Polymers

Question 1

Polymerisation

Answer 1

Many monomer molecules react together chemically to form a polymer

Question 2

Molecular weight

Answer 2

The number of monomers in the chain multiplied by the molecular weight of the polymer

Question 3

Chain network of thermoplastics

Answer 3

Linear or branched chain so melt on heating

Question 4

Chain network of elastomers

Answer 4

Few crosslinks so remain elastic

Question 5

Chain network of thermosets

Answer 5

Many crosslinks which add rigidity and resistance to melting

Question 7

Forces in linear polymers

Answer 7

Van der Walt’s and hydrogen bonding between chains

Question 8

Density of branched polymers

Answer 8

Less dense

Question 9

Cross linking

Answer 9

Adjeacent linear chains are joined one to another at various points by covalent reactions

Question 10

Vulcanization

Answer 10

The crosslinks of rubber elastic materials

Question 11

Semi-crystalline polymers- achieved by some thermoplastics

Answer 11

Contain both crystalline and amorphous regions. Amorphous- linear chains like spaghetti with little to no order to structure. Crystalline- linear chains folding in a repeatable manner to create regular packing between chains- therefore thermosets are amorphous

Question 12

What causes polymers to be amorphous

Answer 12

Long branch chains that prevent crystalline regions from forming

Question 13

Bonding in polymers

Answer 13

Covalent along chain. Weak van der waals or hydrogen between them.

Question 14

Why are thermoplastics characterised as low stiffness and relatively low strength

Answer 14

As force is applied chains will disentangle and chain slippage occurs. However chains may eventually straitened out and stretch and the strength and stiffness is controlled by strength of c-c bond.

Question 16

Characteristics of thermosets and elastomers

Answer 16

Higher stiffness but lower toughness than thermoplastic (more brittle). Elastomers can have low stiffness

Question 17

Stress strain curve of different types of polymers

Question 18

As it is hard to measure young modules of polymers due to non linear region, what is used

Answer 18

Secant or tangent modulus used. Secant line typically drawn at 0.2-1% strain.

Question 19

As yield stress and tensile strength is hard to measure for in polymers due to non linear behaviour what is used

Answer 19

Yield stress defined as local maximum load following yield and the tensile strength as the stress at fracture

Question 20

Impact of temp on polymers

Answer 20

At lower temps they become stiffer, more brittle snd stengthen

Question 21

Stress strain graph for polymers at different temps

Question 22

Graph of specific volume of polymers against temp

Question 23

Why is there an increase in the volume of a polymer with temp

Answer 23

For crystalline- molecules are tightly packed then breakdown into an amorphous liquid (Tm) at melting. For amorphous- more free volume. As temp increases there is an increase in rate of change of specific volume due to melting of secondary bonds and is known as glass transition (Tg). At Tg it is soft and rubbery. A above Tg behaves as a viscous liquid. Semi crystalline- crystalline regions behave as crystalline, amorphous behave as amorphous. Will have mix of characteristics of Tg and Tm. Shows smaller reduction in stiffness as Tg than Tm (dominantly dependent on crystalline)

Question 24

Graph for strain response of an elastic material, viscoelastic and viscous material