ENS 205 Materials Science I Chapter 13: Polymers

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ENS 205Materials Science IChapter 13 Polymers
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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)
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Basics II
linear polymer
crosslinked polymer
star polymer
branched polymer
dendrimer
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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 90-95 ).
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Polymer Structure II
Shish-kebab morphology
SEM image of a spherulite
SEM image of a chain having shish-kebab morphology
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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
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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!
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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
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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
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Polymer Structure VI
Intermolecular forces and crystallinity
(Aromatic ring stacking)
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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
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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!
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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 ?
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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 wont 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).

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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
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Polymer Structure XII
Poly (1,4-butadiene)
Crosslinking
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)
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Mechanical properties
Now, I am sure that you can rationale the
mechanical behavior of various types of polymers
shown in this image.
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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)
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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
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Configuration (Chain Structure)
Copolymer (repeating units are more than one
monomer type)
homopolymer (repeating unit is always same
monomer)
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Examples
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Examples
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Mechanical Testing
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Temperature Dependence of Deformation
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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?vvSabFFQUr9E Compres
sion molding http//www.ticona.com/index/tech/pro
cessing/compression_molding.htm