ENG2000: R.I. Hornsey Poly: 1 ENG2000 Chapter 5 Polymers

ENG2000: R.I. Hornsey Poly: 2 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: 3 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

ENG2000: R.I. Hornsey Poly: 4 Hydrocarbon molecules Hydrocarbons  hydrogen and carbon, bonded covalently Simplest are methane, ethane, propane, butane  C n H 2n+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, C 2 H 4 And triple bonds are also possible  e.g. acetylene, C 2 H 2 CC= H|H| H|H| |H|H |H|H H – C  C – H

ENG2000: R.I. Hornsey Poly: 5 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)

ENG2000: R.I. Hornsey Poly: 6 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|H| |H|H H|H| |H|H H – C – C – C – H H|H| |H|H H|H| |H|H H|H| |H|H H – C – H H|H|

ENG2000: R.I. Hornsey Poly: 7 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 – |||||||||||| ||||||||||||

ENG2000: R.I. Hornsey Poly: 8 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· CC= H|H| H|H| |H|H |H|H CC= H|H| H|H| |H|H |H|H R· +  R – CC ·– H|H| H|H| |H|H |H|H ‘spare’ electron

ENG2000: R.I. Hornsey Poly: 9 The active (spare) electron is transferred to the end monomer, and the molecule grows The 3-D structure is CC= H|H| H|H| |H|H |H|H +  R – CC ·– H|H| H|H| |H|H |H|H R – CC –– H|H| H|H| |H|H |H|H C · H|H| |H|H H|H| |H|H C – ooks/hillchem3/medialib/media_portfol io/text_images/CH09/FG09_17.JPG

ENG2000: R.I. Hornsey Poly: 10 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| H|H| | Cl |H|H C H|H| | Cl H|H| |H|H C –C – H|H| H|H| | Cl |H|H C H|H| | Cl H|H| |H|H C –– C mer unit

ENG2000: R.I. Hornsey Poly: 11 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

ENG2000: R.I. Hornsey Poly: 12 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 of polymer molecular weight number average, weight average, M i is mean weight in size range, i x i is the fraction of total number of chains in size range, i w i is the fraction of total weight in size range, i

ENG2000: R.I. Hornsey Poly: 13 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: 14

ENG2000: R.I. Hornsey Poly: 15 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

ENG2000: R.I. Hornsey Poly: 16 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

ENG2000: R.I. Hornsey Poly: 17 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

ENG2000: R.I. Hornsey Poly: 18 Stress-strain relation There are three typical classes of polymer stress- strain characteristic strain stress (MPa) brittle plastic highly elastic – elastomeric

ENG2000: R.I. Hornsey Poly: 19 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

ENG2000: R.I. Hornsey Poly: 20 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