CHAPTER 4: POLYMER STRUCTURES

1. CHAPTER 4:POLYMER STRUCTURES Spherulite, rubber specimen. Chain-folded lamellar crystallites, ~10 nm thick, 30,000×

2. c04cof01 ISSUES TO ADDRESS... • What are the general structural and chemical characteristics of polymer molecules? • What are some of the common polymeric materials, and how do they differ chemically? • How is the crystalline state in polymers different from that in metals and ceramics ?

3. 4.1 Structures of Polymers • Introduction and Motivation • Polymers are extremely important materials (i.e. plastics) • Have been known since ancient times – cellulose, wood, rubber, etc.. • Biopolymers – proteins, enzymes, DNA … • Last ~50 years – tremendous advances in synthetic polymers • Just like for metals and ceramics, the properties of polymers • Thermal stability • Mechanical properties Are intimately related to their molecular structure …

4. 4.1 Ancient Polymers Originally natural polymers were used: • Wood • Rubber • Cotton • Wool • Leather • Silk Oldest known use: Rubber balls used by Incas Noah used pitch (a natural polymer) for the ark Noah's pitch Genesis 6:14 "...and cover it inside and outside with pitch." gum based resins extracted from pine trees

5. 4.2 Polymer Composition Most polymers are hydrocarbons – i.e., made up of H and C • Saturated hydrocarbons • Each carbon singly bonded to four other atoms • Example: • Ethane, C2H6

6. 4.2 Unsaturated Hydrocarbons • Double & triple bonds somewhat unstable • Thus, can form new bonds • Double bond found in ethylene or ethene - C2H4 • Triple bond found in acetylene or ethyne - C2H2

7. 4.2 Structures of Polymers • about hydrocarbons • Why? Most polymers are hydrocarbon (e.g. C, H) based • Bonding is highly covalent in hydrocarbons • Carbon has four electrons that can participate in bonding, hydrogen has only one • Saturated versus unsaturated • Unsaturated – species contain carbon-carbon double/triple bonds • Possible to substitute another atom on the carbon • Saturated – carbons have four atoms attached • Cannot substitute another atom on the carbon Saturated Unsaturated

8. 4.2 Hydrocarbon Molecules c04eqf02 Acetylene Ethyne Ethylene Ethene Hydrocarbons have strong chemical bonds, but interact only weakly with one another (van der Waals’ forces) (normal) butane isobutane c04eqf02

9. 4.2 Isomerism compounds with same chemical formula can have quite different structures for example: C8H18 • normal-octane  Isomerism – compounds of the same chemical composition but different atomic arrangements (i.e. bonding connectivity) • 2,4-dimethylhexane

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12. c04eqf03 4.3 Polymer Molecules Molecules are gigantic Macromolecules Repeat units Monomer

13. 4.3 Polymers • Polymer molecules • what is a polymer? • Polymers are molecules (often called macromolecules) formed from a series of building units (monomers) that repeat over and over again • polymers can have a range of molecular weights • There are many monomers • Can make polymers with different monomers, etc.. n is often a very large number! e.g. can make polyethylene with MW > 100,000! ~3600 mers ~7200 carbons

14. Chemistry of polymer molecules Example: ethylene • Gas at STP • To polymerize ethylene, typically increase T, P and/or add an initiator Initiation Propagation After many additions of monomer to the growing chain… R* = initiator; activates the monomer to begin chain growth • Initiator: example - benzoyl peroxide

15. 4.4 Polymer chemistry • Polymers are chain molecules. They are built up from simple units called monomers. • E.g. polyethylene is built from ethylene units: which are assembled into long chains: Polyethylene or polythene (IUPAC name poly(ethene)) is a thermoplastic commodity heavily used in consumer products (notably the plastic shopping bag). Over 60 million tons of the material are produced worldwide every year.

16. Tetrafluoroethylene monomer polymerize to form PTFE or polytetrafluoroethylene c04eqf08 poly(tetrafluoroethene) or poly(tetrafluoroethylene) (PTFE) is a synthetic fluoropolymer. PTFE is the DuPont brand name Teflon. Melting: 327C Vinyl chloride monomer leads to poly(vinyl chloride) or PVC PVC: manufacturing toys, packaging, coating, parts in motor vehicles, office supplies, insulation, adhesive tapes, furniture, etc. Consumers: shoe soles, children's toys, handbags, luggage, seat coverings, etc. Industrial sectors: conveyor belts, c04eqf09 c04eqf08 printing rollers. Electric and electronic equipment: circuit boards, cables, electrical boxes, computer housing.

17. Chemistry and Structure of Polyethylene Adapted from Fig. 4.1, Callister & Rethwisch 3e. Note: polyethylene is a long-chain hydrocarbon - paraffin wax for candles is short polyethylene • Polymer = many mers Adapted from Fig. 14.2, Callister 6e.

18. Polymer chemistry • In polyethylene (PE) synthesis, the monomer is ethylene • Turns out one can use many different monomers • Different functional groups/chemical composition – polymers have very different properties! Monomers

19. Homopolymer and Copolymer • Polymer chemistry • If formed from one monomer (all the repeat units are the same type) – this is called a homopolymer • If formed from multiple types of monomers (all the repeat units are not the same type) – this is called a copolymer • Also note – the monomers shown before are referred to as bifunctional • Why? The reactive bond that leads to polymerization (the C=C double bond in ethylene) can react with two other units • Other monomers react with more than two other units – e.g. trifunctional monomers

20. The Top 10 Bulk or Commodity

21. low M 4.5 MOLECULAR WEIGHT Molecular weight, M: Mass of a mole of chains. high M Not all chains in a polymer are of the same length i.e., there is a distribution of molecular weights

22. Molecular weight • The properties of a polymer depend on its length • synthesis yields polymer distribution of lengths • Define “average” molecular weight • Two approaches are typically taken • Number average molecular weight (Mn) • Weight-average molecular weight (Mw)

23. MOLECULAR WEIGHT DISTRIBUTION Adapted from Fig. 4.4, Callister & Rethwisch 3e. Mi= mean (middle) molecular weight of size range i xi= number fraction of chains in size range i wi= weight fraction of chains in size range i

24. Molecular weight Are the two different? Yes, one is essentially based on mole fractions, and the other on weight fractions They will be the same if all the chains are exactly of the same MW! If not Mw > Mn Get Mn from this Get Mw from this

25. Molecular weight • Other ways to define polymer MW • Degree of polymerization • Represents the average number of mers in a chain. The number and weight average degrees of polymerization are m is the mer MW in both cases. In the case of a copolymer (something with two or more mer units), m is determined by fjand mj are the chain fraction and molecular weight of mer j

26. Number average MW (Mn) Example Problem 4.1 • Given the following data determine the • Number average MW • Number average degree of polymerization • Weight average MW • How to find Mn? • Calculate xiMi • Sum these!

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28. Example Problem 4.1 • How to find Mw? • Calculate wiMi • Sum these! Number average degree of polymerization • (MW of H2C=CHCl is 62.50 g/mol) Weight average molecular weight (Mw)

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30. Degree of Polymerization, DP DP = average number of repeat units per chain DP = 6 Chain fraction mol. wt of repeat unit i

31. 4.6 Polymers – Molecular Shape Molecular Shape (or Conformation) – chain bending and twisting are possible by rotation of carbon atoms around their chain bonds • note: not necessary to break chain bonds to alter molecular shape Adapted from Fig. 4.5, Callister & Rethwisch 3e. • C-C bonds are typically 109° (tetrahedral, sp3 carbon) • If you have a macromolecule with hundreds of C-C bonds, this will lead to bent chains

32. Structures of Polymers • Molecular shape • Taking this idea further, can also have rotations about bonds • Leads to “kinks”, twists • “the end-to-end distance of a polymer chain in the solid state (or in solution) is usually much less than the distance of the fully extended chain! • This is not even taking into account that you have numerous chains that can become entangled!

33. 4.7 Molecular structure • Physical properties of polymers depend not only on their molecular weight/shape, but also on the difference in the chain structure • Four main structures • Linear polymers • Branched polymers • Crosslinked polymers • Network polymers

34. secondary bonding Linear B ranched Cross-Linked Network 4.7 Molecular Structures for Polymers Adapted from Fig. 4.7, Callister & Rethwisch 3e.

35. Linear polymers • – polymers in which the mer units are connected end-to-end along the whole length of the chain • These types of polymers are often quite flexible • Van der waal’s forces and H-bonding are the two main types of interactions between chains • Some examples – polyethylene, teflon, PVC, polypropylene

36. Branched polymers • Polymer chains can branch: • Or the fibers may aligned parallel, as in fibers and some plastic sheets. • chains off the main chain (backbone) • This leads to inability of chains to pack very closely together • These polymers often have lower densities • These branches are usually a result of side-reactions during the polymerization of the main chain • Most linear polymers can also be made in branched forms

37. Crosslinked polymers • Molecular structure • adjacent chains attached via covalent bonds • Carried out during polymerization or by a non-reversible reaction after synthesis (referred to as crosslinking) • Materials often behave very differently from linear polymers • Many “rubbery” polymers are crosslinked to modify their mechanical properties; in that case it is often called vulcanization • Generally, amorphous polymers are weak and cross-linking adds strength: vulcanized rubber is polyisoprene with sulphur cross-links:

38. Network polymers – polymers that are “trifunctional” instead of bifunctional • There are three points on the mer that can react • This leads to three-dimensional connectivity of the polymer backbone • Highly crosslinked polymers can also be classified as network polymers • Examples: epoxies, phenol-formaldehyde polymers

39. POLYMER MICROSTRUCTURE • Covalent chain configurations and strength: Direction of increasing strength Adapted from Fig. 14.7, Callister 6e. 2

40. 4.8 Molecular configurations Classification scheme for the characteristics of polymer molecules isomerism – different molecular configurations for molecules (polymers) of the same composition Stereoisomerism Geometrical Isomerism

41. 4.8 Molecular Configurations Repeat unit R = Cl, CH3, etc c04eqf23 Configurations – to change must break bonds Stereoisomers are mirror images – can’t superimpose without breaking a bond A A C C E E B B D D mirror plane

42. c04eqf11 Head to-tail • Typically the head-to-tail configuration dominates Head to-head

43. Structures of Polymers • Stereoisomerism • Denotes when the mers are linked together in the same way (e.g. head-to-tail), but differ in their spatial arrangement • This really focuses on the 3D arrangement of the side-chain groups • Three configurations most prevalent • Isotactic • Syndiotactic • Atactic

44. ISOTACTIC • Stereoisomerism • Isotactic polymers • All of the R groups are on the same side of the chain Isotactic configuration • Note: All the R groups are head-to-tail • All of the R groups are on the same side of the chain • Projecting out of the plane of the slide • This shows the need for 3D representation to understand stereochemistry!

45. SYNDIOTACTIC • Stereoisomerism • Syndiotactic polymers • The R groups occupies alternate sides of the chain Syndiotactic configuration • Note: The R groups are still head-to-tail • R groups alternate – one of out of the plane, one into the plane

46. ATACTIC • Stereoisomerism • Atactic polymers • The R groups are “random” Atactic configuration • R groups are both into and out of the plane, no real registry • Two additional points • Cannot readily interconvert between stereoisomers – bonds must be broken • Most polymers are a mix of stereoisomers, often one will predominate

47. Stereoisomerism—Head-to-tail c04eqf12 isotactic configuration Syndiotactic conformation Atactic conformation

48. cis/trans Isomerism cis cis-isoprene (natural rubber) H atom and CH3 groupon same side of chain trans trans-isoprene (gutta percha) H atom and CH3 group on opposite sides of chain

49. c04eqf18 Geometrical Isomerism c04eqf18

50. 4.9 Plastics • variety of properties due to their rich chemical makeup • They are inexpensive to produce, and easy to mold, cast, or machine. • Their properties can be expanded even further in composites with other materials.