolymer (SBS)

Question 12

Tacticity:

Answer 12

Stereoregularity or spatial arrangement of R units along chain

Question 13

What are the four types of co-polymerisation

Answer 13

random, alternating, block, graft

Question 14

Crystalline Structure of Polymers

Answer 14

• Areas in polymer where chains packed in a regular way.
• Both amorphous and crystalline areas in same polymer.
• Crystalline - regular chain structure - no bulky side groups.
• More crystalline polymer - stronger and less flexible.

Question 15

What are the conditions for crystallisation?

Answer 15

• Symmetrical chain structure

- When the side groups are small enough to fit into a crystal lattice

Question 16

What conditions or factors that affect crystallinity?

Answer 16

• Slower cooling promotes crystal formation and growth
• Mechanical deformation, as in the stretching of a heated
  thermoplastic, tends to align the structure and increase
  crystallization
• Plasticizers (chemicals added to a polymer to soften it) reduce
  crystallinity

Question 17

Polymers can crystallise upon…

Answer 17

cooling from the melt, mechanical

stretching or solvent evaporation

Question 18

Advantages of Nylon (Polyamide):

Answer 18

• Good strength
• Good toughness
• Fair heat resistance
• Good chemical resistance

Question 19

Limitations Nylon (Polyamide):

Answer 19

• Strong acidic environments
• Areas where moisture absorption is of
  concern
• Areas experiencing high operating
  temperatures

Question 20

Applications of Nylon (Polyamide):

Answer 20

• Film
• Automotive
• Electrical/electronics
• Consumer goods

Question 21

Glassy state and glass-rubber transition

Answer 21

Amorphous polymers do not have a specific melting point. At low temp., they are hard and glassy, at high temp. they are rubbery and leathery.

Question 22

What is the glass transition temp.?

Answer 22

The temperature at which the transition between glassy state and rubbery state occurs

Question 23

Structure and behaviour of thermoplastics above melting temp.

Answer 23

Liquid: easy movement of chains

Question 24

Structure and behaviour of thermoplastics below melting temp

Answer 24

• Amorphous solid: movement of chains under stress

- Crystalline solid: difficult movement of chains

Question 25

Structure and behaviour of thermoplastics below glass temp.

Answer 25

Glassy state: - Below the glass-transition temperature (a range), polymers become hard, brittle, and glass-like – Glassy polymers have poor ductility and formability, but good strength, stiffness & creep resistance

Question 26

Degradation Temp:

Answer 26

- At very high temperatures (the degradation temperature), the covalent bonds in polymer chains may be destroyed - In thermoplastics, degradation occurs in liquid state. In thermosets, degradation occurs in solid state. - Other materials are often added to polymers to retard burning. - Polymers may also degrade slowly when exposed to bacteria, UV radiation, etc

Question 27

Viscoelasticity

Answer 27

The deformation of a material by elastic | deformation and viscous flow of the material when stress is applied. The deformation under stress is time dependent.

Question 28

Relaxation time

Answer 28

Polymer properties are related to the rate | at which stress relaxation occurs

Question 29

Mech. properties of thermoplastics

Answer 29

Most thermoplastics exhibit non-Newtonian and viscoelastic behavior: – Non-Newtonian refers to a non-linear stress-strain diagram – Viscoelastic means the material undergoes both plastic and elastic deformation upon applied stress Elastic Behavior: – Applied stress causes the covalent bonds in polymer chains to stretch temporarily – Stress application also distorts entire segments of polymer chains. These may take a long time to return to original positions after stress is removed

Question 30

Viscous materials:

Answer 30

Viscous flow is not recoverable. When the stress is removed from a viscous fluid the strain remains

Question 31

Viscoelasticity:

Answer 31

- If the imposed mechanical stress is held constant then the resultant strain will increase with time, i.e. the polymer creep. - If a constant deformation is imposed then the induced stress will relax with time (stress relaxation)

Question 32

A higher degree of crystallinity leads to...

Answer 32

higher modulus and strength

Question 33

Typical elastomers:

Answer 33

- Amorphous and do not easily crystallize during processing | - Low glass-transition temperature

Question 34

Vulcanisation

Answer 34

Cross-linking elastomer chains by introducing | sulfur or other chemicals

Question 35

When an elastomer contains no cross-links, the application of a force...

Answer 35

causes both elastic and plastic deformation; after the load is removed, the elastomer is permanently deformed.

Question 36

When an elastomer contains cross-links, the application of a force...

Answer 36

leads to large elastic deformation, however the elastomer returns to its original shape when the load is removed

Question 37

Thermoplastic elastomers:

Answer 37

- This special group does not rely on cross-linking to produce elastic behavior - These copolymers behave as elastomers at low temperatures and thermoplastics at high temps - Easier to recycle than pure elastomers

Question 38

Thermosetting polymers

Answer 38

``` Thermosets are highly cross-linked polymer chains that form a 3-D network structure: - Not reusable/easily recyclable - High strength, stiffness, and hardness - Poor ductility and impact properties - High glass-transition temperature ```

Question 39

Why are manufacturing techniques for thermosets more limited?

Answer 39

because after cross-linking has occurred thermosets cannot be formed

Question 40

Recycling of plastics:

Answer 40

– Thermoplastics are easily & widely recycled, and have a numbering system to identify various types – Thermosets and elastomers are more difficult to recycle. It often happens in a secondary fashion (using shredded tires for road surfaces)

Question 41

General facts about thermoplastics:

Answer 41

- Smaller modulus, stress and temp. application - - Larger fracture toughness Kc - - Easier to form and recycle

Question 42

General facts about thermosets:

Answer 42

- Larger E, stress, Tapplication | - Smaller Kc