Characteristic properties of different material types

Question 1

What are polymers?

Answer 1

Large hydrocarbon molecules composed of many monomers

Question 2

Describe the bonding within the chains of a polymer

Answer 2

The atoms are covalently bonded and van der Waals forces exist between the chains

Question 3

Identify the different types of polymers

Answer 3

• linear
• branched
• crosslinked
• network

Question 4

Describe linear polymers

Answer 4

• repeat units joined end to end in single chains extensive van der Waals and hydrogen bonding between chains example: polyethylene, PVC
• greater packing efficiency than branched
• high density

Question 5

Describe branched polymers

Answer 5

• side branches result from reactions during polymer synthesis
• packing efficiency reduced
• lower density
  
  • LDPE-branched, weaker and cheaper than HDPE

Question 6

Describe crosslinked polymers

Answer 6

• adjacent chains joined by covalent bonds at various points
• crosslinking occurs during synthesis or chemical reactions by additive atoms or molecules during covalent bonds with the polymer chains

Question 7

Describe network polymers

Answer 7

• 3D networks formed when molecules form 3 or more covalent bonds
• can be brittle
• very rigid and hard
• retain properties at high temperatures

Question 8

Which properties of polymers are affected by chain length?

Answer 8

melting or softening temperature viscosity tensile strength young’s modulus

Question 9

Calculating the number average molecular weight

Answer 9

Formula:

Question 10

Calculating the weight average molecular weight

Answer 10

Formula:

Question 11

Identify the different types of plastics

Answer 11

thermoplastics thermosetting elastomers/rubbers

Question 12

Characteristics of thermoplastics

Answer 12

composed of long polymer chains with/without branches limited cross-linking chains tangled but easily detangled when tensile stress is applied emorphous or crystalline soften and melt on heating easily recycled

Question 13

Characteristics of thermosetting

Answer 13

long polymer chains substantial amount of cross-linking between chains amorphous structure usually stronger than thermoplastics more brittle decompose on heating difficult to reprocess after formation difficult to recycle

Question 14

Characteristics of elastomers/rubber

Answer 14

capable of sustaining very large plastic deformations thermoplastics or lightly cross-linked thermosets

Question 15

What happens in a polymer when the temperature is greater than the melting temperature?

Answer 15

the polymer is a liquid and the chains have enough energy to move easily

Question 16

Define crystallisation

Answer 16

an ordered or partially ordered crystalline solid phase forms

Question 17

Define a nuclei

Answer 17

regions of the polymer that become ordered and aligned thickness remains constant increased spherulitic radius

Question 18

Describe what happens at the glass transition temperature

Answer 18

Some polymers cool below the melting temperature to an amorphous solid state which is stable, rigid but not ordered

Question 19

Draw a graph showing polymer transitions

Answer 19

Graph

Question 20

Describe the transition of glass or amorphous polymer

Answer 20

it becomes more viscous and rubbery during cooling elastic modulus, specific volume and thermal expansion change significantly around the glass transition temperature

Question 21

Describe the transition of semi-crystalline polymers

Answer 21

change is limited to the amorphous regions the glass transition temperature corresponds to the temperature at which rotation about single bonds is possible due to the increased temperature

Question 22

Describe the transition of crystalline solids

Answer 22

at the melting temperature, there is a molecular rearrangement from ordered to a disordered state

Question 23

Vulcanisation of rubbers

Answer 23

crosslinking in rubbers using sulphur compounds stiffness is determined by the degree of cross-linking

Question 24

Why is cured rubber hard to recycle

Answer 24

sulphur compounds link the long-chain molecules irreversibly

Question 25

Define an amorphous material

Answer 25

It is a material with no long-range periodicity in its structure(no unit cell)

Question 26

How is glass made?

Answer 26

It is made by melting the components then quickly cooling the melt it then solidifies but the atoms do not have time to crystallise a disordered solid state is formed

Question 27

Atomic environments in glass

Answer 27

Picture

Question 28

Characteristics of vitreous silica

Answer 28

• the main component of optical glass and the archetypal glass-forming system
• seems disordered but has a lot of local order
• Si almost always has oxygen neighbours arranged in a rough tetrahedron each oxygen bonds to 2*SO4 tetrahedra
• the disorder appears in the Si-O-Si bond angle distribution

Question 29

Automotive glass

Answer 29

float glass for all glazing

almost all glass is laminated to prevent shattering

Other properties

• tinted
• self-heating
• structural
• reflect infrared-control temperature

Question 30

Define a hybrid material

Answer 30

A material that contains more than one underlying material

Question 31

Different hybrid materials

Answer 31

Question 32

Sandwiches are useful due to their…..

Answer 32

weight strengthening(particularly bending) sound and thermal expansion

Question 33

Particulates can be used in……

Answer 33

cutting and grinding electrical contracts dispersion strengthened metals polymers-filling agents

Question 34

Carbon brake disks SiC with carbon fibres are used to improve……..

Answer 34

resilience heat conduction longevity operating conditions/temperature weight

Question 35

Foam

Answer 35

highlight unrealisable material properties very low density used in insulation

Question 36

Segmented materials

Answer 36

Uses: capacitors ropes and wires bimetallics car windscreens