eng2000 chapter 5 polymers

ENG2000: R.I. Hornsey Poly: 1

ENG2000 Chapter 5Polymers


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


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


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 with another carbon atom a double bond hence ethylene, C2H4

• And triple bonds are also possible e.g. acetylene, C2H2

C C=

H|

H|

|H

|H

H – C C – H


• 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)


Isomerism• Molecules with identical chemical compositions

may have more than one bonding arrangement e.g. butane, and isobutane

• 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

H – C – C – C – C – H

H|

|H

H|

|H

H|

|H

H|

|H

H – C – C – C – H

H|

|H

H|

|H

H|

|H

H – C – H

H|


Polymer molecules• Sometime called macromolecules because of

their huge size, polymers consist of chains of carbon atoms which form the backbone of the molecule

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

– C – C – C – C – C – C – C – C –C – C – C – C – | | | | | | | | | | | |

| | | | | | | | | | | |


Chemistry of polymers• So how is a polymer formed from the monomer?• Consider ethylene (a gas) again

the polymer form is polyethylene, which is a solid at room temperature

• The reaction is initiated by an initiator, R·

C C=

H|

H|

|H

|H

C C=

H|

H|

|H

|H

R· + R – C C ·–

H|

H|

|H

|H

‘spare’ electron


• The active (spare) electron is transferred to the end monomer, and the molecule grows

• The 3-D structure is

C C=

H|

H|

|H

|H

+ R – C C ·–

H|

H|

|H

|H

R – C C ––

H|

H|

|H

|H

C ·

H|

|H

H|

|H

C –

http://cwx.prenhall.com/bookbind/pubbooks/hillchem3/medialib/

media_portfolio/text_images/CH09/FG09_17.JPG


• 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)

C ––

H|

H|

|Cl

|H

C

H|

|Cl

H|

|H

C –C C ––

H|

H|

|Cl

|H

C

H|

|Cl

H|

|H

C –– C

mer unit


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


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

amount ofpolymer

molecular weight

number average,

€

M n = xiMi∑

weight average,

€

M w = wiMi∑

Mi is mean weight in size range, ixi is the fraction of total number of chains in size range, iwi is the fraction of total weight in size range, i


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

• 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

109°

ENG2000: R.I. Hornsey Poly: 14http://www.accelrys.com/consortia/polymer/permod/polypai.jpg


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


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


• 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


Stress-strain relation• There are three typical classes of polymer stress-

strain characteristic

0 2 4 6 8strain

stress (MPa)

0

2

4

6 brittle

plastic

highly elastic – elastomeric


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


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

eng2000 chapter 5 polymers

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