ENG2000 Chapter 5 Polymers

1. ENG2000 Chapter 5Polymers

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

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

4. H | H | C = C | H | H 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 H – C  C – H

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)

6. H | H – C – H H | H | H | H | H | H | H | H – C – C – C – C – H H – C – C – C – H | H | H | H | H | H | H | H 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

7. – C – C – C – C – C – C – C – C –C – C – C – C – | | | | | | | | | | | | | | | | | | | | | | | | 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

8. H | H | H | H | C C = = C C | H | H | H | H 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· ‘spare’ electron H | H | R· + R – C – C ·  | H | H

9. H | H | H | H | H | H | R – C – C ·  R – C – C – C – C · + | H | H | H | H | H | H H | H | C = C | H | H • The active (spare) electron is transferred to the end monomer, and the molecule grows • The 3-D structure is http://cwx.prenhall.com/bookbind/pubbooks/hillchem3/medialib/media_portfolio/text_images/CH09/FG09_17.JPG

10. H | H | H | H | H | H | H | H | C – C – C – C – C – C – C – C | H | Cl | H | Cl | H | Cl | H | Cl mer unit • 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)

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

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 number average, amount ofpolymer 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, molecular weight

13. 109° 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

14. http://www.accelrys.com/consortia/polymer/permod/polypai.jpg

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

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 http://www.lboro.ac.uk/departments/ma/gallery/molecular/Molecular/pollat.gif

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

18. stress (MPa) 6 brittle 4 plastic highly elastic – elastomeric 2 0 strain 0 2 4 6 8 Stress-strain relation • There are three typical classes of polymer stress-strain characteristic

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

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