Lecture 3: Structure of Materials… (continue)

1. Lecture 3: Structureof Materials…(continue) Structure of Polymer

2. H H H H H H H H H H H H H H H H H H C C C C C C C C C C C C C C C C C C H H H H H H H H H H CH3 H CH3 H CH3 Cl Cl Cl Polyethylene (PE) Polyvinyl chloride (PVC) Polypropylene (PP) What is a polymer? Polymer manyrepeat unit repeat unit repeat unit repeat unit Adapted from Fig. 14.2, Callister 7e.

3. 2 Categories- Naturally occurring polymers which derives from plants and animals: wood, natural rubber, cotton, silk Other natural polymer-proteins, enzymes, starches, cellulose Synthetic polymers i.e. plastics, synthetic rubbers, and fibrous materials Properties of polymers are intricately related to the structural elements of materials INTRODUCTION

4. Most polymeric materials are composed of very large molecules-mostly composed of hydrogen and carbon Intramolecular bonds are covalent Saturated hydrocarbon -all bonds are single These molecules have strong covalent bond but weak hydrogen & van der Waals bonds between molecules Unsaturated hydrocarbon -have double & triple covalent bond. Possibility of other atom or group of atoms to attached to original molecule Hydrocarbon Molecules

5. Saturated Hydrocarbon Most polymers are hydrocarbons – i.e. made up of H and C • Saturated hydrocarbons • Each carbon bonded to four other atoms CnH2n+2

7. Unsaturated Hydrocarbons • Double & triple bonds relatively reactive – can form new bonds • Double bond – ethylene or ethene - CnH2n • 4-bonds, but only 3 atoms bound to C’s • Triple bond – acetylene or ethyne - CnH2n-2

8. Isomerism • Isomerism • two compounds with same chemical formula can have quite different structures/atomic structures Ex: C8H18 • n-octane • 2-methyl-4-ethyl pentane (isooctane) 

9. Polymer Molecules • Polymers are referred as macromolecules due to their giant size • The molecules are in the form of long & flexible chains • The backbone consist from string of carbon atoms, many times each carbon atom singly bonds to two adjacent carbon atoms on either side

10. The long molecules are composed of structural entities called repeat unit which are successively repeated along the chain. • Repeat unit also called a mer. • Mer originates from Greek word meros, which mean parts, thus, polymer meansmany partsor many mers/repeat unit • Monomer refers to stable molecule where the polymer is synthesized Adapted from Fig. 14.2, Callister 6e.

11. The Chemistry of Polymer Molecules Consider ethylene(C2H4) gas at ambient temperature & pressure. How this hydrocarbon monomer transformed into polyethylene, which is a solid polymeric material?

12. The Chemistry of Polymer Molecules • The process happens when the initiator/catalyst react with the ethylene mer unit (a) (b) • The polymer chain then forms by the sequential addition of polyethylene monomer units to this active initiator-mer center. • The active site or unpaired electron is transfered to each successive end monomer. The final result is the polyethylene molecules

13. Chemistry of Polymers Adapted from Fig. 14.1, Callister 7e. The angle between the singly bonded carbon atoms is not 180° but rather close to 109°. The C – C bond length being 0.154 nm. • Note: polyethylene is just a long HC • - paraffin is short polyethylene

14. Bulk or Commodity Polymers

17. Other Terms • Homopolymer-when all the repeating units along a chain are of the same type • Copolymers-Polymers’ chains may be composed of two or more different mer units • Bifunctional-they have 2 active bonds which form 2 dimensional molecular structure • Trifunctional-have 3 active bonds, which results in 3 dimensional molecular structure

18. During polymerization, not all polymer chains have a same length which results in distribution of molecular weights An average molecular weight is specified from various physical properties The no. average molecular weight Mn=ΣxiMi The weight average molecular weight Mw=ΣwiMi MOLECULAR WEIGHT

19. • Molecular weight, Mi: Mass of a mole of chains. Lower M higher M MOLECULAR WEIGHT Fraction of the total number of chains The mean molecular weight within the size range The weight fraction of molecules Adapted from Fig. 14.4, Callister 7e.

20. Degree of Polymerization, n - An alternative way of expressing average chain size of polymer n = number of repeat units per chain ni = 6 Chain fraction mol. wt of repeat unit i

21. Polymer chain molecules are illustrated to be straight for easy understanding A more accurate model is shown with 109o between carbon atoms that forms a zigzag pattern MOLECULAR SHAPE Figure (b) shows a straight chain segment resulted when the chain atoms are well positioned

22. Chain bending & twisting are possible when there is chain atoms rotation into other positions as depicted in (c) This could result as in picture below: Polymers consist of large no. of molecular chains, each of which may bend, coil & kink as in the picture Molecular Shape

23. End to End Distance, r This distance is much smaller than the total chain length Adapted from Fig. 14.6, Callister 7e.

24. The shape of the polymer chain molecule are responsible for a number of important characteristics • Some mechanical & thermal characteristics of polymer are a function of the ability of the chain segments to experience rotation in response to applied stress or thermal vibrations • Rotational flexibility is dependent on mer structure& chemistry • For example: The region of a chain segment that has a double bond (C=C) is rotationally rigid • Also, the introduction of a bulky or large side group of atoms restricts rotational movement

25. Molecular Structure • Four general molecular chain structures: • Linear polymers • Branched polymers • Cross-linked polymers • Network polymers

26. Linear Polymers • The mer units are joined together end to end in single chains represents as a mass of spaghetti • May have extensive van der Waals & hydrogen bonding between chains • Examples: PE, PVC, polystyrene, PMMA

27. Branched Polymers • Side branch chains are connected to the backbone • The branches resulted from side reactions that occur during the polymer synthesis • The chain packing efficiency is reduced by the formation of side branches which lowers the polymer density

28. Cross-linked Polymers • Adjacent linear chains are joined one to another at various positions by covalent bonds • Crosslinking process is achieved either by synthesis or nonreversible chemical reaction carried out at elevated temperature • Is accomplished by additive atoms or molecules covalently bonded to the chains

29. Network Polymers • Trifunctional mer units, having 3 active covalent bonds to form 3-D networks • A polymer that is highly crosslinked maybe classified as a network polymer • These materials have distinctive mechanical & thermal properties • Polymers can have more than one distinctive structural type

30. Consider the mer unit where R represents an atom or side group other than H (e.g., Cl, CH3) A possible arrangement when the R side groups of successive mer units bond to alternate carbon atoms. This is designated as a head -to-tail configuration Its complement the head-to-head configuration occurs when R groups bond to adjacent chain atoms MOLECULAR CONFIGURATIONS

31. ISOMERISM-polymer molecules with same composition but different atomic arrangements • STEREOISOMERISME-denotes the situation inwhich atoms are linked together in the same order (head-to-tail) but different in their spatial arrangement • ISOTACTIC CONFIGURATION • All the R groups are situated on the same side of the chain

32. Syndiotactic Configuration • The R groups alternate sides of the chain • Atactic Configuration • Random positioning of R groups • Conversion of one stereoisomer to another will involve severing of bonds reformation after appropriate rotation

33. GEOMETRICAL ISOMERISM • Also possible within mer units having double bond between chain carbon atoms • Bonded to the carbon atoms participating with double bond is a single sided bonded atom or radical • Let’s say the CH3 group & the H atom on the same side. This is a CIS structure • For the trans structure, the CH3 & H reside on opposite chain sides. This isomerism also can’t be convert by simple rotation because the chain double is too rigid cis-isoprene (natural rubber) trans-isoprene (gutta percha) – has properties that are distinctly different from natural rubber

34. Classification scheme based on the materials behavior with rising temperature Thermoplastic polymer have linear & branched structures; they soften when heated & harden when cooled. The process reversible & can be repeated It is not reversible if the temperature is raised to a point where molecular vibrations are violent enough to break the primary covalent bonds THERMOPLASTIC & THERMOSETTING POLYMERS

35. THERMOPLASTIC & THERMOSETTING POLYMERS • Thermosetting polymer, are cross linking and network polymer • Become permanently hard during their formation, and do not soften upon heating. • At initial heat treatment, covalent cross-links are formed between adjacent molecular chains to anchor the chains together which resist vibrations & rotations at high temperature • Crosslinking is usually extensive • Thermoset polymers are harder & stronger than thermoplastics

36. COPOLYMERS Composed of two repeat units Have different sequencing arrangement along the polymer chain a) Random copolymer – two different units are randomly dispersed along the chain b) Alternating copolymer – the 2 mer units alternate chain positions c) Block copolymer – the identical mers are clustered in blocks along the chain d) Graft copolymer – homopolymer side branches of one type maybe grafted to homopolymer main chains that are composed of a different mer

37. Styrene-butadiene rubber (SBR) is a common random copolymer which use to make automobile tires.

38. Atomic arrangement in polymer crystals is more complex than in metals or ceramics (unit cells are typically large and complex). Polymer crytallinity is the packing of molecular chains to produce an ordered atomic array More crystallinity: higher density, more strength, higher resistance to dissolution and softening by heating! POLYMER CRYSTALLINITY Polyethylene

39. Degree of crystallinity is determined by: Rate of cooling during solidification: time is necessary for chains to move and align into a crystal structure Mer complexity: crystallization less likely in complex structures, simple polymers, such as polyethylene, crystallize relatively easily Chain configuration: linear polymers crystallize relatively easily, branches inhibit crystallization, network polymers almost completely amorphous, crosslinked polymers can be both crystalline and amorphous Isomerism: isotactic, syndiotactic polymers crystallize relatively easily - geometrical regularity allows chains to fit together, atactic difficult to crystallize Copolymerism: easier to crystallize if mer arrangements are more regular - alternating, block can crystallize more easily as compared to random and graft.

40. How to calculate crystallinity?

41. POLYMER CRYSTALS Polymer molecules are often partially crystalline (semicrystalline), with crystalline regions dispersed within amorphous material. Crystalline Amorphous Fringed-micelle model

42. Polymer Crystallinity Polymers rarely 100% crystalline • Too difficult to get all those chains aligned crystalline region • % Crystallinity: % of material that is crystalline. -- TS and E often increase with % crystallinity. -- Annealing causes crystalline regions to grow. % crystallinity increases. amorphous region Adapted from Fig. 14.11, Callister 6e. (Fig. 14.11 is from H.W. Hayden, W.G. Moffatt, and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc., 1965.)

43. Recently : Studies focus on Polymer single crystals grown from dilute solution Each platelet will consist of a number of molecule; however the average chain length is much greater than the thickness of the crystallite

44. Photomicrograph of spherulite structure of polyethylene Spherulites: Aggregates of lamellar crystallites ~ 10 nm thick, separated by amorphous material. Aggregates approximately spherical in shape.

45. Schematic representation of the detailed structure of a sperulite.