1. ENS 205 Materials Science I Chapter 13: Polymers

2. Basics I
vinyl chloride
A polymer is a substance composed of molecules with large molecular mass composed of repeating structural units (called monomers), connected by covalent chemical bonds.
Poly (vinyl chloride)

3. Basics II linear polymer crosslinked polymer star polymer
branched polymer
dendrimer

4. Polymer Structure I
Why most? Because a few types of polymers may do under certain circumstances (we will see how)
semi-crystalline
polymer
amorphous
polymer
MOST CRYSTALLINE POLYMERS ARE NOT ENTIRELY CRYSTALLINE !!!
They are either amorphous or semi-crystalline.
The degree of crystallinity varies (From 0 to %).

5. Polymer Structure II Shish-kebab morphology SEM image of a spherulite
SEM image of a chain having shish-kebab morphology

6. Polymer Structure III Crystallinity of polymers is affected by:
Chemical structure
Stereochemistry
Molecular weight
Temperature
Processing conditions
All about the chain properties
Hmm…
External effects

7. Is polystyrene flat like this?
Polymer Structure IV
STEREOCHEMISTRY
(Tacticity)
This is the white, plastic coffee cup used everywhere like Fassane. Strafor kopuk
Is polystyrene flat like this?
Absolutely NO!

8. Polymer Structure V STEREOCHEMISTRY (Tacticity)
In the previous picture you see all the phenyl groups are located on the same side of the polymer chain. But they don't have to be this way. To illustrate let's look at a chain of polystyrene from above. You can see that the pendant phenyl groups can be either on the right or left side of the chain.
All phenyl groups on the same side
Phenyl groups on alternating sides
Phenyl groups distributed randomly

9. Polymer Structure V STEREOCHEMISTRY (Tacticity)
The question is, how tacticity helps crystallinity
CRYSTALLINITY ↔ LONG RANGE ORDER ↔ PACKING
Syndiotactic polystyrene: Highly crystalline
Atactic polystyrene: Highly amorphous
Similarly, a linear polymer can pack well, whereas a branched isomer cannot
Highly crystalline
Highly amorphous

10. Intermolecular forces and crystallinity
Polymer Structure VI
Intermolecular forces and crystallinity
(Aromatic ring stacking)

11. Polymer Structure VII Fibers Can you stretch this structure???
Polymers with regular structure can align themselves very closely for effective utilization of the secondary intermolecular bonding forces.
Already stretched out fully, up to 95% crystallinity
High symmetry, high intermolecular forces.
Characterized by high modulus, high tensile strength, and moderate extensibilities
Can you stretch this structure???
High density polyethylene
Polypropylene
Nylon
Polyester
Kevlar and Nomex
Polyacrylonitrile
Cellulose
Polyurethanes

12. Polymer Structure VIII
Elastomers (rubbers)
Polymers with irregular structure, weak intermolecular attractive forces and flexible chains.
Can undergo local mobility, but gross mobility of chains is restricted.
Characterized by high extensibility, low initial modulus in tension but they stifen when strecthed.
stretch
leave
ENTROPY WORK!

13. Polymer Structure IX Plastics
Fall between the elastomers and fibers. However there is no exact boundary
Harder to stretch than elastomers (Because of crystalline regions?). But preserve their shape when stretched unlike elastomers (Strain induced crystallization, stiff chains)
They are pliable, that is, they can be shaped and molded easily
Thermoplastics: Melt when heated and can be melted again after cooling
Thermosets: Undergoes crosslinking when heated, so does not melt again, decomposes if heated further
Flexible plastics: Plastics above their Tg. Flexible, soft
Rigid plastics: Plastics below their glass transition temperature (Tg). Brittle, hard
What are Tg, crosslinking and “melting for polymers” ?

14. Polymer Structure X Glass transition temperature (Tg)
Different polymers have different segments on their backbones. The ease of movement of these segments (portions of the chain) depends on the structure, physical environment of the chain etc. of the segment.
Any movement of these segments require energy which is kinetic in this case, right? Then each different polymer would have different energy requirement for the movement of these segments (different polymer = different structure, different physical environment of the chain etc).
Below glass transition temperature, these segments do not have sufficient energy to move. So, if you apply some stress, say if you try to bend a polymer which is below its Tg then the segments won’t be able to move into new positions to relieve the stress which you have placed on them; which will make the polymer brittle. Above Tg they would, so they would be flexible.
Always keep this in mind: Tg IS A PROPERTY RELATED WITH THE AMORPHOUS REGIONS OF THE POLYMER, NOT CRYSTALLINE!
So it should now be obvious that elastomers are elastomers above their Tg. Below, they are not elastomers, they are glassy, because they are not flexible anymore (Remember my experiment with rubber glove and liquid nitrogen during the lecture).

15. Polymer Structure XI Melting
Melting is a transition which occurs in crystalline polymers.
Melting happens when the polymer chains fall out of their crystal structures, and become a disordered liquid.
Always keep this in mind: MELTING IS A PROPERTY RELATED WITH THE CRYSTALLINE REGIONS OF THE POLYMER! So do you think you can melt atactic polystyrene? (No, because it is not crystalline)
Question: What if I see both melting and glass transition in the differential scanning calorimeter (DSC) spectrum of a polymer sample???
It is absolutely OK. Remember, most polymers are semi-crystalline, i.e. have both amorphous and crystalline regions

16. Crosslinking with sulphur
Polymer Structure XII
Crosslinking
Poly (1,4-butadiene)
Crosslinking with sulphur
(vulcanization)
Synthetic rubber
This is the tire of your car
Thus, it is possible to produce elastomers via crosslinking!
(In fact, it is not only possible but also the very common way of making elastomers, i.e. rubber)

17. Mechanical properties
Now, I am sure that you can rationale the mechanical behavior of various types of polymers shown in this image.

18. Poly(dimethyl siloxane)
How to make polymers
Step growth
(Condensations)
Chain growth
(Addition)
Monomer
Monomer
Monomer (ethylene)
Initiator
H2O out
Monomer
Polyethylene
This is the plastic bag given in the supermarket
(PET)
Free radical
Ionic
This is the bottle of your 1 lt coke
Ring opening
This is the breast implant of your favorite female model
Poly(dimethyl siloxane)

19. Molecular weight
Not all of the chains of a polymer are of same length. Their size differ most of the time.
Remember: A chain is a polymer molecule (in fact the chain is the polymer itself), so the molecular weight of a polymer should in fact be the molar mass of a single polymer chain. However, since chains have different lengths (that is why we call polymers as polydisperse) we can only talk about averages
Number average molecular weight =
Weight average molecular weight =
Polydispersity index = Mw / Mn
Ni is the number of chains having molecular weights Mi

20. Configuration (Chain Structure)
Copolymer
(repeating units are more than one monomer type)
homopolymer
(repeating unit is always same monomer)

21. Examples

22. Examples

23. Mechanical Testing

24. Temperature Dependence of Deformation

25. Processing
Injection molding:
Blow molding:
Compression molding: