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

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 ?

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 … End of lecture 1

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

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

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

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 End of lecture 1 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 Unsaturated Saturated

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

4.2 Isomerism 2,4-dimethylhexane 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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c04eqf03 4.3 Polymer Molecules Molecules are gigantic Macromolecules Repeat units Monomer

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.. End of lecture 1 n is often a very large number! e.g. can make polyethylene with MW > 100,000! ~3600 mers ~7200 carbons

Chemistry of polymer molecules Example: ethylene Gas at STP To polymerize ethylene, typically increase T, P and/or add an initiator Initiation End of lecture 1 Propagation After many additions of monomer to the growing chain… R* = initiator; activates the monomer to begin chain growth Initiator: example - benzoyl peroxide

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.

Tetrafluoroethylene monomer polymerize to form PTFE or polytetrafluoroethylene c04eqf08 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 printing rollers. Electric and electronic equipment: circuit boards, cables, electrical boxes, computer housing.

Chemistry and Structure of Polyethylene Adapted from Fig. 4.1, Callister & Rethwisch 3e. Polymer- can have various lengths depending on number of repeat units Note: polyethylene is a long-chain hydrocarbon - paraffin wax for candles is short polyethylene • Polymer = many mers Adapted from Fig. 14.2, Callister 6e.

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 End of lecture 1

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 End of lecture 1

The Top 10 Bulk or Commodity Relatively few polymers responsible for virtually all polymers sold – these are the bulk or commodity polymers

4.5 MOLECULAR WEIGHT Molecular weight, M: Mass of a mole of chains. low M Simple for small molecules All the same size Number of grams/mole Polymers – distribution of chain sizes high M Not all chains in a polymer are of the same length i.e., there is a distribution of molecular weights

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) End of lecture 1

MOLECULAR WEIGHT DISTRIBUTION Adapted from Fig. 4.4, Callister & Rethwisch 3e. Simple for small molecules All the same size Number of grams/mole Polymers – distribution of chain sizes 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

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 End of lecture 1 Get Mn from this Get Mw from this

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 End of lecture 1 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 fj and mj are the chain fraction and molecular weight of mer j

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! Number average MW (Mn) End of lecture 1

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Example Problem 4.1 Number average degree of polymerization (MW of H2C=CHCl is 62.50 g/mol) How to find Mw? Calculate wiMi Sum these! Weight average molecular weight (Mw) End of lecture 1

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

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

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! End of lecture 1

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

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

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

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

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: End of lecture 1

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 End of lecture 1

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

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 End of lecture 1

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

c04eqf11 Head to-tail Head to-head Typically the head-to-tail configuration dominates Head to-head

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 End of lecture 1

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! End of lecture 1

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 End of lecture 1

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 End of lecture 1

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

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

c04eqf18 Geometrical Isomerism c04eqf18

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.

Glass phase (hard plastic) Glass-rubber-liquid Amorphous plastics have a complex thermal profile with 3 typical states: Glass phase (hard plastic) Temperature 3 9 6 7 8 4 5 Leathery phase Log(stiffness) Pa Rubber phase (elastomer) Liquid

THERMOPLASTICS Thermosetting and thermoplastic polymers Another way to categorize polymers – how do they respond to elevated temperatures? Thermoplastics – these materials soften when heated, and harden when cooled – this process is totally reversible This is due to the reduction of secondary forces between polymer chains as the temperature is increased Most linear polymers and some branched polymers are thermoplastics End of lecture 1

THERMOSETS Thermosetting and thermoplastic polymers Thermosets – these materials harden the first time they are heated, and do not soften after subsequent heating During the initial heat treatment, covalent linkages are formed between chains (i.e. the chains become cross-linked) Polymer won’t melt with heating – heat high enough it will degrade Network/crosslinked polymers are typically thermosets Polymers which irreversibly change when heated are called thermosets. Most often, the change involves cross-linking which strengthens the polymer (setting). Thermosets will not melt, and have good heat resistance. They are often made from multi-part compounds and formed before setting (e.g. epoxy resin). Setting accelerates with heat, or for some polymers with UV light. End of lecture 1

Thermoplastics Polymers which melt and solidify without chemical change are called thermoplastics. They support hot-forming methods such as injection-molding and FDM.

THERMOPLASTICS VS THERMOSETS --little cross linking --ductile --soften w/heating --polyethylene (#2) polypropylene (#5) polycarbonate polystyrene (#6) • Thermosets: --large cross linking (10 to 50% of mers) --hard and brittle --do NOT soften w/heating --vulcanized rubber, epoxies, polyester resin, phenolic resin Adapted from Fig. 15.18, Callister 6e. (Fig. 15.18 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.) 3

4.10 Structures of Polymers Copolymers Idea – polymer that contains more than one mer unit Why? If polymer A has interesting properties, and polymer B has (different) interesting properties, making a “mixture” of polymer should lead to a superior polymer “Random” copolymer – exactly what it sounds like End of lecture 1 “Alternating” copolymer – ABABABA…

Structures of Polymers Copolymers Idea – polymer that contains more than one mer unit Why? If polymer A has interesting properties, and polymer B has (different) interesting properties, making a “mixture” of polymer should lead to a superior polymer “Block” copolymers. Domains of “pure” mers End of lecture 1 “Graft” copolymers. One mer forms backbone, another mer is attached to backbone and is a sidechain (it is “grafted” to the other polymer)

Copolymers two or more monomers polymerized together Adapted from Fig. 4.9, Callister & Rethwisch 3e. two or more monomers polymerized together random – A and B randomly positioned along chain alternating – A and B alternate in polymer chain block – large blocks of A units alternate with large blocks of B units graft – chains of B units grafted onto A backbone A – B – random alternating block graft

Copolymers Polymers often have two different monomers along the chain – they are called copolymers. With three different units, we get a terpolymer. This gives us an enormous design space…

4.11 Polymer structure The polymer chain layout determines a lot of material properties: Amorphous: Crystalline:

Crystallinity in Polymers Adapted from Fig. 4.10, Callister & Rethwisch 3e. Ordered atomic arrangements involving molecular chains Crystal structures in terms of unit cells Example shown polyethylene unit cell Polymers can be crystalline (i.e. have long range order) However, given these are large molecules as compared to atoms/ions (i.e. metals/ceramics) the crystal structures/packing will be much more complex

Structures of Polymers Polymer crystallinity (One of the) differences between small molecules and polymers Small molecules can either totally crystallize or become an amorphous solid Polymers often are only partially crystalline Why? Molecules are very large Have crystalline regions dispersed within the remaining amorphous materials Polymers are often referred to as semicrystalline End of lecture 1

Structures of Polymers Polymer crystallinity Another way to think about it is that these are two phase materials (crystalline, amorphous) Need to estimate degree of crystallinity – many ways One is from the density End of lecture 1

Structures of Polymers 4.11 Polymer crystallinity What influences the degree of crystallinity Rate of cooling during solidification Molecular chemistry – structure matters Polyisoprene – hard to crystallize Polyethylene – hard not to crystallize Linear polymers are easier to crystallize Side chains interfere with crystallization Stereoisomers – atactic hard to crystallize (why?); isotactic, syndiotactic – easier to crystallize Copolymers – more random; harder to crystallize End of lecture 1

4.11 Polymer Crystallinity (cont.) Polymers rarely 100% crystalline Difficult for all regions of all chains to become aligned crystalline region • Degree of crystallinity expressed as % crystallinity. -- Some physical properties depend on % crystallinity. -- Heat treating causes crystalline regions to grow and % crystallinity to increase. 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.)

4.11 MOLECULAR WEIGHT & CRYSTALLINITY • Molecular weight, Mw: Mass of a mole of chains. • Tensile strength (TS): --often increases with Mw. --Why? Longer chains are entangled (anchored) better. • % Crystallinity: % of material that is crystalline. --TS and E often increase with % crystallinity. --Annealing causes crystalline regions to grow. % crystallinity increases. 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.) 4

4.12 Polymer Crystallinity 4.12 Polymer crystals Chain folded-model Many polymers crystallize as very thin platelets (or lamellae) Idea – the chain folds back and forth within an individual plate (chain folded model) End of lecture 1 Crystalline regions thin platelets with chain folds at faces Chain folded structure

4.12 Single Crystals Electron micrograph – multilayered single crystals (chain-folded layers) of polyethylene Single crystals – only for slow and carefully controlled growth rates Adapted from Fig. 4.11, Callister & Rethwisch 3e.

4.12 Semicrystalline Polymers Some semicrystalline polymers form spherulite structures Alternating chain-folder crystallites and amorphous regions Spherulite structure for relatively rapid growth rates Spherulite surface Adapted from Fig. 4.13, Callister & Rethwisch 3e.

Structures of Polymers Polymer crystals More commonly, many polymers that crystallize from a melt form spherulites One way to think of these – the chain folded lamellae have amorphous “tie domains” between them These plates pack into a spherical shape Polymer analogues of grains in polycrystalline metals/ceramics End of lecture 1

Photomicrograph – Spherulites in Polyethylene Cross-polarized light used -- a maltese cross appears in each spherulite Adapted from Fig. 4.14, Callister & Rethwisch 3e.

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