ENGINEERING MATERIALS ENT 112/4 LECTURE 3 (continue) 3.4 Structure of Polymer

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

2.5.1 INTRODUCTION 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

2.5.2 Hydrocarbon Molecules 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

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

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

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) 

2.5.3 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

Monomer refers to stable molecule where the polymer is synthesized 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 means many parts or many mers/repeat unit Monomer refers to stable molecule where the polymer is synthesized Adapted from Fig. 14.2, Callister 6e.

2.5.4 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?

2.5.4 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

Chemistry of Polymers Note: polyethylene is just a long HC 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. Polymer- can have various lengths depending on number of repeat units Note: polyethylene is just a long HC - paraffin is short polyethylene

Bulk or Commodity Polymers Relatively few polymers responsible for virtually all polymers sold – these are the bulk or commodity polymers

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

2.5.5 MOLECULAR WEIGHT 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 • Molecular weight, Mi: Mass of a mole of chains. Lower M higher M Fraction of the total number of chains The mean molecular weight within the size range Simple for small molecules All the same size Number of grams/mole Polymers – distribution of chain sizes The weight fraction of molecules Adapted from Fig. 14.4, Callister 7e.

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

Computation of Average Molecular Weights & Average Degrees of Polymerization Assume that the molecular weight distributions are for poly (vinyl chloride): (a) (b) Compute: a) the no. average molecular weight b) the no. average degree of polymerization c) the weight average molecular weight

The data taken is from (a) and presented in the above table, the summation of all xiMi products yields the no. average molecular weight, which is 21,150 g/mol No average degree of polymerization is computed from mer molecular weight. For PVC each mer has-2 carbon atoms, 3 hydrogen atoms, 1 chlorine atoms.

Atomic weights: C= 12, H= 1, Cl=35.5, thus m= 2(12 g/mol)+3(1 g/mol)+35.5 g/mol = 62.5 g/mol & nn =Mn/m = (21,150 g/mol)/(62.5 g/mol) = 338 (c) The data taken is from (b), the summation of all wiMi products yields the weight average molecular weight, which is 23,2000 g/mol

2.5.6 MOLECULAR SHAPE 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 Figure (b) shows a straight chain segment resulted when the chain atoms are well positioned

2.5.7 Molecular Shape 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

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

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

2.5.7 Molecular Structure Four general molecular chain structures: Linear polymers Branched polymers Cross-linked polymers Network polymers

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

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

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

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

2.5.8 MOLECULAR CONFIGURATIONS 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

ISOMERISM-polymer molecules with same composition but different atomic arrangements STEREOISOMERISME-denotes the situation in which 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

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

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

2.5.9 THERMOPLASTIC & THERMOSETTING POLYMERS 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

2.5.9 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

2.5.10 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

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

2.5.11 POLYMER CRYSTALLINITY 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! Polyethylene

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.

How to calculate crystallinity?

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

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.)

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

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

Schematic representation of the detailed structure of a sperulite.