Topic 2

Question 1

Describe polymers in the solid state

Answer 1

Crystalline regions; amorphous regions; rarely fully crystalline

Question 2

What is Tg?

Answer 2

Glass transition temperature (temperature at which a material undergoes a transition from a glassy to a rubbery state (or vice versa)). When the glass temperature has been reached, the stiffness stays the same for a while until the temperature exceeds Tm and the material melts

Question 3

What is Tm?

Answer 3

Melting transition temperature (or crystalline melting temperature); S –> L

Question 4

What is an elastomer/rubber?

Answer 4

Material with the mechanical (or material) property that it can undergo much more elastic deformation under stress than most materials and still return to its previous size without permanent deformation. Materials are generally soft - sometimes crosslinked.

Question 5

What is the Tg for useful rubbers?

Answer 5

< 0 degrees C

Question 6

What is plastic/glass?

Answer 6

Polymers in the glassy state are stiff immobile materials that do not readily undergo elastic deformation. Plastic is a material capable of being shaped through plastic flow by application of deforming forces. Usually a high MW organic material

Question 7

What is the Tg for useful plastics?

Answer 7

> 60 degrees C (i.e. doesn’t deform at room temperature)

Question 8

Describe the transitions for polymers with different morphology

Answer 8

Amorphous - only Tg (unable to crystallise); completely crystalline - only Tm; semi crystalline - both (Tg and Tm)

Question 9

True or false - “melted” polymeric materials are always above their Tg.

Answer 9

True - a polymer is defined as being melted when it is above its Tg.

Question 10

What types of transition are Tg and Tm?

Answer 10

Tg - endothermic, 2nd order; Tm - endothermic, 1st order

Question 11

Why does a rubber band get hot when stretched?

Answer 11

Due to crystallisation

Question 12

What is the rate of change in the molecular weight of a step growth polymer at lower conversion and higher conversion respectively?

Answer 12

Decreases and then increases

Question 13

How can you determine the Tg point on a graph?

Answer 13

Change in slope occurs

Question 14

How does free volume affect Tg?

Answer 14

Free volume = the space in a solid/liquid which is not occupied by polymer molecules. In a liquid, free volume is high so conformations can change readily. Reduction in temperature reduces amount of thermal energy. As temperature is lowered, free volume is lowered until there is not enough free volume to allow for molecular translation/rotation. This temperature is the Tg as the polymer is frozen

Question 15

What factors affect Tg?

Answer 15

Chain flexibility/stiffness; sterics; tacticity; molecular architecture (MW, branching, crosslinking); polarity; symmetry; rate of heating/cooling

Question 16

How does stiffness affect Tg?

Answer 16

Stiff chains = higher Tg as more energy needed to allow them to move past each other and achieve rubbery state. Chain stiffness is increased when a ring is incorporated into the backbone (reduces rotation and no of conformations)

Question 17

How does sterics affect Tg?

Answer 17

Size of R important. Bigger R/side group = higher Tg

Question 18

How does MW affect Tg?

Answer 18

Initial linear relationship between MW and Tg but then levels off i.e. dependence only for low MW/short chain polymers

Question 19

How does symmetry affect Tg?

Answer 19

Increase in symmetry reduces Tg. Free volume effect. Harder to pack, larger free volume so less energy to create free volume for chain rotation (higher free volume reduces Tg)

Question 20

How does polarity affect Tg?

Answer 20

Increased polarity increases Tg. Increased amount of polar bonding, chains held tighter together so need more energy to break to rubbery state

Question 21

How does branching affect Tg?

Answer 21

Low levels reduces Tg due to changes in free volume (branches lead to less end groups). High levels = Tg raises as same effect as side chains in restricting chain mobility

Question 22

How does crosslinking affect Tg?

Answer 22

Reduces specific volume of polymer and hence free volume is reduced so Tg is raised as molecular motions are more difficult

Question 23

How does MW affect Tg?

Answer 23

Key assumption that there is more free volume associated with a chain end than a segment in the middle part of a chain. Shorter polymer = more ends

Question 24

What happens at the melting point?

Answer 24

Liquid and solid phases are in thermodynamic eqm

Question 25

What factors affect Tm?

Answer 25

Symmetry; intermolecular interactions; flexibility; tacticity; branching; MW

Question 26

How does symmetry affect Tm?

Answer 26

Increased symmetry reduces conformation freedom and increases Tm

Question 27

How do intermolecular interactions affect Tm?

Answer 27

Increased interactions = higher Tm

Question 28

How does flexibility affect Tm?

Answer 28

Rigidity increases Tm

Question 29

How does tacticity (steric arrangement) affect Tm?

Answer 29

Same as Tg - larger side group = higher Tm

Question 30

How does branching affect Tm?

Answer 30

A small amount lowers Tm

Question 31

How does MW affect Tm?

Answer 31

Same as Tg

Question 32

How does chain flexibility/stiffness affect Tm?

Answer 32

Chain flexibility increases as Tm increases

Question 33

How do polar groups affect Tm?

Answer 33

Polar groups raise Tm as intermolecular bonding increases assists in crystallisation

Question 34

Describe the relationship between Tg and Tm

Answer 34

Tg (reported in K) < Tm. Generalisation: Tg/Tm = 0.5 for symmetrical polymers; Tg/Tm = 0.75 for unsymmetrical polymers; Tg = 0.5 to 0.8 Tm. Tm more dependent on molecular symmetry. Tg more dependent on secondary forces and chain flexibility

Question 35

When do you get a glassy polymer?

Answer 35

When Tg is above room temperature

Question 36

What is DSC?

Answer 36

Differential scanning calorimetry. Measure the change in heat capacity of a sample as a function of temperature