PolymersThe term polymer implies many "mers" or the building blocks....similar to the unit cell in metals.A polymer is a chemical compound or mixture of compounds formed by a process called polymerization, a chemical reaction in which two or more molecules combine to form larger molecules.Generally speaking, polymers refer to the intermediate stage before the final plastic product is produced.

• Natural polymers– derived from plants and animals

• wood, rubber, cotton• wool, leather and silk

– biological polymers• protein, enzymes, starches, cellulose

• Synthetic polymers– huge expansion since WWII

Historical Classification

• Plastics are derived from organic materials and are in abundance.• Raw materials commonly used in the production of polymers are coal, air, water, wood, petroleum, limestone, and salt.• Most common material used is petroleum.• These materials contain the basic elements that are used in forming polymers...carbon, hydrogen, oxygen, nitrogen, chlorine, and fluorine.

Basic Building Blocks

Hydrocarbon MoleculesHydrocarbon Molecules• Most polymers are organic

– composed of H and C• each C has 4 bonds• each H has 1 bond

– bonds are covalent• Bonds between carbons can

– single (e.g. ethane)– double (e.g. ethylene or

ethene)– triple (e.g. acetylene or

ethyne)

C

HH

HH

methane,simplest hydrocarbon form CC

CCCC

≡=−

Single BondSingle Bond

Unsaturated Hydrocarbon MoleculesUnsaturated Hydrocarbon Molecules• Ethane

– C2H6- single bond• Ethylene

– C2H4 -double bond• Acetylene

– C2H2 -triple bond

C C

H H

H H

C CH H

Molecules that have double or triple bonds are termed unsaturated

C

HH

HHC

H

H

Chemistry of Polymer MoleculesChemistry of Polymer Molecules

Unsaturated hydrocarbons may permit the addition of another atom or group of atoms. Example ethylene C2H4, which is a gas.

C CH H

H HR•+ C C

H H

H HR- •

C CH H

H HR- • C C

H H

H H+ C C

H H

H HR- C C

H H

H H•

R• - free radical (unpaired electron) in initiator - catalyst

C CH H

H HR- C C

H H

H H• R•+ C C

H H

H HR- C C

H H

H H- R….termination

initiation

growth

Structure of Polymer MoleculesStructure of Polymer Molecules

Polymer composed of “mers” (repeat unit)

Single unit called “monomer”

C C C C C C[ ]mer

C CH H

H H

monomern

Ethylene (C2H4) - gasPolyethylene (PE) - solid

polymeric material• carbons are 109° to

each other (tetrahedral bond angle for sp3

hybridization• “zigzag” structure

n = the degree of polymerization

Example

• Polymers are gigantic compared to hydrocarbon molecules– called macromolecules

• For most polymers– long, flexible chains with a string

of carbon atoms in the “backbone”

– remaining electrons can be involved in side bonding with atoms or groups of atoms

– structural entities are called “mers”

Common PolymersCommon Polymers

C C C C C C

F F F F F FMer unit

F F F F F F

C C C C C CH H H H H H

H Cl H Cl H Cl

Mer unit

C C C C C CH H H H H H

H CH3 H

Mer unit

CH3 H CH3

Polypropylene (PP)

Polyvinylchloride (PVC)

Polytetrafluoroethylene(PTFE) Trade name Teflon

Molecular StructureThe physical characteristics of a polymer depends not only on its molecular weight and shape but also on differences in the structure of the molecular chains.

Types of Molecular Structures

• Linear – Linear polymer are those in which the mer units are joined together end to end in single chains. There is only Van der Waals bonding between chains. Examples polyethylene, nylon.

• Branched – Polymers where side-branch chains are connected to the main ones. The chain packing efficiency is reduced compared to linear polymers – lower density.

• Cross linked – Adjacent linear chains are joined one to another at various positions by covalent bonds. Many rubbers have this structure.

• Network – Trifuntional mer units having three active covalent bonds, form three dimensional networks. Example: epoxy, phenol-formaldehyde

IsomersIsomers

• Hydrocarbons with the same composition but different atomic arrangements are called isomers (ex: Butane and Isobutane - C4H10)

• These isomers have different properties (e.g. b.p.)• Two types of isomerism are possible: stereoisomerism

and geometrical isomerism.

CH

HH

CH

HHC

H

HCH

H CH

HH

CH

HCH

H

CH

H H

Butane Isobutane

StereoisomerismStereoisomerism: atoms are linked together in the same order, but can have different spatial arrangement

1 Isotactic configuration: all side groups R are on the same side of the chain.

2 Syndiotactic configuration:side groups R alternate sides of the chain.

3 Atactic configuration: random orientations of groups R along the chain.

Geometrical isomerismGeometrical isomerism: consider two carbon atoms bonded by a double bond in a chain. H atom or radical R bonded to these twoatoms can be on the same side of the chain (cis structure) or on opposite sides of the chain (trans structure).

Cis-polyisoprene

Trans-polyisoprene

Summary: Size – Shape -Structure

Thermoplastic and Thermosetting Polymers

The response of a polymer to mechanical forces at elevated temperatures is related to its dominant molecular structure.

Thermoplast (thermoplastics: Polymer that soften when heated (and eventually liquefy) and harden when cooled – processes that are totally reversible and may be repeated. Example (polyethylene, most linear polymers.

Thermoset (thermosetting polymers: Polymers became permanently hard when heat is applied and do not soften upon subsequent heating. Examples: vulcanized rubbers, epoxies, phenolics, etc.

Copolymers (composed of different mers)

Copolymers: at least two different types of mers, can differ in the way the mers are arranged:

Random copolymer

Alternating copolymer

Block copolymer

Graft copolymer

Synthetic rubbers are copolymers

Polymer CrystallinityThe crystalline state may exist in polymeric materials. Atomic arrangement in polymer crystals is more complex than in metals or ceramics (unit cells are typically large and complex).

Polymer molecules are often partially crystalline (semi-crystalline), with crystalline regions dispersed within amorphous material.

Polyethylene

Crystalline polymers are denser than amorphous polymers, so the degree of crystallinity can be obtained from the measurement of density:

ρc: Density of perfect crystalline polymer

ρA: Density of completely amorphous polymer

ρs: Density of partially crystalline polymer that we are analyzing

( )( ) 100% xityCrystallin

ACS

ASC

ρρρρρρ

−−

=

Polymer Crystals

Thin crystalline platelets grown from solution - chains fold back and forth: chain-folded model

Polyethylene

The average chain length is much greater than the thickness of the crystallite

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

Photomicrograph spherulite structure of polyethylene

Mechanical Behavior of PolymersThe mechanical properties of polymers are specified with many of the same parameters used for metals. But polymers are highly sensitive to the rate of deformation (strain rate), the temperature and the environment.

The stress-strain behavior can be brittle (A), plastic (B), and highly elastic (C)

Deformation shown by curve C is totally elastic (rubber-like elasticity). This class of polymers -elastomers

A: Brittle Polymer B: Plastic Polymer C: Elastomer

Modulus of elasticity – defined as for metalsDuctility (%EL) – defined as for metalsYield strength - For plastic polymers (B), yield strength is defined

by the maximum on curve just after the elastic region (differentfrom metals)

Tensile strength is defined at the fracture point and can be lower than the yield strength (different from metals)

Moduli of elasticityPolymers: ~ 10 MPa - 4 GPaMetals: ~ 50 - 400 GPaTensile strengthsPolymers: ~ 10 - 100 MPaMetals: 100’s - 1000’s MPaElongationPolymers: up to 1000 % in

some cases Metals: < 100%

Temperature increase leads to:Decrease in elastic modulusReduction in tensile strengthIncrease in ductility

polymethyl methacrylate(PMMA)

ViscoelasticityAmorphous polymer: glass at low temperatures, rubber at

intermediate temperatures, viscous liquid at high T.Low temperatures: elastic deformation at small strains (σ = Eε).

Deformation is instantaneous when load is applied. Deformation is reversible.

High temperatures: viscous. Deformation is time dependent and not reversible.

Intermediate temperatures: viscoelastic behavior. Instantaneous elastic strain followed by viscous time dependent strain.

Viscoelastic behavior determined by rate of strain (elastic for rapidly applied stress, viscous for slowly applied stress)

Load is applied at ta and released at tr

Elastic

ViscousViscoelastic

Forming Techniques for PlasticsVarious techniques are employed in the forming of polymeric materials:

Injection Molding Compression Molding Transfer Molding Rotational Molding Extrusion Blow Molding

Blown film extrusion Thermoforming Calendaring FiberingFoaming Laminating

• Process for forming thermosets by applying heat and pressure.• A measured amount of thermoset powder, granules or pellets, is fed into the mold cavity.• Heat softens the material and pressure fills the cavity, then the material is cured.• Heat actually causes the polymer to transform into a highly cross-linked and networked structure.• Process is of limited use for thermosets because of the cooling timerequired of the mold.• Typical products include electrical insulators, pot handles, and some automotive parts.

Compression Molding

• Associated with processing thermoplastics.• However, with development of the reciprocating screw type equipment, thermosets can also be injection molded.• The basic process includes plasticizing, injection, cooling, and ejection.• Granules are feed from a hopper into to a screw that rotates to feed the material into a heated chamber to allow the material to change to a molten state.• The material is then forced through a nozzle into the mold cavity.• A cooling time is necessary to allow the polymer to become solid, and then is ejected from the mold by mechanical ejector pins.

Injection Molding

• Used to produce thin film hollow tubes.• Somewhat of a combination of extrusion, blown molding and calendaring.• As material is extruded, air is forced through the center of a die, causing the material to expand to the diameter of the mold.• Mold is open at the end, and the material is continuously taken up on rollers.• During the take up process, the walls on the "tube may be seamed welded and perforated such as the case with garbage bags.

Blown Film Extrusion

• Continuous flow of molten material is forced through a die.• Shape of the final product is determined by the shape of the die opening.• Thermoplastic material is fed from a hopper, similar to the configuration of the screw system in injection molding.• The screw forces the material through a tapered opening in the die.• Heat and friction causes plasticizing to occur, softens the material, and forces it through the die opening.• Material is cooled by either air or water.

Extrusion

• Rate of cooling can be controlled and further forming is possible.• Example, PVC pipe is extruded as electrical conduit. If allow to be immersed in hot water, the conduit can be bent at 90 degree angles.• Products that are extruded include tubing, rods, bars, moldings, sheets and films.• Extrusion is also used for coating wire and cable.

Making Fiber Optic Cable