Transcript Chapter 14. Polymer Structures (1)

Introduction to Materials Science, Chapter 14, Polymer Structures
Chapter Outline: Polymer Structures
 Hydrocarbon and Polymer Molecules
 Chemistry of Polymer Molecules
 Molecular Weight and Shape
 Molecular Structure and Configurations
 Copolymers
 Polymer Crystals
 Optional reading: none
Greek polumeres, which means `having many
parts'
Google Polymers
http://www.psrc.usm.edu/macrog/index.htm
"I am inclined to think that the development of
polymerization is perhaps the biggest thing chemistry
has done, where it has had the biggest impact on
everyday life."
-Lord Todd (b. 1907), president of the Royal Society of London
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Introduction to Materials Science, Chapter 14, Polymer Structures
Polymers: Introduction
 Polymer:large molecule consisting of repeated
chemical units (`mers') joined like beads on a
string. Polymers usually contain many more
than five monomers; some contain hundreds or
thousands of monomers in each chain.
 Polymers are natural, such as cellulose or DNA,
or synthetic, such as nylon or polyethylene.
Silk fibre produced by
silk worms in a cocoon
to protect it while it
metamorphoses into a
moth.
Many of the most important current research
problems involve polymers. Living organisms are
mainly composed of polymerized amino acids
(proteins) nucleic acids (RNA and DNA), and other
biopolymers. Our brains are mostly a complex
polymer material soaking in salty water!
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Introduction to Materials Science, Chapter 14, Polymer Structures
Hydrocarbon molecules (I)
 Most polymers are organic, and formed from
hydrocarbon molecules
 Each C atom has four e- that participate in
bonds, each H atom has one bonding eExamples of saturated (all bonds are single ones)
hydrocarbon molecules:
Methane, CH4
Ethane, C2H6
Propane, C3H8
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Introduction to Materials Science, Chapter 14, Polymer Structures
Hydrocarbon molecules (II)
Double and triple bonds can exist between C atoms
(sharing of two or three electron pairs). These
bonds are called unsaturated bonds. Unsaturated
molecules are more reactive
H-CC-H
Ethylene, C2H4
Acetylene, C2H2
Isomers are molecules that contain the same atoms
but in a different arrangement. An example is
butane and isobutane:
Butane  C4H10  Isobutane
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Introduction to Materials Science, Chapter 14, Polymer Structures
Hydrocarbon molecules (III)
Other organic groups can be in a polymer.
R represent radical: organic groups that
remain a unit during reactions
(e.g. CH3, C2H5, C6H5)
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Introduction to Materials Science, Chapter 14, Polymer Structures
Polymer molecules
 Polymer molecules
macromolecules
are
very
large:
 Polymers are long and flexible chains
with a string of C atoms as a backbone.
 Side-bonding: C to an H or a radical
 Double bonds possible in both chain and
side bonds
 Repeat unit (“unit cell”) is a mer
 A single mer is called a monomer
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Introduction to Materials Science, Chapter 14, Polymer Structures
Plastics
Some ubiquitous `mers' are ethylene,
styrene and acrylamide.
Each polymerized to make:
polyethylene (the soft clear plastic that plastic
bags are made of),
polystyrene (stiffer, usually white plastic that
the covers for soft-drink cups are made of)
polyacrylamide (very tough, clear plastic that
compact discs are made from).
On bottom of recyclable plastic bottle - you
will see PE (polyethylene) or PS (polystyrene)
Examples of what happens to polymers when
they solidify: chains are entangled and packed
together to make light, tough, flexible
materials
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Introduction to Materials Science, Chapter 14, Polymer Structures
A way to think about some of these materials is to
think of what a big glob of cooked spaghetti is like.
Stretch it a bit, it is kind of elastic, but if you pull
hard, the noodles start to slide past one another
and the whole glob starts to permanently deform.
Does this remind you of what happens to a PE
plastic bag when you stretch it? Think about what
must be happening to the microscopic spaghetti
that the bag is made up of!
If you heat up PE or PS to moderate temperatures,
if the chains have not been chemically stuck
together (`cross-linked') they will melt, and turn
into goopy liquids, which are called polymer melts.
Some polymers are melts even at room
temperature. Remembering that paper is made of
cellulose, which is a polymer of biological origin, if
you look around the room that you are in, you will
see that a good fraction of the stuff in it is made of
polymers. And of course, you are, too!
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Introduction to Materials Science, Chapter 14, Polymer Structures
Chemistry of polymer molecules (I)
 Ethylene (C2H4) a gas at room T and P
 Ethylene-->polyethylene (solid) though active
mer formed by reaction with initiator / catalytic
radical (R.)
 (.) denotes unpaired electron (active site)
Polymerization:
1.Initiation reaction:
2. Rapid propagation ~1000 mer units in 1-10 ms:
3. Termination: two active chain ends meet or active
chain end meet with radical (R.)
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Introduction to Materials Science, Chapter 14, Polymer Structures
Chemistry of polymer molecules (II)
Replace hydrogen
in polyethylene
polytetraflouroethyl
ene (PTFE) – Teflon
Replace
every
fourth hydrogen in
polyethylene with
Cl
 polyvinyl
chloride
Replace
every
fourth hydrogen in
polyethylene with
CH3 methyl group
 polyproplylene
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Introduction to Materials Science, Chapter 14, Polymer Structures
Chemistry of polymer molecules (III)
 When all mers are the same, molecule is called
homopolymer
 When there is more than one type of mer
present, the molecule is a copolymer
 Mer units that have 2 active bonds to connect
with other mers are called bifunctional
 Mer units that have 3 active bonds to connect
with other mers are called trifunctional. Form
3-D molecular network structures.
Polyethilene
(bifunctional)
Phenol-formaldehyde
(trifunctional)
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Introduction to Materials Science, Chapter 14, Polymer Structures
Molecular weight (I)
 Final molecular weight (chain length) controlled
by relative rates of initiation, propagation,
termination steps
 Formation
of
macromolecules
during
polymerization results in distribution of chain
lengths and molecular weights
 Average molecular weight obtained by
averaging masses with fraction of times they
appear (number-average molecular weight) or
with mass fraction of the molecules (weightaverage molecular weight).
number-average:
Mn   x i Mi
weight-average:
M w   w i Mi
wi is weight fraction of chains of length i
xi is number fraction of chains of length i
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