ENS 205
Materials Science I
Chapter 13: Polymers
Basics I
vinyl chloride
Poly (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.
Basics II
linear polymer
crosslinked polymer
star polymer
branched polymer
dendrimer
Polymer Structure I
Why most? Because a few
types of polymers may do
under certain circumstances
(we will see how)
amorphous
polymer
semi-crystalline
polymer
MOST CRYSTALLINE POLYMERS ARE
NOT ENTIRELY CRYSTALLINE !!!
They are either amorphous or semi-crystalline.
The degree of crystallinity varies (From 0 to 90-95 %).
Polymer Structure II
Shish-kebab morphology
SEM image of a chain having
shish-kebab morphology
SEM image of a spherulite
Polymer Structure III
Crystallinity of polymers is affected by:
All about the
chain properties
Chemical structure
Stereochemistry
Molecular weight
Temperature
Processing conditions
Hmm…
External effects
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!
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
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
Polymer Structure VI
Intermolecular forces and crystallinity
(Aromatic ring stacking)
Polymer Structure VII
• Fibers
– 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
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!
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” ?
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).
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 semicrystalline, i.e. have both
amorphous and crystalline regions
Polymer Structure XII
Crosslinking
Poly (1,4-butadiene)
Synthetic rubber
Crosslinking
with sulphur
(vulcanization)
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)
Mechanical properties
Now, I am sure that you can
rationale the mechanical behavior
of various types of polymers
shown in this image.
How to make polymers
Step growth
(Condensations)
Chain growth
(Addition)
Initiator
Monomer
Monomer (ethylene)
Monomer
H2O out
Monomer
• Free radical
• Ionic
Polyethylene
This is the plastic bag
given in the supermarket
Ring opening
(PET)
This is the bottle of your 1 lt coke
This is the breast implant of your
favorite female model
Poly(dimethyl siloxane)
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
Configuration (Chain Structure)
Copolymer
(repeating units are more than
one monomer type)
homopolymer
(repeating unit is always
same monomer)
Examples
Examples
Mechanical Testing
Temperature Dependence of
Deformation
Processing
Injection molding: http://www.bpf.co.uk/downloads/files/InjectionMoulding.swf
Blow molding: http://www.pct.edu/prep/bm.htm
http://www.youtube.com/watch?v=vSabFFQUr9E
Compression molding: http://www.ticona.com/index/tech/processing/compression_molding.htm