Transcript ENG2000 Chapter 2 Structure of Materials

ENG2000 Chapter 5
Polymers
ENG2000: R.I. Hornsey
Poly: 1
Overview
• In this chapter we will briefly discuss the material
properties of polymers
 starting from the basic construction of a polymer molecule
 and finishing with the stress-strain relationship
• A full treatment of the chemistry and the
mechanical properties of polymers it too
extensive for this course
 further reading can be found in Callister chapters 14 and 15
ENG2000: R.I. Hornsey
Poly: 2
Polymers
• You may think of polymers as being a relatively
modern invention
 however naturally occurring polymers have been used for
thousands of years
 wood, rubber, cotton, wool, leather, silk
• Artificial polymers are, indeed, relatively recent
and mostly date from after WWII
 in many cases, the artificial material is both better and
cheaper than the natural alternative
• We start by considering the basics of organic
molecules
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Hydrocarbon molecules
• Hydrocarbons
 hydrogen and carbon, bonded covalently
• Simplest are methane, ethane, propane, butane
 CnH2n+2, the paraffin family
 where each carbon shares an electron either with another
carbon or with a hydrogen
• Alternatively, a carbon can share two electrons
H H
with another carbon atom
 a double bond
 hence ethylene, C2H4
• And triple bonds are also possible
 e.g. acetylene, C2H2
ENG2000: R.I. Hornsey
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C= C
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H H
H–CC–H
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• Most hydrocarbon molecules are unsaturated
 i.e. have less than the maximum of 4 neighbouring atoms
(either H or C)
 in unsaturated molecules, other atoms may be attached
without removing existing atoms, because there are
‘available’ bonds
• Saturated molecules have entirely single bonds
 and no other atoms may be attached without first removing
an existing atom
• Bonds between the hydrocarbon molecules are
the weak van der Waals bonds
 so the boiling point is very low (e.g. -164°C for methane)
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Poly: 5
Isomerism
• Molecules with identical chemical compositions
may have more than one bonding arrangement
 e.g. butane, and isobutane
H H H H
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H–C–C–C–C–H
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H H H H
H
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H–C–H
H H H
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H–C–C–C–H
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H H H
• Physical properties of isomers are different
 e.g. boiling point for normal butane is -0.5°C, whereas that
for isobutane is -12.3°C
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Polymer molecules
• Sometime called macromolecules because of
their huge size, polymers consist of chains of
carbon atoms
 which form the backbone of the molecule
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– C – C – C – C – C – C – C – C –C – C – C – C –
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 each of the two remaining valence electrons may bond with
other atoms, side chains, or form double bonds, etc
• Since poly-mer means “many mers”, the basic
unit is known as a mer
 which comes from the Greek for ‘part’
 monomers are the stable molecules from which polymers
are synthesised
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Chemistry of polymers
• So how is a polymer formed from the monomer?
• Consider ethylene (a gas) again
H H
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C= C
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H H
 the polymer form is polyethylene, which is a solid at room
temperature
• The reaction is initiated by an initiator, R·
H H
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R· + C = C
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H H
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
H H ‘spare’ electron
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R – C –C ·
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H H
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• The active (spare) electron is transferred to the
end monomer, and the molecule grows
H H
H H
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R – C –C · + C= C
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H H
H H
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H H
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R–C –C–
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H H
H H
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C–C·
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H H
• The 3-D structure is
http://cwx.prenhall.com/bookbind/pubb
ooks/hillchem3/medialib/media_portfol
io/text_images/CH09/FG09_17.JPG
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Poly: 9
• The angle between the bonded C atoms is close
to 109°, and the bond length is 1.54Å
• We can replace all the H atoms in polyethylene by
fluorine atoms
 which also have one valence electron
• The result is polytetrafluoroelthyene (PTFE)
 marketed with the trade name teflon
 this type of material is a fluorocarbon
• Anothe common polymer is polyvinyl chloride
(PVC)
H H H H H H H H
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C –C–
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H Cl
ENG2000: R.I. Hornsey
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C – C– C – C –
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H Cl H Cl
mer unit
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C– C
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H Cl
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Other polymer forms
• The materials we have considered so far are
homopolymers
 all the mer units are identical
• Copolymers consist of mers of two or more types
• Polymers may also grow in three dimensions
 called trifunctional
 polyethylene is bifunctional and grows in 2-D
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Molecular weight
• Very large molecular weights are common for
polymers
 although not all chains in a sample of material are the same
length, and so there is a distribution of molecular weights
number average,
amount of
polymer
M n   xi M i
Mi is mean weight in size range, i
xi is the fraction of total number of chains in size range, i
wi is the fraction of total weight in size range, i
weight average, M w
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  wi M i
molecular weight
Poly: 12
Molecular shape
• If the form of the molecule was strictly
determined, polymers would be straight
 in fact, the 109° bond angle in polyethylene gives a cone of
rotation around which the bond lies
109°
• Hence the polymer chain can bend, twist, and
kink into many shapes
 and adjacent molecules can intertwine
 leading to the highly elastic nature of many polymers, such
as rubber
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http://www.accelrys.com/consortia/polymer/permod/polypai.jpg
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Molecular structure
• Linear polymers
 long, ‘straight’, flexible chains with some van der Waals or
hydrogen bonding
• Branched polymers
• Crosslinked polymers
 cross linkage happens either during synthesis or in a
separate process, typically involving addition of impurities
which bond covalently
 this is termed vulcanisation in rubber
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Crystallinity in polymers
• Although it may at first seem surprising,
Polymers can form crystal structures
 all we need is a repeating unit
 which can be based on molecular chains rather than
individual atoms
• Polyethylene forms an orthorhombic structure
http://www.lboro.ac.uk/departments/ma/gallery/molecular/Molecular/pollat.gif
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• Small molecules tend to be either crystalline
solids or amorphous liquids throughout
 e.g. water, methane
• This is more difficult to achieve with very large
polymer molecules
 so a sample tends to be a mixture of crystalline and
amorphous regions
 [this is true of most materials in any form other than thin films
because it is hard to freeze a whole lump of material quickly
enough to make it all amorphous]
• Linear polymers more easily form crystals
because the molecules can orient themselves
readily
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Stress-strain relation
• There are three typical classes of polymer stressstrain characteristic
stress (MPa)
6
brittle
4
plastic
highly elastic – elastomeric
2
0
0
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2
4
6
8
strain
Poly: 18
Viscoelastic deformation
• An amorphous polymer can display a number of
characteristics, depending on the temperature
 glass at low T
 rubbery solid at intermediate T
 viscous liquid at high T
• Some materials display a combination of elastic
and viscous properties at an intermediate
temperature
 these are termed viscoelastic
 ‘silly putty’ is a common example, which can be elastic (ball
bounces), plastic (slow deformation) or brittle (sudden force)
 depends on rate of strain
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Summary
• Polymers are formed of one or more repeating
‘mers’
 typically based on a carbon backbone
• These molecules can be long and have a complex
three-dimensional structure
• Three forms are common
 linear
 branched
 cross-linked
• Crystalline forms of polymers are also possible
• Stress-strain curves show a number of different
behaviours, depending on the conditions and the
material
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Poly: 20