Polymer 2

Question 1

Influence of the chemical composition of a polymer

Answer 1

chemical composition of a polymer influences the forces of attractions between polymer chains and how closely the chains can pack. These can have a fundamental impact on the physical properties of the polymer

Question 2

The physical properties of polymers

Answer 2

·Therefore the physical properties of polymers are almost infinitely variable i.e. they can be

Øtough or brittle

Østrong and durable

Ørigid and inflexible

Øflexible and rubbery

Øhard or soft.

·

·These properties can also vary with temperature – this allows polymers to be processed

Question 3

Below Tg

Above Tg

Answer 3

Below Tg polymer chains are effectively ‘frozen’ in place and the polymer is described as glassy.

Motion is restricted to short-range vibrations and rotations. The polymer is hard, inflexible, and brittle.

Above Tg the chains possess enough thermal energy to allow long range molecular motion, chains can move past each other.

Polymer chains are rubbery, can flow as a viscous liquid and be processed.

Question 4

Tg rule

Answer 4

REMEMBER the glass transition is a phenomena associated with amorphous polymer or the amorphous domains of semi-crystalline polymers

Question 5

What determines the Tg of a polymer?

Answer 5

The Tg increases with increasing chain stiffness (the stiffer the chains are, the harder it will be for the chains to move).

Chain stiffness is induced in polymer chains that have aromatic units in the polymer backbone or polymers with bulky substituents

Question 6

Tg increases when

Answer 6

The Tg also increases with increasing forces of intermolecular attraction (the stronger the forces holding the chains in place, the more energy required for the chains to move)

Question 7

Tg of crosslinked networks

Answer 7

It is obvious that linear and branched polymers have a glass transition temperature but what about crosslinked networks? Discuss!

Depends on chemical composition of polymer chains and crosslink density!

Question 8

Chain and crosslink in elastomers and thermosets

Answer 8

Question 9

intermolecular forces between chains in crystalline regions

Answer 9

·In the crystalline regions of semi-crystalline polymers, the intermolecular forces between chains are optimized, and these forces hold the chains firmly in place

Tm is always higher than Tg

This is despite the fact that 100% crystalline material would have the lowest energy, i.e. it is the most thermodynamically stable form (because all the intermolecular forces have been optimized).

Question 10

Why don’t we get 100% crystalline polymers?

Answer 10

·In the melt the polymer chains will be in a disordered, random conformation - ‘random coils’.

·As polymer melt cools to below Tm, it starts to crystallize. This occurs at Tc, the crystallization temp.

·However, it would take eons for all of chains to unravel, align and form a 100% crystalline polymer.

·Hence crystallization is under kinetic rather than thermodynamic control, and the degree of crystallinity that is obtained depends upon the crystallization conditions (cooling rate)

Question 11

Semi-crystalline polymers and Crystallinity

rapidly reduce the temperature of the polymer melt:

cool the polymer more slowly:

Typical values of crystallinity range:

Why are Semi-crystalline polymers important materials

Answer 11

So if we rapidly reduce the temperature of the polymer melt (called quenching), the degree of crystallinity will be low because the chains will have little time to rearrange into ordered crystalline regions before Tg is reached and all the chains become frozen in place.

On the other hand if we cool the polymer more slowly, chains have more time to unravel and the degree of crystallinity is enhanced

Typical values of crystallinity range from ~30-70%depending upon the nature of the polymer and the crystallisation conditions.

Semi-crystalline polymers are important materials due to their superior mechanical properties and better chemical, wear and heat resistance than their amorphous analogues.

Semi-crystalline polymers are used for making fibres for clothing and textiles, as well as bulk plastic products.

Question 12

What determines the Tm of a polymer?

Answer 12

Question 13

Chain branching in semi-crystalline polymers

Answer 13

Question 14

Tacticity

Answer 14

refers to the arrangement of side groups along a polymer chain.

Question 15

3 types of tacticity

Answer 15

Question 16

Packing of isotactic and syndiotactic

Answer 16

·Isotactic and syndiotactic polymers both have a regularorientation of side groups and so can pack in a regular manner to form crystalline regions

à gives rise to semi-crystalline polymers

Question 17

packing in atactic

Answer 17

·Atactic polymers have a random orientation of side groups which prevents regular packing

·

à gives rise to amorphous polymers

Question 18

Properties of Atactic

Answer 18

Atactic polypropylene is an amorphous, soft waxy polymer that becomes very sticky upon heating; it has no commercial uses.

Question 19

Properties of isotactic

Answer 19

Isotactic polypropylene is a semi-crystalline polymer (c. 65% crystalline, m.pt ~ 160o C) that is tough and has excellent chemical resistance.

It is used widely for packaging, textiles, carpets fibres, automotive parts and much more.

Question 20

Structural/chemical features which effect Tg and Tm

Answer 20

Ø Intermolecular forces

Ø Chain stiffness

Ø Chain packing – tacticity and chain branching